Wires and wiring harnesses are the arteries of the vehicle’s electrical system, and as such they need to be kept in good condition, free of any damage or corrosion. They carry the electrical power and signals through the vehicle to control virtually all of the systems on a vehicle. As technology in vehicles has increased, so, too, has the number of wires and cables installed on these vehicles Figure 36-53. Although wireless communication is being used in some vehicle security, entertainment, and tire pressure monitoring systems, wires are still the dominant signal carriers in a vehicle. To help protect wires and keep them organized, they are bundled together in a wiring harness. A number of wiring harnesses are located throughout the vehicle.

Electrical wires are used to conduct current around the vehicle Figure 36-54. Wire can also be referred to as cable, although cable typically refers to large-diameter wire. Automotive wire is commonly a multistranded copper core wrapped with seamless plastic insulation. Copper is typically used as it offers low electrical resistance and remains flexible even after years of use. The insulation is designed to protect the wire and prevent leakage of the current flow so that it can get to its intended destination. Ribbon cable is a series of wires that are formed side by side and joined along the wire insulation; they are flat like a ribbon Figure 36-55. Ribbon cable works well when several wires run from one component to another. The ribbon design groups them so they can be routed neatly and easily. Ribbon cable is often found inside computers and other electronic components. It is used for connecting between printed circuits or between printed circuits and other components. Some wires, especially signal wires and communication wires, are shielded, which helps to prevent electromagnetic interference, also referred to as “noise.”
Shielding

In certain locations within a vehicle and in environments where strong electromagnetic interference (EMI) is present, wiring harnesses are subject to a situation where unwanted electromagnetic induction occurs. This interference is referred to as electrical noise or EMI noise. To prevent noise, some vehicles use shielded wiring harnesses. The type of shielding used can be one of three forms: twisted pair, Mylar tape, and drain lines.
Twisted Pair

Twisted pair uses two wires delivering signals to a common component. The wires are uniformly twisted through the entire length of the harness and end at a terminating resistor. The twisted wires along with the terminating resistor have the effect of canceling any noise that occurs in the wires, reducing the loss of data in the transmitted signals. The controlled area network, or CAN , bus in a modern vehicle may use one or more twisted pairs to connect all the vehicle control units with common data line(s) to share information.

Mylar Tape

Mylar tape is an electrically conductive material that is wrapped around a wiring harness inside the outer harness layer. Any noise that attempts to reach the wires inside the shield are absorbed by the Mylar, where it will be conducted to ground via a ground connection. The shielding is important to prevent electrical noise penetrating into the electrical wiring. If the harness is exposed, the Mylar will have to be rewrapped so that noise cannot penetrate into the harness.
Drain Lines

A drain line is a noninsulated wire that is wrapped within a wiring harness. The drain wire is connected to ground at the harness source end and conducts any noise to ground, negating the noise effect. If the drain wire is cut, it will be inoperative, so it is important that the wire not be cut or left disconnected.

Wire size is very important for the correct operation of electrical circuits. Selecting a wire gauge that is too small for an application will have an adverse effect on the operation of the circuit. This will cause voltage drop and poor performance, or, in extreme cases, the wire will get hot enough to melt the insulation. Selecting a wire gauge that is too large increases costs and weight and the size of wiring harnesses.

The resistance of a wire affects how much current it can carry. Even good conductors have a slight amount of resistance. The resistance of a wire is determined by its length, diameter, construction material, and temperature. The longer the wire and the smaller the diameter, the higher the resistance. The shorter the wire and the larger the diameter, the lower the resistance.

There are two scales used to measure the sizes of wires: the metric wire gauge and the American wire gauge (AWG) table 36-1. The metric system measures the cross-sectional area of the conductor in square millimeters. The AWG system uses a rating number; the larger the rating number, the smaller the wire and the lower its current-carrying capability. Most countries use the metric scale. American manufacturers are split, with some using AWG and others using the metric scale.

To select the correct wire gauge for any given application, it is best to refer to a wire chart. Manufacturers and standards bodies use wire gauge charts to define how much current each wire gauge can carry safely and efficiently. A vehicle uses a variety of wire sizes depending on the requirements of each particular circuit.

The correct wire size for an application can be looked up on a wire size chart if you know the amperage of the circuit and the length of the wire. But be careful of the chart you use, since many of them allow up to a 10% voltage drop over the length of the wire, which is way more than is allowed in most automotive circuits. For example, if a 12-volt circuit is designed for a maximum current flow of 10 amps and is approximately 20´ (6.1 meters) long, using the AWG table as a reference, you can determine that the correct wire gauge to use is 12 AWG table 36-2.

There are two different methods of describing the conductor size within these standards. A wire may be described in metric size as 5.0, indicating it has a cross sectional area of 5.0 millimeters squared (mm²). It can also be expressed as 10/0.5, indicating there are 10 strands of wire, each with a cross-sectional area of 0.5 mm². The same system can be applied to the AWG rating.
Length Versus Resistance

Copper is used to conduct electrical current because of its low resistance value. However, it does offer some resistance, and as the length of the wire increases, so, too, does the resistance within the wire. To overcome the effect of resistance, the greater the length of the wire, the larger the cross-sectional area needs to be. Increasing the cross-sectional area overcomes the resistance and maintains the current-carrying capacity of the circuit. Refer to Table 36-2 for information on wire size and current-carrying capacity.

Terminals are installed to the ends of wires to provide low-resistance termination to wires. They allow electricity to be conducted from the end of one wire to the end of another wire. In many cases, they allow the wires to be disconnected and reconnected. They come in many different types and sizes to suit various wire sizes and termination requirements Figure 36-56. For example, there are push-on spade terminals, eye ring terminals to accommodate screws, butt connectors, and male and female terminals that are designed to be separated and reconnected. Most terminals are the crimp type, which require the use of special tools to crimp the terminal to the end of the wire. They can be insulated or noninsulated. Some solder-type terminals, which require the use of a soldering iron and solder, are still in use and require the use of electric or gas soldering irons, flux, and solder to make the connection. When soldering wiring, always use a rosin or rosin-core solder; never use an acid-core solder, since acid can cause corrosion and high resistance over time.

Terminals can be installed as a single terminal on a wire or grouped together in a wiring harness with a connector housing, also called wiring harness connectors. Connector housings have male and female sides and are usually shaped so that they can be connected in only one way. They will often incorporate a locking mechanism so the plug cannot accidentally work loose. Many of these connectors are weatherproofed to keep moisture out. Special tools are usually needed to insert and remove the terminals from the connector housing.

Wiring harnesses, also known as wiring looms or cable harnesses, are used throughout the vehicle to group two or more wires together within a sheath of either insulating tape or tubing Figure 36-57. Often, harnesses on modern vehicles contain many wires, each terminating at crimped terminals inserted into connector or harness plugs. There are usually a number of harnesses within the vehicle interconnecting with various connector plugs, as required to form the wiring system of the vehicle. Wiring harnesses run around the engine bay, through the dash and interior cabin, and to the rear of the vehicle. They are attached to the vehicle with harness fasteners such as body clips or wire ties, and rubber sealing grommets are used when the harness passes through the metal bodywork

Wiring diagrams, also known as electrical schematics or electrical diagrams, use abstract graphical symbols to represent electrical circuits and their connection or relationship to other components in the system. They are essentially a map of all of the electrical components and their connections Figure 36-58. The wiring on modern vehicles is very complex, with many wires and components interconnected. A single wiring diagram of the whole vehicle would be very difficult to read. To make it easier, the wiring diagrams are split up into systems and subsystems to reduce the complexity on each page. For example, there is a wiring diagram for the starter system, a separate one for the charging system, and others for the engine, transmission, anti-lock brakes, headlights, taillights, and so on.

Wiring diagrams contain large amounts of information in the form of lines and symbols Figure 36-59. The technician will need to decode this information by interpreting the symbols and connections and relating them to the actual components on the vehicle. To assist in understanding the wiring diagrams, manufacturers supply keys on the diagrams, which are lists of the component symbols and their names, wiring color codes, harness connectors, and pin numbers. Armed with all of this information, the technician can read the wiring diagram and identify circuits as they relate to the actual components and circuits on the vehicle.


Printed/Integrated Circuits

Printed circuits or circuit boards are insulated boards approximately 1/16 (25 mm) thick that are designed to hold electronic components such as microprocessors and their associated components. Conductive tracks on the surface of the printed circuits connect the leads of the electronic components to each other to form complex circuits. Printed circuit boards can be very complex with many high-density integrated circuit components installed. Many boards have more than one layer of conductive tracks. They often have two layers and as many as 16 to accommodate the complex circuitry required to connect many integrated circuits.

Technician Tip

Modern vehicles have many electrical components, wiring connectors, and wires. To work on vehicles, you need an understanding of how all the components are assembled together and arranged in circuits. Electrical symbols and circuit diagrams are a way to provide this information in a logical way that represents the physical wiring harness and components attached to vehicles. Every electrical device and component has a corresponding electrical symbol. Many of the symbols are standardized and used universally by manufacturers, although in some cases, variations may exist. Manufacturers’ diagrams and manuals contain keys to identifying the various symbols used and their meaning.

Wires are generally trouble free and long lasting. But they can be damaged. Generally speaking, any issues with wiring are more likely to be with the terminals than with the wires themselves. Terminals can corrode, lose their tension, or push back up inside the connector, leading to poor connections and voltage drops. If you suspect a problem with a wire, first inspect the ends. If no problems are found, look for mechanical damage to the wire or wiring harness itself Figure 36-62. When a wire is damaged, it is usually due to one of several conditions. One possibility is that the wire has been physically broken, such as a wire that was not disconnected when removing a major component such as the engine or transmission. Another issue is when a wire gets pinched between components such as between the engine and the transaxle when replacing a clutch. The pinched wire can cause either a short circuit or an open circuit. Wires can also be misrouted so that they lay on a hot surface such as the exhaust manifold, which melts the insulation and causes the wires to short. In all of these cases, the problem can typically be spotted visually. One problem that may be harder to spot is if a circuit shorts out and melts one or more wires together within a harness. In this case, you may have to open up the wiring harness and inspect the wires. Next, we will cover how to repair wires, terminals, and connectors.

