Understanding the basic theory of electricity makes electrical troubleshooting much easier. Several gauges are used in electrical troubleshooting to see inside the circuit being tested. Without a basic understanding, it will be difficult to understand testing procedures.
Electricity is the flow of electrons, hypothetical particles thought to constitute the basic "stuff'' of electricity. In a comparison with water flowing in a pipe, the electrons would be the water. As the flow of water can be measured, the flow of electricity can be measured. The unit of measurement is amperes, frequently abbreviated amps. An ammeter will measure the actual amount of current flowing in the circuit.
Just as the water pressure is measured in units such as pounds per square inch, electrical pressure is measured in volts. When a voltmeter's two probes are placed on two live portions of an electrical circuit with different electrical pressures, current will flow through the voltmeter and produce a reading which indicates the difference in electrical pressure between the two parts of the circuit.
While increasing the voltage in a circuit will increase the flow of current, the actual flow depends not only on voltage, but on the resistance of the circuit. The standard unit for measuring circuit resistance is an ohm, measured by an ohmmeter. The ohmmeter is somewhat similar to an ammeter, but incorporates its own source of power so that a standard voltage is always present.
An actual electric circuit consists of four basic parts. These are: the power source such as a generator or battery; a "hot'' wire, which conducts the electricity under a relatively high voltage to the component supplied by the circuit; the load, such as a lamp, motor, resistor, or relay coil; and the ground wire, which carries the current back to the source under very low voltage. In such a circuit, the bulk of the resistance exists between the point where the hot wire is connected to the load, and the point where the load is grounded. In an automobile, the vehicle's frame, which is made of steel, is used as a part of the ground circuit for many of the electrical devices.
Remember that the voltmeter is connected in parallel with the circuit being tested (without disconnecting any wires) and measures the difference in voltage between the locations of the two probes. The ammeter is connected in series with the load (the circuit is separated at one point and the ammeter inserted so it becomes a part of the circuit). Since the ohmmeter is self-powered, all the power in the circuit should be off before connecting the meter's probes to each end of the circuit (or circuit segment) being measured.
For any electrical system to operate, it must make a complete circuit. This simply means that the power flow from the battery must make a complete loop. When an electrical component is operating, power flows from the battery to the component, passes through the component causing it to perform its function (such as illuminating a light bulb), and then returns to the battery through the ground of the circuit. This ground is usually (but not always) the metal part of the truck on which the electrical component is mounted.
Perhaps the easiest way to visualize this is to think of connecting a light bulb with two wires attached to it to your battery. The battery in your truck has two posts (negative and positive). If one of the two wires attached to the light bulb is attached to the negative post of the battery and the other wire is attached to the positive post of the battery, you would have a complete circuit. Current from the battery would flow out one post, through the wire attached to it and then to the bulb, where it would pass through, causing it to light. It would then leave the bulb, travel through the other wire, and return to the other post of the battery.
The normal automotive circuit differs from this simple example in two ways. First, instead of having a return wire from the bulb to the battery, the light bulb returns the current to the battery through the chassis of the vehicle. Since the negative battery cable is attached to the chassis and the chassis is made of electrically conductive metal, the chassis of the vehicle can serve as a ground wire to complete the circuit. Secondly, most automotive circuits contain switches to turn components on and off.
Some electrical components which require a large amount of current to operate also have a relay in their circuit. Since these circuits carry a large amount of current, the thickness of the wire in the circuit (gauge size) is also greater. If this large wire were connected from the component to the control switch on the instrument panel, and then back to the component, a voltage drop would occur in the circuit. To prevent this potential drop in voltage, an electromagnetic switch (relay) is used. The large wires in the circuit are connected from the battery to one side of the relay, and from the opposite side of the relay to the component. The relay is normally open, preventing current from passing through the circuit. An additional, smaller wire is connected from the relay to the control switch for the circuit. When the control switch is turned on, it grounds the smaller wire from the relay. If you were to disconnect the light bulb (from the previous example of a light bulb being connected to the battery by two wires) from the wires and touch the two wires together (please take our word for this; don't try it), the result will be a shower of sparks. A similar thing happens (on a smaller scale) when the power supply wire to a component or the electrical component itself becomes grounded before the normal ground connection for the circuit. To prevent damage to the system, the fuse for that circuit blows to interrupt the circuit, protecting the components from damage. Because grounding a wire from a power source makes a complete circuit, minus the required component to use the power, the phenomenon is called a short circuit. The most common causes of short circuits are: the rubber insulation on the wire breaking or rubbing through (exposing the current-carrying core of the wire to a metal part of the truck), or a shorted switch.
Some electrical systems on the truck are protected by a circuit breaker which is, basically, a self-repairing fuse. When either of the above described events takes place in a system which is protected by a circuit breaker, the circuit breaker opens the circuit the same way a fuse does. However, when either the short is removed from the circuit or the surge subsides, the circuit breaker resets itself and does not have to be replaced as a fuse does.
The final protective device in the chassis electrical system is a fuse link. A fuse link is a wire that acts as a fuse. It is usually connected between the starter relay and the main wiring harness for the truck. This connection is found under the hood, usually very near a similar fuse link which protects the engine electrical system. Since the fuse link protects all the chassis electrical components, it is the probable cause of trouble when none of the electrical components function, unless the battery is disconnected or dead.
Electrical problems generally fall into one of three areas:
Problems that fall into the first category are by far the most complicated. It is the current supply system to the component which contains all the switches, relays, fuses, etc.
The electrical system can be checked with a test light and a jumper wire. A test light is a device that looks like a pointed screwdriver with a wire attached to it. It has a light bulb in its handle. A jumper wire is a piece of insulated wire with an alligator clip attached to each end.
If a light bulb is not working, you must follow a systematic plan to determine which of the three causes is the villain.
- Turn on the switch that controls the inoperable bulb.
- Disconnect the power supply wire from the bulb.
- Attach the ground wire on the test light to a good metal ground.
- Touch the probe end of the test light to the end of the power supply wire that was disconnected from the bulb. If the bulb is receiving current, the test light will go on.
If the bulb is one which works only when the ignition is turned ON (such as a turn signal), make sure the key is ON when checking for voltage.
If the test light does not go on, then the problem is in the circuit between the battery and the bulb. As mentioned before, this includes all the switches, fuses, and relays in the system. Follow the wire that runs back to the battery. The problem is an open circuit between the battery and the bulb. If the fuse is blown and, when replaced, immediately blows again, there is a short circuit in the system which must be located and repaired. If there is a switch in the system, bypass it with a jumper wire. This is done by connecting one end of the jumper wire to the power supply wire going into the switch, and the other end of the jumper wire to the wire coming out of the switch. Again, consult the wiring diagram. If the test light illuminates with the jumper wire installed, the switch or whatever was bypassed is defective.
Never substitute the jumper wire for the bulb, as the bulb is the component required to use the power from the power source.
- If the bulb in the test light goes on, then the current is getting to the bulb that is not working in the truck. This eliminates the first of the three possible causes. Connect the power supply wire and connect a jumper wire from the bulb to a good metal ground. Do this with the switch which controls the bulb turned on, and also the ignition switch turned on if it is required for the light to work. If the bulb works with a jumper wire installed, it has a bad ground. This is usually caused by the metal area on which the bulb mounts to the truck being coated with some type of foreign matter.
- If neither test located the source of the trouble, the light bulb itself is defective.
The above test procedure can be applied to any of the components of the chassis electrical system by substituting the component that is not working for the light bulb. Remember that for any electrical system to work, all connections must be clean and tight.