To spark interest, I'd love to say electrical damage has become more common recently because of the sophistication of modern vehicles. But damage to electrical wiring and components has occurred frequently in accidents because of the crushing nature of a collision.
With the introduction of hybrid vehicles, we have a broader range of voltages with which to contend. Though electrical components always have had an element of danger, it has become more pronounced with the new higher voltages.
From testing for power with a test light to the more complex issues of continuity, parasitic load, shorts, and voltage drop, these tests are the cornerstone of automotive electrical testing for standard 12-volt systems and in intermediate 24/36 voltages used by some hybrid systems to run electrical air-conditioning and power steering.
Definitions
Three terms refer to the values of electricity in a simple circuit: voltage, current and resistance.
Picture a vessel filled with water with a pipe coming out the bottom connecting it to another vessel that has only a little water in it (Fig 1). The water will flow from the full vessel to the other. Its pressure is its voltage (measured in volts). The small pipe at the bottom where the water flows through is its current (measured in amps), or the electrical flow through wires. The pipe diameter restricts the amount of water that can flow through that pipe, which offers resistance (measured in ohms). A valve in the small connecting pipe represents a switch turning on or off the flow (current) of water.
These three measurements – volts, amps and ohms – are used through testing procedures. By knowing their relation to the circuit, a technician can diagnose the problem and understand the fault to perform appropriate repairs.
A simple automotive circuit needs power, such as a battery. Then the circuit's potential pressure (voltage) can flow through a path (conductor) provided by the wires connecting the battery to a protective device – a fuse in this case. This device protects the circuit from overload, and will blow or stop the flow of electricity from flowing beyond it. The current flows through the fuse to a switch that can be placed in the "on" position so current is allowed to flow through it to the load (in our example a motor) with a path from the load back to the battery. In the case of most automotive circuits, the vehicle body acts as a universal grounding for the battery, which also is grounded to the vehicle body. This simple circuit contains five of the most common components of an automotive electrical circuit (Fig 2).Components
A switch is the device that allows current to flow (when it's on) through the conductor to the load and back to the power source. It can be placed in the circuit at any point in that circuit. If it's placed before the load, it's referred to as a power-side switch, which allows current to flow to (when on) or stops the current flow (when off) to the load then back to the battery. If placed in the circuit after the load, it's referred to as a ground-side switch. A common ground-side switch (but not always found there) could be the horn switch. When the horn button is pushed, the ground is connected, and the current flows to the horn. This circuit is a series because it has only one path for the current to flow. If I had more than one load, such as two or three lights, and the current was interrupted at any point, the current would not flow, and all the lights would go out. Older Christmas tree lights operated with a series circuit. If one light blew out, all the lights would go out and that bulb would need to be discovered and replaced for the entire string to relight (Fig 2). A parallel circuit has more than one path for the power (Fig. 3) and will allow the current to flow around a blown-out light (load) to the others in that circuit. In newer Christmas tree lights, for example, the circuit is parallel: If one light goes out, the remainder will continue to be lit, so the blown one is easy to find. Automotive circuits are often complex and use a combination of series and parallel (Fig 4) within a single circuit. Understanding how the two circuits operate can make a complex combination circuit approachable.Tools
Though there's a staggering array of tools used to diagnose automotive electrical flats—which can range from inexpensive to beyond the reach of a technician—there are three tools that can be obtained for reasonable amounts that will test the majority of electrical (not electronic) damage from collision damage.Test for power
Using a test light is simple. Attach the cable end to a good ground and use the pointer end to touch the conductor where power should be (Fig. 8). A test light can be used to diagnose the fault's location. By moving back in the circuit path, the exact point of the fault can be found.
Often in collision repair, electrical problems can be diagnosed by observation and inspection of the wires (looking for breaks or exposed wires); loose fittings jarred apart from the collision; and loose, incomplete or disconnected grounds. When reassembling a vehicle, a technician may find the ground didn't make good contact or just has neglected to be reconnected. In connecting a ground to freshly painted parts, a good ground must be made.
Another cause for electrical faults in collision repair is damaged or corroded fittings. The damage caused from the collision can often be found on inspection, and then the end can be repaired or replaced. Corrosion may have started when the vehicle was dissembled and developed quickly while in storage.
Some techs advocate the use of dialectic grease, which is nonconductive grease that will lubricate the plastic or rubber connector but won't help with conduction. The part must be cleaned before reconnecting or applying the lubricant grease. WD-40 is a good solvent and corrosion cleaner designed to establish conduction. It can be used to clean mild to moderately corroded connectors. Dialectic grease is suited better for application to spark plug boots or trailer light connections, so they attach and detach easily.