Stripping Wire Insulation

An insulating layer of plastic covers electrical wire used in automotive wiring harnesses. When electrical wire is joined to other wires or connected to a terminal, the insulation needs to be removed. Wire stripping tools come in various configurations, but they all perform the same task. The type of tool you use or purchase will depend on personal preference and the amount of electrical wire repairs you perform. A good pair of wire strippers removes the insulation without damaging the wire strands. Never use a knife or other type of sharp tool to cut away the insulation, as it often cuts away some of the strands of wire as well. This is known as ringing the wire, which effectively reduces the current-carrying capacity of the wire

To strip wire insulation, follow the steps in Skill Drill 36-1:
1
Choose the correct stripping tool for the wire you are stripping. (Photo 1)
2
Most wire strippers can remove the insulation from different gauges of wire, so select the hole in the stripper that is closest to the diameter of the wire to be stripped.
3
Place the wire in the hole and close the jaws firmly around it to cut the insulation. If you have selected the right gauge, the tool will cut through the insulation but not through the copper core. Only remove as much insulation as is necessary to do the job. Insufficient bare wire may not achieve a good connection, and excessive may expose the wire to a potential short circuit with other circuits or to ground. Also, removing more than 0.5" (12.7 mm) of insulation at a time can stretch and damage the core. (Photo 2)
4
Some strippers automatically cut and remove the insulation. Others just make the cut and hold the wire tightly; you then need to pull firmly on the wire to remove the insulation and expose the copper core. To keep the strands together after the insulation is stripped away, give them a light twist. (Photo 3)



Installing a Solderless Terminal

Solderless terminals are used by the factory throughout the vehicle, primarily at connectors. If a wire itself needs to be repaired, it should be soldered back together instead of using solderless terminals to reconnect the wires. Solderless terminals are quick to install and effective at conducting electricity across joints that are designed to be disconnected. Solderless terminals require a clean, tight connection. It is important to make sure the wire and the connection are clean before attaching any terminals. You should use connections that match the size of the wire. Many solderless connectors are color coded for the size of wire they are designed to work with, such as yellow 12-10 AWG, blue 16-14 AWG, and red 22-18 AWG. Use the correct wire stripper to strip only as much insulation off as needed to allow the wire to fully engage the terminal. To keep the wires together after stripping them, give them a slight twist. Do not twist the wire too much; otherwise you risk a poor wire-to-terminal connection. Use the correct crimping tool for the connection. Using the wrong type of tool will cause the connection to have a poor grip on the wire.

To install a solderless terminal, follow the steps in Skill Drill 36-2:
1
Select the terminal for the wire and connector you are using. There are different types and sizes of wire terminals, but the procedure for installing all of them is the same. Make sure you have the correct size of terminal for the wire to be terminated and that the terminal has the correct volt/amp rating for the job it is to perform.
2
Remove an appropriate amount of the protective insulation from the wire with the proper stripping tool. (Photo 1)
3
You will get a better connection if you do not twist the strands together tightly before placing them through the terminal, as this gives the terminal more surface area to come in contact with the wires when crimped. However, it can be difficult to insert the wires into the terminal if they are loose strands. Twist them together just enough to help you insert them cleanly. Place the solderless terminal onto the wire. It is important that the stripped end of the wire be flush with, or only slightly beyond, the crimping portion of the terminal. (Photo 2)
4
Use the proper crimping pliers for the terminal you are installing. Do not use standard pliers, as they have a tendency to cut through the connection, which can cause trouble during service. Select the proper anvil for the connector or terminal selected. (Photo 3)
5
Crimp the core section first. Use firm pressure so that a good electrical contact will be made, but not excessive force, as this can deform the pin or terminal. (Photo 4)
6
If crimping an uninsulated terminal, lightly crimp the insulation tabs so that they hold the insulation firmly. (Photo 5)
7
A note about alternative terminals: Some types of crimp terminals do not have an insulation component fixed to them. These come in two parts, and the insulator is supplied as a separate component. In these cases, always make sure the core of the wire to be crimped extends through the core tabs in the terminal.

Soldering Wires and Connectors

Solder used in automotive electrical applications is an alloy typically made up of 60% tin and 40% lead. Solder needs to change from a solid state into liquid easily and return to its solid state quickly. Solder is available as solid or flux cored. Solid solder requires an external flux to be applied in the soldering process. Flux is needed to prevent the metals from being joined due to oxidization when they are heated. Flux-cored solder has a bead of flux within the center of the solder. Flux in flux-cored solder can have either an acid base or a rosin base. Acid flux is designed to be used on nonelectrical metal joints such as radiators and must be removed after the soldering process so that the joint does not corrode. Rosin flux solder is used on electrical connections because it is much less likely to corrode the metals than acid flux. Acid flux and rosin flux also come in paste form that can be brushed onto the joint if using solid-core solder.

Solder is applied with a hot soldering iron. The soldering iron is heated electrically or by an external source such as a butane or oxyacetylene torch. The soldering iron tip absorbs heat that is then applied to the materials to be joined. Once they are hot enough, solder can be melted between the components. It solidifies as it cools, “gluing” the metal pieces together.

For a connection to be successful, the soldering iron needs to be clean and “tinned.” Cleaning may be as simple as heating the tip and wiping it on a damp cloth. Or, with the soldering iron cold, you may need to use a file to remove oxidized metal and reshape it so it can effectively transfer heat to the wires. The tinning process assists in transferring heat to the wire, by leaving a small amount of liquid solder on the tip which increases the surface area where the tip contacts the wires. To tin the soldering iron, the tip is heated and a small amount of solder is applied to the tip. Excess solder is removed with a cloth rag. The soldering iron tip is heated and then applied to the wire so heat is transferred to the wire. The solder is then applied to the wire opposite the soldering iron. Once the wire is up to soldering temperature, it will melt the solder and pull the solder into the strands of wire producing a strong, effective joint. Do not apply too much heat to the wire or two things will happen. First, the solder will be drawn too far up the strands of wire making a very long, nonflexible joint that is subject to breaking. The second problem is that the insulation may overheat and melt.

To solder wires and connectors, follow the steps in Skill Drill 36-3:
1
When using a soldering iron, you must be careful not to burn yourself or any part of the vehicle you are working on. The tip of the soldering iron has to be hot enough to melt metal solder, so make sure it is in a safe position and not touching anything while it is heating up.
2
While the soldering iron is heating, remove an appropriate amount of the protective insulation from the wires. Always use a proper stripping tool that is in good condition. If you intend to seal the joint with a heat-shrink sleeve, cut a section of this tubular material long enough to overlap the wire insulation on both sides of the joint and slide it over the end of one of the wires before joining them. (Photo 1)
3
Twist the wires together to make a good mechanical connection between them. If there are impurities in the solder, and the wires are not directly touching each other, then although there may be a strong physical connection, there may not be a good electrical connection. It is also very important that the surfaces be very clean before soldering or there will be a poor connection. (Photo 2)
4
Tin the soldering iron tip, and use the soldering iron to gently heat up the wires while placing the solder opposite of the soldering iron. Allow the solder to be drawn into the joint, ensuring that just enough solder runs smoothly into the wires. (Photo 3)
5
Be careful not to use too much solder. Also, if you apply too much heat, you will melt the wire insulation. When you have finished soldering, clean any excess flux from the joint with a damp rag. (Photo 4)
6
Once the electrical connection has been made and it has cooled enough for you to be able to handle it, slide the insulator sleeve over the joint. There are different types of sleeves. The most popular type is heat-shrink tubing, which shrinks when heat is applied to it with a heat gun. Another type contains a glue, which when heated with the heat gun melts into and seals the joint. If there is no heat-shrink tubing available, then it is possible to seal and protect the splice with electrical insulating tape. (Photo 5)
7
To solder a wire to a terminal connector, it is best to crimp it in place as before and just use the solder to “glue” the joint together.
8
Place the heated iron onto the terminal to get it hot enough to melt the solder applied to the end of the crimped wire tabs. Some solder will be pulled between the terminal and the wire. Be careful not to use too much solder. If you get the terminal too hot, the wire insulation will start to melt.
9
Once the electrical connection has been made and it has cooled down enough for you to be able to handle it, you can place the heat-shrink tubing over the terminal, heat it up with a heat gun, and place the connection into service. (Photo 6)


Soldering Wires and Connectors

Solder used in automotive electrical applications is an alloy typically made up of 60% tin and 40% lead. Solder needs to change from a solid state into liquid easily and return to its solid state quickly. Solder is available as solid or flux cored. Solid solder requires an external flux to be applied in the soldering process. Flux is needed to prevent the metals from being joined due to oxidization when they are heated. Flux-cored solder has a bead of flux within the center of the solder. Flux in flux-cored solder can have either an acid base or a rosin base. Acid flux is designed to be used on nonelectrical metal joints such as radiators and must be removed after the soldering process so that the joint does not corrode. Rosin flux solder is used on electrical connections because it is much less likely to corrode the metals than acid flux. Acid flux and rosin flux also come in paste form that can be brushed onto the joint if using solid-core solder.

