There are few collision repair challenges more vexing and curse inducing than those linked to electrical problems. You're probably familiar with the following scenario:
Your shop has just finished a job and rolled out another freshly painted masterpiece. While performing the final inspection, a dashboard warning light comes on or the vehicle shuts off soon after it starts. Worse, the customer brings the vehicle back a day later because a power window doesn't work or the heater failed. You're stuck spending time and money – impacting your cycle times – to handle a problem in which the source is simple or complicated.
It's time to put together a troubleshooting plan. The difficult part about troubleshooting electrical problems is the number of possible causes. When a vehicle is involved in a collision, electrical components suffer stretching, bending and other trauma that test the patience of even the most experienced tech. Because the clock is ticking to have these problems solved quickly, consider the most likely causes and your best solutions.
Pre-existing problems
One of the surest signs of an electronic failure is a warning light. Before breaking out diagnostic tools, consider if the light is accurate and if it's a result of the collision. There have been plenty of faulty electronic gauge clusters that indicated problems that didn't exist. When investigating any warning light, check with the vehicle owner if you think the light is in error.
Several years ago, a shop handled a repair that involved replacing a collision-damaged transmission. When the vehicle was ready to be returned, the dash temperature gauge showed the vehicle to be overheating. Techs spent two hours testing the cooling system and attempting to run down a potentially faulty sensor. Their efforts turned up nothing. Finally, the manager called the vehicle owner who said the temperature gauge had been signaling incorrect readings for the past three years.
Battery central
Considering the complexity of modern electrical systems, shops can fall into the trap of expecting electrical problems that have sources that are difficult to trace. These problems are just as likely to have predictable causes. Start with the obvious. For electrical issues, including those in addition to a vehicle not starting, look at the battery first. Look for corrosion, especially near the terminals, and proper contact with the battery cables. Check the cables because a front-end collision can crack and damage them.
To evaluate the battery properly, do more than a simple visual inspection. It can be tempting to determine the customer's 72-month battery has been in service for only half that time and assume it still has plenty of life. Don't make this mistake.
Extreme hot or cold climates can cut battery life in half. Always use a digital Volt-Ohm meter to determine if the battery is holding a sufficient charge.
Should you need to replace the battery, test the rest of the vehicle's electrical system. The battery may be just one part of a systematic problem. If the battery or any of the components near it have been compromised, or if the vehicle shows signs of other potential electrical problems, address them. Because the body of work is complete, that doesn't mean your job is over. If you uncover other problems, notify the customer.
Alternator alterations
The other prime source for electrical problems is the alternator. Potential collision damage and other issues related to alternators can create difficulties when solving problems.
Conduct a thorough inspection of the system and determine if its power output is within operating guidelines. If an alternator isn't delivering a sufficient power output, it's going to set off a warning light and/or present other problems.
Modern vehicle electrical systems are overtaxed so often they can set off a warning light regularly, even when nothing is wrong mechanically with the system. Modern vehicles use 80 to 100 percent of their electrical capacity, requiring the alternator to work extra hard much of the time, hence the shortened lives of alternators. Also, heat can limit an alternator to producing only 70 percent of its rated output.
General Motors notes any car can exhibit a low-voltage reading or issues such as dimming lights when electrical loads are heavy, especially when a vehicle is idling. This means you could be attempting to diagnose a problem that doesn't exist.
If the diagnostics indicate reduced power output, determine if the vehicle has sustained front-end damage or has been in a violent collision, which can loosen belt tension, impact pulleys and cause other collateral damage. Be mindful alternators are overtaxed regularly, so even minor changes to belt tension or other seemingly minor issues can have a significant impact on the alternator's performance.
Check OEM tech bulletins and other literature to determine if the vehicle exhibits alternator problems, including false warning lights and dimming headlights, regularly.
Also, the gauge might be damaged, especially if the dash gauge is indicating an alternator problem. Keep in mind that some gauges function differently.
Always check the fuse that controls the light circuit. In some vehicles, if the fuse is blown, the dash light will come on but may not go off. In others – for example, some GM vehicles – a burned-out fuse can cause the warning light work in reverse order. When the vehicle is turned on, the light remains off. Once the engine starts and the alternator begins charging, the light will come on.