Shorts
A short circuit is caused by a conductor touching a ground or other conductor where it shouldn't. They come in many forms. One is short to ground, which means if the ground occurs before the load, the short will cause the fuse or other circuit protector (fusible link) to blow. If the short occurs after the load but before a ground-side switch, the load will work, but the switch won't (e.g., the horn blows after the accident and won't turn off). A short to ground after the load and the switch may not affect the circuit.
One of the most difficult shorts to find is an intermittent one. This will be caused by a short to ground that only happens when the vehicle hits a bump or is jostled. If an intermittent short is suspected on inspection, wiggle the harness, which may trigger the short of a loose fitting. Intermittent faults can be caused from a poor ground, such as seen in a flickering headlight as the car bounces down the road.
To test for a short or open circuit, attach the clip to the circuit's ground and test the probe on the battery. If the light flows, the ground is good. Then test the circuit before and after the fuse to test it and its connectors. (I've seen fuse boxes that weren't obvious damaged in a collision.) If all is well, move to before and after the switch to test it. Then test before the load. This will isolate the area of the fault.
Using a test light isn't without drawbacks. Many test lights have a sharp probe that can be used to pierce insulation of a conductor for a quick test. This hole, which creates a place for moisture to enter, allows corrosion to start. Use this method only when the probe can't be placed on the conductor by other means, such as in through the connector or removing the connector and testing the open connector. A hole in the insulation will invite eventual corrosion and failure.
Continuity test
A continuity test checks if the conductor of circuit has a complete path for the current to flow. If the conductor has been broken, a continuity test can reveal it. The crushing nature of a collision often can squeeze a wire in a harness, severing it, with little or no visible damage to its exterior. However, the continuity test may be misleading or damaging. The test tool, if it's a dedicated continuity tester, or a DVOM with continuity testing capacity, applies low voltage to a conductor between the two ends to identify if the conductor is complete (Fig 10).Most conductors in a vehicle are multifilament (many wires twisted together), but a continuity test may show connection with only one wire making contact. Because of this possibility for mistaken diagnosis, many techs will use the voltage drop test instead of the continuity test.
When testing electronic circuits, the continuity test isn't advised because even the low voltage sent along the conductor to test it may be more than the circuit can bear and overload the delicate electronics. Because of these two potentially damaging or incorrect results, most techs voltage drop test instead.
Parasitic load
All modern vehicles will have a slight parasitic load because of constantly-on electronics such as the radio. The battery, if left for long periods of time without charging, also will show a slight discharge. If the discharge is more rapid, there may be a greater-than-normal parasitic drain that must be diagnosed. To do this, use the following procedure. Make sure the vehicle has a fully charged battery and all circuits are turned off (including the engine bay light being removed). Disconnect the negative battery cable so it can be removed easily. Connect the clamp of your test light to the negative battery cable and the probe on the battery post. Then remove the cable (Fig 11). The contact shouldn't be broken. If a parasitic load exists, the test light will come on. With the light on, remove and replace visible fuses one by one until the light goes out, showing the circuit where the parasitic load exists. Also, a low amp probe or the DVOM's amp function can accurately measure the amount of drain.Voltage drop
Some argue the voltage drop is the most significant test that a technician can perform. A DVOM is utilized to measure the resistance in the circuit. All of the circuit components have some resistance, even the conductors (wire). So some voltage drop can be detected in all circuits. It's when the voltage drop is greater than what's expected that indicates a fault. A circuit's load is likely to have the most resistance, but it can be calculated.As an example, a 12v circuit with three lights each with a 4v drop will add up to 12v voltage drop at the ground of that circuit. A voltage drop of greater than 4v at any point of that circuit would indicate a problem. This test measures the internal resistance of a wire or component. If you suspect a switch isn't working properly or a motor is running but at a slower speed than it should, a voltage drop test could indicate the amount of resistance inside that component.
Voltage drop is a much faster test than disconnecting the suspected component and using an ohm meter to test its resistance. The circuit can remain powered and the voltage checked before the component is checked. Then compare the voltage after the component. If the drop is greater than it should be, the component is defective and must be repaired or replaced. You'll need to know the correct resistance of the device you're testing.
For example, you're testing a heater blower switch with three settings and you note a large voltage drop with the motor running slowly. If the switch is set on low, the large voltage drop may be what the switch is intended to do. Restrict the voltage to the motor so it'll run slower on low, with less resistance on medium, and with little resistance (at the switch) for high.
Testing for power, a good ground, finding a short, parasitic load and voltage drop are simple, quick and useful electrical tests all technicians should know. The tools required are simple and inexpensive. Some of the most common electrical faults can be found with good inspection.
By the way, snapping a cell phone photo while disconnecting the circuit (to remember the roughing during reassembly) and checking for a good ground when assembling the vehicle will go a long way toward eliminating many problems.