Solder is applied with a hot soldering iron. The soldering iron is heated electrically or by an external source such as a butane or oxyacetylene torch. The soldering iron tip absorbs heat that is then applied to the materials to be joined. Once they are hot enough, solder can be melted between the components. It solidifies as it cools, “gluing” the metal pieces together.

For a connection to be successful, the soldering iron needs to be clean and “tinned.” Cleaning may be as simple as heating the tip and wiping it on a damp cloth. Or, with the soldering iron cold, you may need to use a file to remove oxidized metal and reshape it so it can effectively transfer heat to the wires. The tinning process assists in transferring heat to the wire, by leaving a small amount of liquid solder on the tip which increases the surface area where the tip contacts the wires. To tin the soldering iron, the tip is heated and a small amount of solder is applied to the tip. Excess solder is removed with a cloth rag. The soldering iron tip is heated and then applied to the wire so heat is transferred to the wire. The solder is then applied to the wire opposite the soldering iron. Once the wire is up to soldering temperature, it will melt the solder and pull the solder into the strands of wire producing a strong, effective joint. Do not apply too much heat to the wire or two things will happen. First, the solder will be drawn too far up the strands of wire making a very long, nonflexible joint that is subject to breaking. The second problem is that the insulation may overheat and melt.

To solder wires and connectors, follow the steps in Skill Drill 36-3:
1
When using a soldering iron, you must be careful not to burn yourself or any part of the vehicle you are working on. The tip of the soldering iron has to be hot enough to melt metal solder, so make sure it is in a safe position and not touching anything while it is heating up.
2
While the soldering iron is heating, remove an appropriate amount of the protective insulation from the wires. Always use a proper stripping tool that is in good condition. If you intend to seal the joint with a heat-shrink sleeve, cut a section of this tubular material long enough to overlap the wire insulation on both sides of the joint and slide it over the end of one of the wires before joining them. (Photo 1)
3
Twist the wires together to make a good mechanical connection between them. If there are impurities in the solder, and the wires are not directly touching each other, then although there may be a strong physical connection, there may not be a good electrical connection. It is also very important that the surfaces be very clean before soldering or there will be a poor connection. (Photo 2)
4
Tin the soldering iron tip, and use the soldering iron to gently heat up the wires while placing the solder opposite of the soldering iron. Allow the solder to be drawn into the joint, ensuring that just enough solder runs smoothly into the wires. (Photo 3)
5
Be careful not to use too much solder. Also, if you apply too much heat, you will melt the wire insulation. When you have finished soldering, clean any excess flux from the joint with a damp rag. (Photo 4)
6
Once the electrical connection has been made and it has cooled enough for you to be able to handle it, slide the insulator sleeve over the joint. There are different types of sleeves. The most popular type is heat-shrink tubing, which shrinks when heat is applied to it with a heat gun. Another type contains a glue, which when heated with the heat gun melts into and seals the joint. If there is no heat-shrink tubing available, then it is possible to seal and protect the splice with electrical insulating tape. (Photo 5)
7
To solder a wire to a terminal connector, it is best to crimp it in place as before and just use the solder to “glue” the joint together.
8
Place the heated iron onto the terminal to get it hot enough to melt the solder applied to the end of the crimped wire tabs. Some solder will be pulled between the terminal and the wire. Be careful not to use too much solder. If you get the terminal too hot, the wire insulation will start to melt.
9
Once the electrical connection has been made and it has cooled down enough for you to be able to handle it, you can place the heat-shrink tubing over the terminal, heat it up with a heat gun, and place the connection into service. (Photo 6)
SKILL DRILL
36-3
Soldering Wires and Connectors
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1
Safely position the soldering iron while it is heating up. While the soldering iron is heating, remove an appropriate amount of the protective insulation from the wires with wire strippers.
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2
Twist the wires together to make a good mechanical connection between them.
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3
Tin the soldering iron tip and gently heat up the wires while placing the solder opposite of the soldering iron. Allow the solder to be drawn into the joint.
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4
A good solder joint where the solder has been drawn in.
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5
Once the electrical connection has been made and it has cooled enough for you to handle it, slide the insulator sleeve cover over the joint and use a heat gun to shrink the tubing around the joint.
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6
To solder a wire to a terminal connector, it is best to crimp it in place as before and use the solder to “glue” the joint together. Place the heated iron onto the terminal to get it hot enough to melt the solder applied to the end of the crimped wire tabs. Some solder will be pulled between the terminal and the wire. Cover the terminal with heat-shrink tubing.

SAFETY
While soldering is generally thought of as a simple process, it can be very dangerous. The solder, soldering iron, and wires are very hot and can cause severe burns. Be careful what you grab or where you set hot items. Molten solder can be flicked by a springy wire up into your eyes, so always wear safety glasses or goggles.

Technician Tip
One mistake students make is trying to apply the solder directly to the tip of the soldering iron while the iron is heating up the wires. This does melt the solder, but it is likely that the wire is not hot enough for the solder to stick to it; instead the solder just globs on top of the wires, leading to what is called a cold joint. A cold joint has high resistance and the wires are likely to break loose from the solder. One sign that the solder joint is good is that you can clearly see the outline of the wires on the surface of the solder, all the way around the joint.


Repairing a CAN-Bus Harness

Always check manufacturer information for the correct procedures when repairing wiring. For a repair on a CAN-bus system, extreme caution must be taken. The CAN-bus system carries high-speed data information and is susceptible to communication errors if the repair is not completed according to the manufacturer’s recommendations. When measuring resistance or voltage on a CAN-bus, always use a digital multimeter and disconnect the negative battery terminal when measuring resistance.

CAN-bus wires are usually a twisted pair with terminating resistors to reduce noise and interferences. Use the correct-sized wires and terminals (if necessary) when making repairs. The challenge in repairing a twisted pair is that you need to perform the solder repair on wires that are twisted together. Simply put, trying to solder one wire back together when the other wire is very close to it is tough. Also, it is harder to use shrink tubing since the wires are twisted. It is possible to make these repairs; it just takes a bit of practice and patience. If both wires in the twisted pair need to be repaired, it is best to stagger the joints so that both joints are not side by side, which could lead to a short circuit. To prevent damage, ensure that all insulation is replaced and the harness is secured correctly. If using a soldering iron and solder to make repairs, use a rosin-core flux and clean the joint before insulating to finalize the repair. Heat-shrink tubing of the appropriate size ensures a sealed and insulated joint.



Introduction

Digital volt-ohmmeters (DVOMs) and oscilloscopes are electrical measuring tools frequently used to diagnose and repair electrical faults. Like many diagnostic tools, practice is required to understand how the DVOM and oscilloscope are used to take electrical measurements and connect them into electrical circuits to ensure correct readings are obtained. Once a reading is obtained, it needs to be interpreted and applied, in conjunction with knowledge of electrical theory, to diagnose the circuit being tested. This chapter provides an explanation of how to use and set up a DVOM for measuring voltage, amperage, and resistance.

This chapter also has a number of exercises that will expand your knowledge on using a DVOM, allowing you to practice taking readings and apply the results. It also relates the practical circuit examples and DVOM readings to Ohm’s law calculations. Basic oscilloscope use and typical waveforms are also covered. Knowing how to properly use DVOMs and oscilloscopes along with interpreting and applying their readings will allow you to diagnose electrical faults, making you very valuable to your employer.

A digital volt-ohmmeter (DVOM) or digital multimeter (DMM) is a versatile and useful piece of test equipment Figure 37-1. It is called a digital meter because the meter gives a numerical reading on a digital display. An analog meter, by comparison, uses a needle that hovers over a series of scales, requiring the technician to determine the numerical value of the reading. Digital meters are easier to read, which means that a technician is less likely to get the wrong reading. The DVOM tends to be the first test tool selected for electrical diagnosis and repairs. Basic DVOMs can measure alternating current (AC) and direct current (DC) voltage, AC and DC amperage, and resistance. Most modern DVOMs can also measure frequency and temperature and have a dedicated diode test capability.

DVOMs come in a variety of layouts and quality. You will want to get used to the meters in your shop so you will know their capabilities and how to use them. Most DVOMs of average quality are “fused,” meaning that one or more “fast-blow” fuses are included inside the DVOM. If the amperage is too high, the fuse will blow, protecting the meter. If the meter is unfused, it will not be protected and could be damaged if used incorrectly when measuring amperage.

DVOMs and test leads also should have a CAT rating listed on the front. CAT is short for “category.” Each level, or CAT, is designed to work safely on higher-powered electrical systems TABLE 37-1. CAT ratings were not designed initially for automotive meters since most vehicles use low voltage, but with more and more hybrid and electric vehicles on the road, which operate on very high voltages, CAT ratings are becoming important for automotive technicians. Hybrid vehicles typically require meters and test leads rated as CAT III or CAT IV. Another thing to keep in mind if you are working on high-voltage systems is that you need to wear a pair of certified and tested rubber-insulated gloves, most likely with leather protectors over top. Always use the proper CAT-rated meter and leads along with the proper personal protective equipment when working on high-voltage systems.


DVOM Components

There are two main components of a DVOM, the main instrument body and the test leads that connect the DVOM to the circuit being tested. The DVOM main instrument body has a function switch to choose the type of electrical measurement to be taken, digital display to report the readings, and sockets to connect test leads. Test leads are used to connect the DVOM to or into the circuit being tested and come in pairs: one red, the other black. Basic leads have a probe on one end for making the connection with the electrical circuit being tested and a connector on the other end for plugging into the sockets of the DVOM. A wide variety of test leads and adapters are available to make it easier to use the DVOM; for example, alligator clips enable hands-free connection of the leads. Adapters such as temperature probes and inductive current clamps connect to the input sockets of the DVOM and convert temperature or current flow into a voltage that can be measured by the DVOM

DVOMs read very small quantities in the one-ten thousandths of a unit range up to very large quantities in the range of millions of units in the case of resistance measurements. It is not possible for DVOMs to effectively and accurately measure such ranges with only a single range or scale; they must have multiple ranges or scales. But before we can talk about those ranges, we need to understand that the DVOM screen can only display four or five digits. This means that symbols must be used to substitute for some of the digits. TABLE 37-2 shows the common symbols, their prefix, and the factor they represent. You will have to place the appropriate electrical symbol, V, A, or Ω, behind the factor symbol based on what you are measuring. For example, 2168 mV would be the same as 2168 millivolts. It could also be called 2.168 volts, since there are 1000 millivolts in 1 volt. Either designation is correct. The challenge is taking the meter reading and making sense of it, which takes practice.