Drop outs
The road ahead looks like a black hole. It's so dark and dreary even the bravest Saturday mechanic would prefer to be off the highway and safe at home. Unfortunately, you're still hours away from your destination. You can't see anything except the small puddle of light cast by your headlights. And that puddle seems to be getting smaller and more yellow. A quick stop at the convenience store for gasoline and a quart of carrot juice reveals the cause – a voltage drop.
Electricity shouldn't be daunting, especially when it comes to automotive wiring because it's simple direct current (DC). Electrical-system diagnosis, though, is one step removed. You can't see the electricity in the wire. Diagnosing anything more complicated than a burned-out bulb requires a voltmeter or digital multimeter (DMM).
In the case of a dim headlight, resistance in the circuit reduces the voltage available at the headlamp. But you can't use a DMM's ohmmeter scale to find the extra resistance because you're chasing resistances smaller than an ohm. The resistance scale on a DMM probably bottoms out at 200 ohms, making measurement of single-digit values tricky. Instead, use the voltage scale, which, on most DMMs, is accurate down to several millivolts.
Start by turning on the offending circuit – in this case the headlight low beams. Now measure the battery voltage. You need to know the exact number you see when metering across the battery posts not the clamps. It should be around 12.5 to 12.8 volts if the battery is charged completely.
Back-probe the connector on the dim headlight. The black lead on the DMM should be connected to a solid ground, preferably the battery negative post. The voltage you meter at the low-beam lug should be about 11 volts, which is lower than the system voltage at about 12.5 but not low enough to explain the severe dimming. Now probe the ground lug at the bulb connector. The meter reads almost 4 volts, but it should read zero. This indicates a resistance in the ground side of the wiring, leaving only 7 volts for the filament.
Remember electricity always runs in a circle, and the ground side is just as important as the hot side. Also, use a system analysis. If only one headlamp is dim, skip troubleshooting any part of the circuit that's shared with the one that's working.
If you meter the ground side and the voltage on the meter jumps suddenly to 12.5 volts instead of the 11 volts you saw before, you're metering full battery voltage, which means there's a lack of continuity – an open in the circuit between the DMM positive probe and the battery ground. If the open resulted from a burned-out filament or a broken wire on the hot side, you'd see zero volts. The open is on the ground side.
What used to be a resistance, about 1 ohm, in the ground circuit suddenly has become an open with essentially infinite resistance. It's a broken ground wire probably caused by someone poking a pointy test light or meter probe through the wiring to examine a problem years ago. The hole in the insulation has admitted water to the wire inside, turning it into green, high-resistance corrosion, which will cause the wire to fail.
Remember, never poke a hole into a wire to check a circuit. Replace the wire. The problem is solved, at least until you check the lights. Perking up the dim headlight suddenly makes you realize they're both less than brilliant, which is what to expect when you meter 11 volts at the bulb socket instead of the 14 you'd expect when the engine is running. There's still a resistance in the circuit, but this time it's between the battery and bulb. Back to the DMM.
Meter between the battery positive post and the clamp. You should see very little voltage there. With the lights up, the total draw on the battery should be 15 amps or more. Any resistance between the clamp and post will cause a measurable voltage drop. It shouldn't be more than a few millivolts. Chase the circuit toward the lamp, one metal-to-metal junction at a time. Probing between the input and output of the headlamp relay shows a drop of almost a volt. Popping in a new relay reduces the reading to a few millivolts, and both headlamps are blazing. Problem solved.
Warning: math alert
Your 55-watt headlamp bulb draws 4 to 5 amps from the car's electrical system, and you can calculate it has a resistance of about 3 ohms. The trouble light has a resistance of 10 to 12 ohms, meaning that if you poke the trouble-light probe into a circuit, it becomes part of the circuit, changing the values you're trying to diagnose. The DMM has a resistance of more than 10 million ohms, eliminating the possibility that attaching the meter probe will change the voltage in a circuit.
It's important to testing this with the circuit turned on and operating. Imagine your corroded wire was in the positive side of the headlamp circuit, not the ground side, and the battery is a little low, so you pop the connector off the bulb and meter the socket. If the wiring is OK, you'll see full system voltage on the meter, but the damaged wire is in there with an internal resistance of an ohm or three. You'd expect the meter to show reduced voltage, but it's the current flowing in the circuit that produces the voltage drop. The DMM, with its megohm impedance, draws no current, and you'll read full system voltage until the circuit is loaded down. The total drop in any circuit shouldn't be more than 1 volt, whether it's a dome light drawing 500 milliamps or a starter drawing 200.