Once you understand the symbols and what value they represent, you are ready to decide which range to set the meter to. TABLE 37-3 lists a typical set of DVOM ranges; however, there is no single range or scale value used by DVOM manufacturers. The resolution indicates the accuracy of the count within any given range and is different for each range. To achieve the most accurate reading, always select the lowest range possible for the value being measured. For example, if you are measuring 12 volts, you should select the 60-V range, because 6 volts would be too low and 600 volts would be less accurate.

Most modern DVOMs have an automatic ranging capability while maintaining the ability to be used in a manually selected range, which means the user can determine the range. When used in the auto range, the DVOM selects the best range for the value being measured so that the technician does not have to be concerned with manually setting the range. But be careful! The meter does not give you flashing light warnings that it has changed ranges, so it is extremely easy to miss that. Many a technician has been led down the wrong diagnostic path by thinking the 12.6 on the meter was volts when in fact the meter had auto-ranged to millivolts; so instead of having full power, the battery being tested had almost no power. To prevent this mistake, many instructors require their students to use only manual ranging when using their meter.


DVOM Use

DVOMs are used to take many different electrical measurements on electrical circuits and are one of the first tools used when conducting electrical repair or diagnosis work. As a voltmeter, the DVOM can measure electrical voltage within circuits; for example, the available voltage at a fuse, switch, or lamp. The DVOM can also measure resistance of a component, connector, or cable, such as the resistance of an ignition coil to check against specifications. DVOMs can also measure current flow in circuits, such as when the amount of current flow through a fuse needs to be checked against specifications. Clearly, a DVOM is a very versatile tool, explaining why it is the most commonly used electrical diagnosis tool. In the next several sections we will further explore DVOMs and how to use them.



Setting Up a DVOM

To set up a DVOM to take accurate measurements, you need to know if you will be measuring resistance, voltage, or current. You should also know the reading that you are expecting so you can be sure to set up the meter appropriately. If very high voltages are to be measured, it is important to make sure the DVOM and leads match the appropriate CAT rating for use at the voltages you will be testing. All of this information will determine the way in which you set up the DVOM, including the connections you need to make on the DVOM and the range you select. Resistance measurements should be undertaken with the circuit disconnected. If measuring the resistance of components, they should be removed from the circuit.
The following steps describe how to set up a DVOM:

    Know what you are testing—volts, amps, or ohms.
    Know the value you expect to be reading (specification).
    Select leads and probes to suit the measuring task.
    Connect the leads to the DVOM.
    Use the function switch to select the type of measurement to be undertaken (e.g., resistance, volts, or amps; DC or AC).
    Select the correct meter range if you are using a manual range meter.
    Connect the leads to the circuit being tested.
    Read the meter display.

Test Leads: Common and Probing

Many people incorrectly label the red lead as positive and the black lead as negative. However, if you look at your meter near the test lead terminals, you will not see a “+” or a “–” anywhere. What you will see is “A” (typically 10 A), “mA,” “common,” and “V/Ω.” Common just means that the terminal is “common” to all of the functions of the meter. In other words, this lead does not need to be moved when different functions of the meter are typically accessed. On the other hand, the red lead does have to move, depending on what function of the meter is being used. That is why it is labeled with the V/Ω symbol, and not “+.” If you find this distinction questionable, consider the following: When we measure various electrical signals at the same time on an oscilloscope, we need more than just the red lead. In fact, we typically use a yellow, a blue, and a green test lead. In all of these situations, the test lead (no matter the color) acts as a probe into the circuit. So rather than referring to the red lead as the positive lead, it is more accurate to refer to it as the probing lead for the DVOM. Then we can introduce probing leads of other colors when we use an oscilloscope.

The other important note is that the meter screen will always read what the probing lead is touching. For example, if the common lead is touching the battery’s negative post and the probing lead is touching the positive post, the meter screen will display a “+” before the reading. That means that the probing lead is touching something more positive than the common lead. If we reverse the leads, the meter screen will display a “–” before the number, meaning that the probing lead is touching something more negative than the common lead. When you understand this concept, rather than jumping to the conclusion that “the meter leads are hooked up backward,” you will be ready to start diagnosing all kinds of strange electrical problems, especially with ground issues and charging system issues

A probing technique is the way in which the DVOM probes are connected into circuits. There are many different types of probes and probing techniques you can use, depending on the circuit being tested. Some examples are alligator clips, fine-pin probes, and insulation piercing clips Figure 37-2. Make sure you know the voltage limits of the probes you use, since high-voltage measurements require special probes that are designed for that purpose.

Never use excessive force when probing; doing so may bend or damage connectors and terminals. The standard probe leads that are supplied with a DVOM are basic straight metal probes useful for making quick measurements in circuits, but they do require the use of both hands to hold them in place. Leads with alligator clips, which come in various sizes, allow the DVOM leads to be clipped onto the circuit and held in place, freeing up your hands for other tasks. These clips are particularly useful for connecting to larger terminals, such as battery terminals.

Back-probing occurs when the probe is pushed in from the back of a connector to make a connection. To perform this task, very fine pins are used to reduce the possibility of damage. The pins are designed to slip into the back of connectors and provide contact without causing damage. Insulation piercing probes are also available but should be used with caution. They have sharp fine pins that pierce the insulation on conductors to create a connection. Remember to always reinsulate the hole that the probe makes to prevent any corrosion. Use liquid insulation or a similar product to reinsulate; do not use room temperature vulcanizing (RTV) silicone, which attracts moisture as it cures, potentially causing corrosion. Since it may result in damage to the insulation or conductor, this type of probe should be used only as a last resort.


Measuring Volts, Ohms, and Amps

The most common measurements taken with DVOMs are voltage, resistance, and current. To take voltage measurements, the probing lead (red) is connected to the volts/ohms, or V/Ω, terminal, and the common lead (black) is connected to the common, or COM, terminal of the DVOM. An appropriate range or auto range is selected on either AC or DC voltage, depending on the voltage to be measured. The probing lead is typically connected to the positive side of the circuit being tested, and the common lead to the negative side. Watch your screen. If the “+” or “–” is not what you were expecting, check the leads to verify they are connected the way you intended. If you still get an unexpected reading, stop and analyze the situation. Ask yourself, what could cause the meter to read that way? Then brainstorm the options.

Most DVOMs can measure milliamps or 10 to 20 amps directly through the meter. The correct range needs to be selected, along with AC or DC. The red probe is connected to the A terminal, and the black lead is connected to the COM terminals. On some DVOMs, there may be a separate mA terminal that the red probe plugs into to measure milliamps. To measure current, the DVOM is connected in series with the circuit, with the probing lead closest to the positive terminal of the power supply or battery. Quality DVOMs typically have an internal fuse that will blow if excessive current flows through it. This fuse is designed to help prevent damage to the meter.

If larger amperage needs to be measured, then current clamps can be connected to the DVOM, and, depending on their range, they can measure high currents, such as starter motor current draw of 400 amps or more. Current clamps are available in a variety of current-measuring ranges. The current clamp fastens around the conductor and measures the strength of the magnetic field produced from current flowing through the conductor and outputs a voltage that the DVOM reads as voltage, which is directly related to the amount of current flowing in amps. When using current clamps, the DVOM is set to read volts. Current clamps also have the advantage that they clamp around the conductor, so the circuit does not need to be broken into to insert the DVOM in series as you would with standard probes on an ammeter.

To accurately measure the resistance of a component, you should remove or isolate the component from the circuit. Doing so removes the possibility of any parallel circuit resistance affecting the resistance measurement. If you need to measure resistance in a circuit, always make sure the power is disconnected. In order to read resistance, batteries inside the DVOM supply the circuit with power to take the measurement. If power is not removed from the circuit being tested, it disrupts the measurement and can provide a false reading or potentially damage the DVOM. To take resistance measurements, the red (probing) lead is connected to the V/Ω terminal, and the black (common) lead is connected to the COM terminal of the DVOM. You will need to select an appropriate range or auto range to measure resistance. The red probing lead is connected to one side of the component being tested, and the black common lead is connected to the other side.
AppliedMath
AM-1: Whole Numbers: The technician can add whole numbers to determine measurement conformance with the manufacturer’s specifications.

An alternator is being tested to determine if it meets manufacturer’s specifications. If an alternator is damaged due to a blown diode or similar problem, it is usually out of specifications by a wide margin. For this type of alternator the output specification is 95 amperes. The technician tests the alternator that puts out 65 amps. The service material states a good alternator will provide an output that is within 15 amps of its rated value. The technician adds 15 amps to the original 65 amps which is a total of 80 amps. This is below the specifications of 95 amps for this type of alternator.

In this example, we are working with whole numbers. If a number has a negative sign, a decimal point, or a part that’s a fraction, it is not considered a whole number.
AppliedMath
AM-2: Decimals: The technician can add decimal numbers to determine conformance with the manufacturer’s specifications.

A starter has been rebuilt and the technician wants to check the pinion clearance. A feeler gauge will be used to determine this clearance. Manufacturer’s specifications for this clearance are from 0.010" to 0.140", with 0.070" considered the midpoint. The technician’s feeler gauge set only goes to 0.045", which fits too loosely in the gap. He places a 0.025" blade next to the 0.045" blade, which together, fits in the gap with just the right tension. The selected gauges are 0.025" plus 0.045" to equal a total of 0.070".

In this example, we are working with decimal numbers. Decimals are numbers that are expressed using a decimal point.

Technician Tip
A technician recently posted an electrical problem on a technical forum. He said that he had hooked up a voltmeter with the black (common) lead on the negative battery terminal and the red (probing) lead on the vehicle engine ground with the engine running. The meter read a negative number. He asked the forum if he had the meter leads hooked up backward. He received several comments saying yes, he had hooked them up backward. However, those technicians were not correct. His probing lead was registering a reading that was more negative than the common lead. But what could be more negative than the negative post of the battery? When the engine is running, the alternator can be more negative than the negative battery post. So what his DVOM was trying to tell him was that there was a voltage drop between the negative battery post and the alternator frame. If he would have understood that the probing lead was not lying to him, that it was reporting exactly what it was touching compared to what the common lead was touching, then he could have started down the path to diagnosing what it indicated. In this case, he should have been looking for a dirty ground connection between the negative battery post and the engine block.

Technician Tip
DVOMs come in many forms. Always follow the specific manufacturer’s instructions in the use of the DVOM or serious damage either to the DVOM and/or to the electrical circuit could result.



Min/Max and Hold Setting

Many DVOMs have special settings incorporated into their design to assist you in taking measurements of rapidly changing values or to freeze the display so that an individual reading is not lost. In the min/max setting, the DVOM will record in memory the maximum and minimum reading obtained during the time the DVOM is connected to a source to take a reading. The min/max setting is often used to measure vehicle battery voltage while the engine is cranking or the battery is charging. During cranking of the starter motor, current is at its highest, but for only a fraction of a second when the engine initially starts to crank. Cranking is also when the battery voltage is at its lowest. In min/max mode, a DVOM will capture the minimum and maximum battery voltage. A limitation in the use of the DVOM is the sample rate. The sample rate is the speed at which the DVOM can sample the voltage. The DVOM does not continuously sample the voltage; rather, it checks the voltage at regular intervals or at a sample rate. While this occurs quickly—for example, every 100 milliseconds—it does mean that if a transient voltage occurs between samples, it will not be recorded by the DVOM. Where quicker sample rates are required, other tools such as oscilloscopes can be used.

The hold function allows the display to be frozen. When the hold function is activated, the display will hold the value on the display until the function or DVOM is turned off. A variation of the hold function is the “auto hold” function found on some DVOMs. When activated, the auto hold function takes a measurement and freezes or holds the display until the function or DVOM is turned off. This function can be useful when taking measurements in difficult locations, such as underneath a dash where you may not be able to watch the meter display while making the meter connections.


The following voltage exercises are designed to explain the use of the DVOM in taking DC voltage measurements Figure 37-3. Examples are given to show the use of different ranges on the meter display and voltage drops in the series circuits across equal and unequal loads. It is important to understand that the sum of the series voltage drops equals the supply voltage as explained in Kirchhoff’s voltage law.

Typically, a DVOM has both an auto range and a manual range capability. The way in which you select auto range and manual range will vary depending on the DVOM. Different DVOMs have different range settings. For example, one DVOM’s setting could be 6 V, 60 V, and 600 V, and another’s 4 V, 40 V, and 400 V. Figure 37-4 shows a circuit with two resistors in a series with a 12-volt DC supply. Various DVOM ranges can be compared by measuring the voltage drops across each of the resistors. The DVOM has the following ranges: 600.0 mV, 6.000 V, 60.00 V, 600.0 V, and 1000 V. TABLE 37-4 provides results of voltmeter readings and a DVOM’s display to show how different ranges affect the way in which the DVOM readings are displayed.

Voltage Drop

Voltage drop is measured with a voltmeter and is the potential difference between two points in a circuit. The sum of all the voltage drops in a series circuit equals the supply voltage, while the voltage drop across all parallel circuit branches is the same. Voltage drop does occur in all parts of the circuit, but in a correctly working vehicle circuit, the vast majority of voltage drop is across the component or load we want to do work, such as the headlight bulb.

Unwanted voltage drop becomes a problem if it becomes excessive and occurs in parts of the circuit other than the load. For example, ideally the only resistance in the circuit would be the headlight bulb. If this were the case, then all of the battery voltage would be dropped (used up) across the headlight bulb. In practice, however, resistance exists in the cables and connectors within the circuit. In a good circuit, the resistance of the cables, terminals or connectors, and switches is very low, causing small and insignificant amounts of voltage drop. A problem arises when excessive voltage drop occurs in the circuit cables, connectors, and switches, which reduces the efficiency of the circuit. Excessive voltage drop is a fault in the circuit and can cause problems; for example, it will result in yellow or dim headlights. To test for unwanted voltage drop of the conductors, switches, and connectors, measure the voltage across each of these parts of the circuit and add the voltage drops together. In a 12-volt system, the total unwanted voltage drop across each side of the whole circuit should not exceed 0.5 volts or 1.0 volt for a 24-volt circuit. And individual voltage drops across an individual wire, connection, or common switch should be less than 0.2 volts.

To measure voltage drop, the DVOM needs to be used on the voltage range. To perform this measurement process, you will need to set the function switch to “auto range volts DC” on the DVOM (some technicians prefer to use manual range, so they will not be fooled by auto range), and connect the black lead to COM and the red (probing) lead to V/Ω. Voltage drop can be measured across components, connectors, or cables. The probing lead of the DVOM is normally connected to the point in the circuit where you want to know the voltage. For example, if you want to know what the voltage is at the positive post of the battery, then you would connect the red lead to the positive post of the battery and the black lead to the negative post of the battery. The voltmeter would then read the amount of voltage greater than is at the negative terminal.

If you are performing a voltage drop test, for example, on the feed side of the horn circuit, you could connect the black lead to the positive terminal of the battery and the red lead to the input wire of the horn (the wire connected to the horn). When you activate the horn, the voltmeter will read the amount of voltage drop in the feed side of the circuit. For example, it might be −4.2 volts. This means that the voltage is 4.2 volts less at the input of the horn (red lead) than it is at the positive battery post (black lead). In this case, the “–” means less than. So there are 4.2 volts less at the horn than at the positive battery post. Since the voltage drop is more than 0.5 volts, this is an excessive voltage drop in that portion of the circuit, and the voltmeter leads will need to be moved wire by wire closer together until the point of the voltage drop is located.

You could make the same measurement with the DVOM leads reversed. If you place the red lead on the positive battery post and the black lead on the input of the horn, the meter would then read 4.2 volts. In this case it shows positive. This is because the red lead is on the positive post of the battery, which is 4.2 volts higher than the horn input where the black lead is connected. As you can see, voltmeter leads can be hooked up in a couple of ways. Just remember that the meter always reads what the red lead is touching.

Figure 37-5 shows a series circuit of two resistors with a 12-volt battery and switch. In this example of how to measure voltage drop, the voltage in various parts of the circuit will be measured with the switch in the open position. table 37-5 gives an explanation of the voltage measurements and validates the measurements with Ohm’s law calculations with the switch in the open position.

Figure 37-6 shows a series circuit of two resistors with a 12-volt battery and switch. In this example of how to measure voltage drop, the voltage in various parts of the circuit will be measured with the switch in the closed position. table 37-6 gives an explanation of the voltage measurements and validates the measurements with Ohm’s law calculations with the switch in the closed position.

Unwanted voltage drops in vehicle circuits can cause real problems and faults. For example, a corroded or bad chassis ground can cause a voltage drop that reduces the voltage and current available to components. Figure 37-7 shows a simple circuit with a bulb connected via a switch across a 12-volt circuit. In this circuit, a corroded ground connection has caused a resistance that is dropping 2 volts across it. table 37-7 analyzes the voltage drops across the corroded ground connection and explains how it reduces the voltage across the bulb, which in turn will cause poor illumination

To measure voltage drop, the DVOM is used on the voltage range. Select “auto range volts DC” on the DVOM, and connect the black lead to COM and the red lead to V/Ω. Voltage drop can be measured across components, connectors, or cables, but current has to be flowing to get an accurate measurement. Remember, the leads when checking voltage can be placed in either direction. Just remember which way you placed them so you understand what the reading means. We will show the red lead on the most positive side for purposes of the following diagrams.

In Figure 37-8, the resistors in the series circuit each have the same value—3 ohms. TABLE 37-8 lists the circuit voltages and provides an explanation for each.

To measure voltage drop, the DVOM must be set on the voltage range. Select “auto range volts DC” on the DVOM, and connect the black lead to COM and the red lead to V/Ω. Voltage drop can be measured across components, connectors, or cables accurately only when current is flowing. Remember, the leads when checking voltage can be placed in either direction. Just remember which way you placed them so you understand what the reading means. We will show the red lead on the most positive side for purposes of the following diagrams.

Figure 37-9 shows that the resistors in the series circuit have different values; R1 is 4 ohms and R2 is 2 ohms. table 37-9 lists the circuit voltages and provides an explanation for each.

To conduct this exercise, the DVOM must be set to read “DC amps.” The red lead will be connected to the A socket and the black lead connected to the COM socket. If using a manual-range DVOM, select an appropriate range. If unsure which range is appropriate, start with the largest range and work down.

Figure 37-12 shows a circuit with two resistors in series with a 12-volt DC supply. The DVOM can be connected in various parts of the circuit to measure the current flow. table 37-11 shows the results and an explanation of the ammeter readings in a series circuit.

In this example, a relay controlled by a switch will be used to switch the current through a resistor. The compass is used to demonstrate that a magnetic field is produced around the relay winding when the current flows through it. To conduct this experiment, set the DVOM to measure “DC amps.” Connect the red lead to the A socket and the black lead to the COM socket. If using a manual-range DVOM, select an appropriate range.

Figure 37-13 shows a circuit with a relay controlled by a switch and a single resistor with a 12-volt DC supply. The compass is used to show that when energized the relay winding produces a magnetic field. The DVOM will be used to measure current. table 37-12 shows the current flow through the circuit and provides an explanation of the circuit, current flow, and how Ohm’s law calculations can be used.

In this example, a relay controlled by a switch will be used to switch the current through a resistor. The compass is used to demonstrate that a magnetic field is produced around the relay winding when the current flows through it. To conduct this experiment, set the DVOM to measure “DC amps.” Connect the red lead to the A socket and the black lead to the COM socket. If using a manual-range DVOM, select an appropriate range.

Figure 37-13 shows a circuit with a relay controlled by a switch and a single resistor with a 12-volt DC supply. The compass is used to show that when energized the relay winding produces a magnetic field. The DVOM will be used to measure current. table 37-12 shows the current flow through the circuit and provides an explanation of the circuit, current flow, and how Ohm’s law calculations can be used.

In this section, the exercises are designed to explain the use of the DVOM when taking DC current measurements. Undertaking the exercises will improve your understanding of Ohm’s law and current measurements. Examples are given to demonstrate measuring current and to show the magnetic fields produced around a conductor when current flows. It is important to understand that current is the same in all parts of a properly working series circuit. Always remember that an ammeter must be connected in series within the circuit Figure 37-10. That means that the circuit must be broken in two and each end of the ammeter connected to one of the two broken ends. This method will ensure that all of the current flowing through the circuit flows through the ammeter.

In this exercise, voltage and current measurements will be taken. For voltage measurements, select “auto range volts DC” on the DVOM, and connect the red lead to V/Ω and the black lead to COM. For current measurements, select “auto range amps DC” on the DVOM. Connect the red lead to the A socket and the black lead to the COM socket. If using a manual-range DVOM, you will need to select an appropriate range.

Figure 37-11 shows a circuit with a single resistor with a 12-volt DC supply. The DVOM will be used to measure both voltage and current. table 37-10 provides an explanation of the DVOM readings and shows how they relate to Ohm’s law.

In this exercise, a resistance measurement will be taken. For resistance measurements, you need to select “auto range Ω” on the DVOM, and connect the red lead to V/Ω and the black lead to COM. If using a manualrange DVOM, select an appropriate range by starting at the highest range and working your way down. Resistance measurements should only be taken with power disconnected, and ideally, with the component disconnected from the circuit.

Figure 37-15 shows a circuit with a lamp in series with a resistor and a 12-volt DC supply. The DVOM will be used to measure resistance. table 37-13 shows the measurement that can be expected from the circuit.

Electrical circuit testing begins with understanding circuit types and how electricity behaves within them Figure 37-38 Figure 37-39. Add to that the ability to use meters and oscilloscopes to measure the values of voltage, amperage, and resistance, along with understanding how to read wiring diagrams so you will know how the circuits are constructed, and you will be well on your way to diagnosing electrical faults successfully. Those are the concepts we will be exploring in this section. Feel free to refer back to the previous circuits to help you remember how electricity behaves, as well as how meters are hooked up for specific measurements. Let’s kick this off by seeing how Ohm’s law can help us predict the behavior of electricity.

Ohm’s law can be used in two ways to help in diagnosing electrical circuit faults. The first is by using it to perform the math to predict and verify measurements. The second way is by using the relationships it demonstrates to guide you through the diagnosis process.

When using Ohm’s law in the first way, it is used to calculate electrical quantities in a circuit and is valuable in cross-checking actual measured results within the circuit. For example, if the resistance and voltage of a circuit are known, then the theoretical current can be calculated using Ohm’s law. The calculated result can then be compared to the measured results from an ammeter to determine if the circuit is functioning correctly. Technicians will often do a quick calculation, sometimes just in their head, to obtain an approximate value of an electrical quantity before they take actual measurements. Doing so allows them to anticipate what they will be measuring and to set the measuring tool to the correct range. Always remember that a calculation may only yield an approximate value; in actual circuits, variations or tolerances exist in components, causing differences between calculated values and actual measurements.

Using Ohm’s law the second way helps you to understand the relationship between volts, amps, and ohms. For example, if voltage stays the same but resistance decreases, amperage must increase. In the case of a short circuit, the resistance decreases and the amperage increases, potentially blowing the fuse. In the opposite scenario, where resistance increases, current flow decreases. This is the case when a corroded or loose connection introduces excessive resistance to the circuit. It also results in less electrical power (volts and amps) to operate the intended load.

What does Ohm’s law tell us to expect when the voltage changes? If voltage decreases and the resistance stays the same, then amperage will decrease. This results in less power being able to operate the load. If the voltage increases and the resistance stays the same, then amperage will increase. If amperage and voltage both increase, then the electrical power operating the load will also increase. This condition can shorten the life of, or even burn out, the load.

So, how do amperage changes affect volts and amps? That is a good question. But if you think about it, amperage is a result of, or product of, the voltage and resistance. Amperage cannot exist without both voltage and resistance—a means of pushing the amperage (voltage) and a path for the amperage to flow (resistance). If you ask yourself, what is the amperage doing in a circuit? the answer will always be, it is doing whatever the voltage and resistance allow it to do. If the amperage is low, then you know that one of two conditions is present—either the voltage is low or the resistance is high. If the amperage is high, then either the voltage is high or the resistance is low. Understanding this relationship between volts, amps, and ohms will help you know what test you need to perform next during diagnosis.

Since amperage is a product of voltage and resistance, it is a good idea to keep your eye on the amperage. What this means is that if you have a circuit fault, you can generally see what the amperage is doing; it is either high or low. If you truly cannot see what the current is doing, like in a solenoid, you will have to measure it. But in most cases, you can see it; the fuse blew because the current was too high, the lights are dim because the current is too low, etc. If current is low (most common scenario), then Ohm’s law tells you that it is because either the voltage is low or the resistance is high. On the flip side, if your eye determines that the current is high, then either the voltage is high or the resistance is low.

For example, let’s say that the left front headlight is dim. Your eye determines that the current in that circuit is low. Thus, either the voltage is low or the resistance is high. Now you just have to test for those two things. Use a voltmeter to measure the voltage at the battery. If low, determine why the battery voltage is low. If good, check the voltage across both sides of the headlight with the circuit on. It should be within 1.0 volt of the battery voltage. If not, then switch to looking for the high resistance. This is accomplished by first measuring the voltage drop on each side of the headlight. If there is an excessive voltage drop on one side, follow the circuit back toward the battery to identify the cause. If the voltages on both sides of the headlight are within specifications, the problem is most likely the headlight itself. You may be able to check the resistance of the headlight itself and compare it to a known good bulb. Or you may have to measure the current flow through the bulb and compare that to specifications, since its resistance increases greatly due to heat when it is illuminated.

If the current flow appears too high, then the circuit may have too much voltage or too little resistance. Measuring the battery voltage is an easy way to check for too much voltage. Using an ohmmeter to check the resistance of the load and comparing that to specifications will tell you if it is shorted. If it is not, then use the ohmmeter to check the wire harness for any short circuit conditions

To use Ohm’s law to diagnose circuits, follow the steps in skill Drill 37-2:
1
Identify the circuit to be tested and determine the expected voltage, current, and resistance of the component or circuit. Using Ohm’s law, calculate the expected voltage, current flow, or resistance of the circuit.
2
Set up the DVOM for a continuity or resistance check. Make sure there is no power connected to any circuit that you test for continuity. Next prepare the DVOM for testing just like you did for voltage by inserting the black probe into the COM terminal and the red probe into the V/Ω terminal.
3
Turn the rotary dial of the DVOM to the mode for measuring ohms, which also measures continuity. The digital display should now give you an “Out of Limits” reading indicating that there is not a continuous circuit connection between the two probes (some meters show “OL,” and others place “1” on the left of the display). Touch the probe ends together. The display should now give a zero reading, or very close to zero, which indicates no resistance. This means there is a continuous circuit through the probes. Some DVOMs also indicate continuity with an audible tone.
4
Check a fuse. One typical use of the test is to determine whether a fuse needs to be replaced. If the fuse has been overloaded and “blown,” then it will no longer complete a circuit when a DVOM is used to test it. To check this, place the black probe on one end of the fuse and the red probe on the other. If the fuse is functioning correctly, then the reading will be zero, indicating a complete, or closed, circuit. If the fuse is open, then there will be no reading and no tone, indicating an incomplete, or open, circuit.
5
A continuity or resistance test is used to check for a broken circuit caused by a break in a cable or lead or caused by a component becoming disconnected. The same test can also confirm whether there is continuity between components that are not supposed to be connected, a condition known as a short circuit. This test can also be used to check circuits that are suspected to have a high resistance.
6
Compare the test results with the calculated results from step 1. Key things to note are any variations between the calculated and measured results. Determine whether the variations can be accounted for within the tolerances of the components or whether a fault exists.

Technician Tip
When using an ohmmeter to measure resistance or to check continuity with a DVOM, the circuit must be powered down to avoid a wrong reading. The best way to ensure this is by disconnecting the component from the circuit.


Vehicle wiring diagrams or schematics may be available as paper-based manuals, computer programs, or online resources. They are produced by manufacturers and some aftermarket publishing companies. Increasingly, repair information is accessed via the Internet using subscription services that are regularly updated. To use wiring diagrams, an understanding of the symbols, abbreviations, and connector coding used in the diagrams is required. These are usually found on the diagram or in information pages. See the chapter Lighting Systems for examples of some of the common symbols used.

Reading a wiring diagram is like reading a road map. There are a lot of interconnected circuits, wires, and components to decipher. Learning to read wiring diagrams takes a bit of time and experience, but knowing that circuits usually consist of a power source, a switch, a load, and a ground is a good start. Jorge Menchu of AESWave has been promoting a novel approach of using color crayons to help understand how a particular circuit in a wiring diagram operates. The following is a paraphrased version of that process.

Begin by printing out a copy of the wiring diagram for the circuit being diagnosed. Color all of the wires green that are directly connected to “ground.” Color all of the wires red that are “hot” at all times. Color all of the wires orange that are “switched to power.” Color all of the wires yellow that are “switched to ground.” If there are any wires that reverse polarity, such as power window motor wires, mark those with side-by-side orange and yellow lines. Finally, color any variable wires, such as signal wires, blue.

Coloring the wires on the wiring diagram in this way does several things. First, it forces you to determine what each wire in the diagram does, which helps you get the total picture. Second, it helps to organize your thoughts so that you can understand how electricity flows through the circuit. Third, it helps to keep you from losing your place or forgetting what a particular wire does. And fourth, it can give you confidence that you have properly diagnosed the problem when you know why the circuit is not working properly and exactly where the problem is located Figure 37-40.

To use wiring diagrams to diagnose electrical circuits, follow the steps in skill Drill 37-3:
1
Identify the correct wiring diagram for the vehicle and system circuit being repaired and print a copy.
2
Color each wire (using the wiring diagram’s color code key) on the wiring diagram for the circuit that requires diagnosis. Note components, wire coding, and harness connectors.
3
Determine circuit test points and their location on the wiring diagram. Find the same test point on the vehicle and perform the appropriate electrical test.
4
Depending on the results of the test, continue to use the wiring diagram to guide you in performing additional tests on the circuit until the fault has been located.



Using a DVOM to Measure Voltage

The electrical system is becoming increasingly complex on modern vehicles, and measuring voltages with a DVOM is a very common task when diagnosing electrical faults. For most measurements, set the DVOM to auto range for ease of use. Select DVOM leads and probe ends to match the task at hand; for example, if you need to take a measurement but require both hands to be free, use probe ends with alligator clips. Ensure that you do not exceed the maximum allowable voltage or current for the DVOM. If you are measuring high voltages, wear appropriate personal protective equipment, such as high voltage safety gloves, long-sleeved shirts and pants, and protective eyewear, and remove any personal jewelry or items that may cause an accidental short circuit.

When using a voltmeter for measuring voltage, you have a couple of options. One is just a simple voltage test. This typically involves placing the common lead on a good ground and the red probing lead on the input side of an electrical component. Doing so will give you a reading of how much more voltage is at the probing lead than is at the common lead. But that only gives us an indication of voltage. It does not tell us how much voltage we started with or how much voltage did not make it through the circuit. Therefore, be careful when using this test to determine if the voltage is good.

A better test is a voltage drop test. A voltage drop occurs when current flows through a resistance. The higher the resistance, the higher the voltage drop. We could say that a voltage drop test measures for excessive resistance in a circuit. When testing for a voltage drop, always have the circuit turned on. That way, the circuit will have current flowing, thereby making voltage drops evident. Just to be clear, in a real circuit, no current flow means no voltage drop.

There are two ways to do a voltage drop test: the direct method and the indirect method Figure 37-41. The direct method uses both test leads on the same side of the circuit. It will directly read how much voltage is lost between those two points. The indirect method leaves the black lead on the negative battery terminal (or other good ground) all the time. The probing lead is moved from one point (generally the positive battery terminal) to another point (generally the input of the load). The second reading is then subtracted from the first reading to give the amount of the voltage drop. As you can see, the first method requires no math, so it is less prone to errors.

So how do you place the leads in the circuit when performing a direct voltage drop test? Let’s assume we are checking the positive side of the circuit for voltage drops. If we stick with the idea that the red lead is the probing lead, then it makes sense to place the black lead on the positive battery post and probe with the red lead. Let’s pretend we are measuring the voltage drop on the positive side of the circuit feeding the low beam filament on the left headlight. With the headlights on low beam, the voltmeter reads −0.71 volts. Wait a minute, there is that negative reading again. Remember, we are measuring voltage drop, so it makes sense that the meter would read that the voltage is 0.71 volts less than (−) the voltage at the black lead. By the way, a 0.71-volt drop is beyond the 0.5-volt drop maximum allowed on one side of a circuit; thus, each part of the feed side of the circuit needs to be voltage drop tested to find the excessive voltage drop.

And how should the leads be placed when checking voltage drop on the ground side of the circuit? If we are looking to measure voltage drop, then we need to place the black lead on the output terminal of the headlight and the red lead on the negative battery terminal. Turn on the low beam headlights and measure the voltage drop. In this case, the meter reads −0.24 volts. Again, it is telling us that the voltage is dropping, this time 0.24 volts, on the return trip to the negative battery terminal.

To look at it another way, you could place the black lead on the negative battery terminal and the red lead on the output side of the headlight. Turn the headlight on and take the reading again. This time it measures 0.24 volts. Why isn’t it negative this time? Good question. It is telling us that there are 0.24 volts more at the output of the headlight than at the negative post of the battery. In other words, the negative side of the circuit has 0.24 volts more at the start of the return path for current flow than it does at the end of the path, which is the negative battery terminal. Thus, we could hook up the meter either way; we just need to know which way we have it connected in the circuit and what reading we expect, whether it is negative or positive. Just don’t get in the habit of ignoring the + and − signs. These signs are indicators that can help us know what is happening within a given circuit.

The other method of voltage drop testing is the indirect method. This method involves taking two voltage readings and subtracting them from each other to determine the voltage drop. This method is useful when working far away from the battery, where connecting one of the voltmeter leads to the battery is not possible, such as when checking lights at the rear of a vehicle. The indirect method involves first measuring the voltage at the battery with the electrical device turned on. This is the base reading. The second measurement is taken at the load that is being tested while the circuit is on. The leads are placed on the input to the load and the ground near the load. This will give the voltage that is available to the load. The last thing to do is subtract the voltage at the load from the battery voltage; any difference is the voltage drop in the system. Note, however, that the drop could be on the power side or the ground side of the circuit, so further testing must be performed if the voltage drop is excessive.

To use a DVOM to measure voltage, follow the steps in skill Drill 37-4:
1
Prepare the DVOM for testing voltage by connecting the black lead to the COM terminal and the red lead to the V/Ω terminal. (Photo 1)
2
Turn the rotary dial until you have selected the mode for volts DC. The reading on the DVOM should now be at zero. Some DVOMs will automatically sense the correct voltage range when a voltage is detected. On other DVOMs you will need to set the voltage range before using the DVOM. (Photo 2)
3
Connect the black lead to the negative battery terminal and the red lead to the positive battery terminal. Interpret the results. If the battery is fully charged, the DVOM will give a reading that is 12.6 volts or more. If it is not fully charged, the reading will be less than 12.6 volts. (Photo 3)

Checking Circuits with a Test Light

Non-powered test lamps are useful in determining if electrical power is present in a part of a circuit. But you should always first test the test light on a known good power and ground before using it to test a circuit. It is possible that the bulb in the test light is burned out. You want to know that before performing any tests. If the test light illuminates, the two ends of the test light are touching both a power and a ground. If the light does not illuminate, the circuit is missing one or both of those elements or the test light is faulty. Test lights are great to grab to perform simple tests such as testing fuses. The test light lead can be quickly grounded and the probe end touched to each end of the suspect fuse. If both ends light, the fuse itself is good (but the fuse box terminal could be loose). If only one side of the fuse lights the test lamp, then the fuse is blown.

To avoid damaging the test light, make sure the circuit voltage you are testing does not exceed the test light’s rating. Most test lights are rated for 6- or 12-volt systems, and using the light in a 24-volt system will usually blow the bulb. You should not use a test light to test SRS (supplemental restraint systems), as unintended deployment of the airbags could result, a very dangerous and costly mistake. Also, using a test light on a computer circuit designed for very small amounts of current flow can damage the circuit.

To check circuits with a test light, follow the steps in skill Drill 37-5:
1
Connect the end of the light with the clip on it to the negative battery terminal. Touch the probe end of the test light to the positive battery terminal. The light should come on. (Photo 1)
2
Connect the clip to any known good ground close to the area to be tested. A typical known good ground is any unpainted metal surface on the vehicle that is directly attached to the battery ground return system. (Photo 2)
3
Locate the device to be tested and place the probe so that the test light circuit is parallel to it. If there is voltage present, the light will come on. (Photo 3)


Checking Circuits with Fused Jumper Leads

Jumper leads can be used in a number of ways to assist in checking circuits. They can be created by the technician or purchased in a range of sizes, lengths, and fittings, or connectors. They are used to extend connections to allow circuit readings or tests to be undertaken with a DVOM, an oscilloscope, current clamps on fuses, relays, and connector plugs on components. In some circumstances, jumper leads may provide an alternate current or ground source for components being tested. Regardless of their application, it is important that the circuit remain protected by a fuse of the correct size. To determine the correct size of fuse for any particular application, refer to the manufacturer's information.

To check circuits with fused jumper leads, follow the steps in skill Drill 37-6:
1
Identify the circuit to be checked and determine the fuse rating for the circuit.
2
Select appropriate jumper leads with the correct fuse rating.
3
Install the jumper lead into the circuit. Perform any required circuit checks. Never use a jumper lead to jump across a load; doing so bypasses circuit resistance and will likely cause excessive current to flow in the circuit.
AppliedScience
AS-77: Capacitance: The technician can demonstrate an understanding of the role of capacitance in timer circuits such as RC timers or a MAP sensor.

Sensors are the components of the system providing input to the computer making it possible for it to carry out its operations.

The MAP sensor, or manifold absolute pressure sensor, plays a big role in proper engine performance. There are several different types of MAP sensors. Some of the popular styles are the variable voltage MAP sensor, the variable-inductance MAP sensor, and the variable-capacitance MAP sensor. Here, we will focus on the variable-capacitance MAP sensor.

The variable-capacitance MAP sensor consists of two aluminum oxide plates in a chamber that is connected by tubing to the engine's intake manifold. This sensor is capable of generating an output signal in hertz proportional to the change in manifold pressure. This is the key point that the technician should understand. The electrical output signal to the computer is directly proportional to engine load as the manifold pressure changes. At engine idle, the vacuum is at approximately 17–21 in. Hg. at sea level. Under this condition, the MAP sensor hertz will be approximately 95. When the engine is at wide open throttle, the vacuum is almost zero inches of mercury, which is converted to a hertz reading of approximately 160. The MAP sensor sends the proportional signal to the computer as a result in changes in engine load. The computer responds by providing more or less fuel to the injectors as well as performing a number of other vital tasks.

SAFETY
Be very careful how you hook up any type of jumper leads, fused or unfused. If you hook them up to the wrong branch of a circuit, especially electronic circuitry, damage can be extensive. There is the old "magic smoke" saying: "Electrical and electronic components work off of the principle of magic smoke. Once the magic smoke is allowed to escape from the component, the component will never function again." Don't use jumper leads in a way that would let the "magic smoke" out of the circuit.



Locating Opens, Shorts, Grounds, and High Resistance

DVOMs, test lamps, and simulated loads tend to be the tools used most often for locating opens, shorts, grounds, and high-resistance faults. Refer to the chapter Principles of Electrical Systems for more information on opens, shorts, grounds, and high resistance faults. An open circuit is a break in the electrical circuit where either the power supply or ground circuit has been interrupted. Most open circuits can be located by probing along the circuit at various points testing for power and by checking for an effective ground at the ground point. A systematic check of the circuit is required by first performing a voltage drop check on each side of the affected circuit. An open circuit will cause a voltage drop equal to the source voltage. Once the voltage drop is isolated to one side of the circuit, voltage drop testing can continue on that side, working the leads closer together in steps. Also, use your understanding of electrical systems to consider the most likely places for the open circuit, such as a blown fuse or a faulty switch. And don't forget that the load could also be open. If the voltage drop test on each side of the circuit is within specifications, use an ohmmeter to check whether the load is open, if possible. Some loads such as diodes cannot be tested with a standard ohmmeter. In this case, follow the manufacturer's diagnostic procedure.

Shorts, or short circuits, can occur anywhere in the circuit and can be difficult to locate, especially if it is intermittent. A short is a circuit fault in which current travels along an accidental or unintended route and can be thought of as a shorter path for current to flow. The short may occur within the load, such as shorted relay windings, or it can be in the wiring, where a wire is shorted to ground or to supply voltage. A short will typically cause lower than normal circuit resistance. The low resistance fault would cause an abnormally high current flow in the circuit and may cause the circuit protection devices, such as fuses or circuit breakers, to open the circuit. A short to supply voltage may cause the circuit to remain live even after the switch is turned off. For example, a short between a wire with power on all the time and a wire switched by the ignition switch would cause the circuit controlled by the ignition switch to remain on even after the switch is turned off. Just remember that shorts can be caused by faulty components or damaged wiring.

Shorts that happen within components, such as a relay coil, can usually best be tested by comparing the reading of an ohmmeter to specifications. Shorts that occur in wire harnesses are usually best tested by disconnecting each end of the affected harness and using an ohmmeter to test for unwanted continuity between various wires. A reading on the ohmmeter when connected to two separate wires indicates a short circuit between them. A true short between wires would be indicated by a very low ohm reading, typically around 1 ohm or less.

Grounds is a term often used in conjunction with shorts and is usually a reference to a short to ground. An initial test can be conducted by carrying out resistance checks or disconnecting the load. For example, if testing the blower motor, first disconnect the blower motor. If the short is still in place, then the wiring between the fuse or circuit breaker and the load must be at fault. To further narrow down the site of the short to ground, inspect the wiring harness, looking for obvious signs of damage. Another test can be conducted by connecting a test lamp or buzzer in place of a fuse. Current will flow through the test lamp or buzzer and find a ground through the short. Parts of the circuit can then be disconnected along the wiring harness to narrow down the location of the short. Specialized short circuit detection tools are also available. They work by sending a signal through the wiring harness where a short is suspected. A receiving device is then moved along the wire loom and will indicate when a short is located. This type of device can be very useful in situations where it is difficult to access the wiring, such as within large wire looms or under vehicle trim.

Short to power refers to a condition where power from one circuit leaks into another circuit. A short to power situation usually causes strange electrical issues. In some cases, one or more circuits will operate when they should not. Or in the case of sensor wires, the incorrect signals caused by the short to power can cause the computer to make very wrong decisions based on the faulty data. In this case, the engine, transmission, or other computer- controlled component can react strangely. Shorts to power are diagnosed first with a voltmeter to check for the unwanted voltage. Next, an ohmmeter is used to isolate the problem in the wire harness.

High resistance refers to a circuit where there is unintended resistance, which then causes the circuit to not perform properly. It can be caused by a number of faults, including corroded or loose harness connectors, incorrectly sized cable for the circuit current flow, incorrectly fitted terminals, and poorly soldered joints. The high resistance causes an unintended voltage drop in a circuit when the current flows. This drop reduces the amount of voltage that can be used by the load. The highresistance fault will also reduce the current flow in the circuit. The reduction in voltage and current to the load reduces the amount of electrical power to load (Power = Voltage × Current), affecting its performance. Unwanted high resistance can best be located by conducting a voltage drop test in the power and ground circuits.

If the high resistance is within the load, such as a relay coil, then the resistance can be checked with an ohmmeter and compared to specifications. Some devices, such as a fuel injector or ignition coil, may need further testing using an oscilloscope. In this way, the waveform can be evaluated, which can indicate issues that an ohmmeter cannot identify as easily.



Inspecting and Testing Circuit Protection Devices

Protection devices are designed to prevent excessive current from flowing in the circuit. Protection devices like fuses and fusible links are sacrificial, meaning that if excessive current flows, they will blow or trip and have to be replaced. Circuit breakers can be reset. Once they trip, they either reset automatically or require a manual reset by pushing a button or moving a lever. Fuses, fusible links, and circuit breakers are available in various ratings, types, and sizes, and must always be replaced with the same rating and type.

In most vehicles, protection devices are situated in the power or feed side of the circuit. A blown or faulty fuse can be tested using a DVOM or test lamp. A good fuse will have virtually the same voltage on both sides. A blown fuse will typically have battery voltage on one side of the fuse and 0 volts on the other side. They can also sometimes be visually inspected. This may require the removal of the fuse from the fuse holder. The fusible element should be intact and, if measured by an ohmmeter, should have a very low resistance. The contacts on both the fuse and the fuse holder should be clean and free of corrosion and should fit snugly together.

To inspect and test circuit protection devices, follow the steps in skill Drill 37-7:
1
Identify the protection device to be inspected and tested.
2
Conduct a visual inspection.
3
Set up a DVOM to read volts or use a test lamp.
4
Energize the affected circuit, if necessary.
5
Test for voltage on both sides of the circuit protection device. Determine and perform any necessary actions.

Technician Tip
It is fine to condemn fuses if they are obviously blown, but if they appear intact, do not rely on your eyes. Over time, fuses heat slightly and cool. This heating and cooling process can cause the fuse to become brittle and crack. The crack can be very fine, almost invisible, yet not conduct electricity, leading your diagnosis astray.



Inspecting and Testing Switches, Connectors, Relays, Solenoid Solid-State Devices, and Wires

Inspection of electrical devices and wires usually starts with a visual inspection of the electrical circuit and is followed up with electrical testing. The visual inspection looks for breakage, corrosion, or deformity and includes examination of the insulation for any worn or melted spots. In the case of switches, solenoid contacts, and relay contacts, an electrical inspection is necessary. For example, all switches would require voltage drop testing to see if they operate properly without excessive resistance. Additionally, solenoid and relay contacts can wear out and produce excessive resistance, so performing a voltage drop test on them is a valid testing procedure. Some solenoids can be disassembled and visually inspected. In this case, the solenoid cap may be removed and the contacts visually inspected. Typically, if there is an excessive voltage drop across the contacts, the contacts will be pitted and burned. Measuring resistance also comes into play when a shorted relay or solenoid winding is suspected.

DVOMs and test lamps are used for most basic testing, with more specialized test equipment, such as oscilloscopes, being used if necessary. It is important to note that test lamps should not be used on electronic circuits due to their higher current draw, which could overpower the electronic components. Some tests, such as resistance tests, can be conducted on components in or out of the circuit. In-circuit tests are often preferred, as they usually provide the opportunity to test the component under load or during operational conditions. After in-circuit testing, components can be removed for individual testing, if required.

Manufacturers produce diagnostic flowcharts that guide the technician through a diagnostic sequence based on test results. In complex circuits, it is good practice to gather as much information as possible about the operation of the circuit and the customer concern. With that information, and the diagnostic flowchart, formulate a testing sequence for diagnosing the fault. Going through this process will help you to understand the problem and to identify possible causes, and a potential sequence of testing.