How to Diagnose Car Electrical Problems by Tracing Voltage Drops

Diagnose electrical problems by chasing voltage drops, instead of the old-fashioned way — checking continuity. Including Mike Allen’s Physics 101 introduction to Ohm’s law.
The road ahead looks like a black hole. It’s so dark and so 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 yellower. A quick stop at the convenience store for gasoline and a quart of carrot juice reveals the cause–one of your headlights is as yellow as Satan’s toenails.
You’ve got a voltage drop.
Back to Basics
Electricity shouldn’t be daunting, especially when it comes to automotive wiring. It’s simple direct current (DC), and it doesn’t pack enough punch to make your toes tingle–even if you’re standing in wet sneakers. I will grant you, working on an electrical system just isn’t as intuitive as a mechanical system is. Imagine the linkage to a carburetor. Remember carburetors? Carbs are easy to understand. If one end of the throttle linkage moves when you wiggle it and the other end doesn’t, it’s broken. If you wiggle one end and neither end moves, it’s stuck.
And if it’s hard to move, it needs to be oiled. Electrical-system diagnosis, on the other hand, is one step removed–you can’t see the electricity in the wire like you can see the linkage wiggling. Sure, you can do simple electrical diagnosis with nothing more than a trouble light. I have a couple of trouble lights, and I use them all the time. But diagnosing anything more complicated than a burned-out bulb calls for bigger guns. You need a voltmeter. Or, more technically, a digital multimeter, or DMM. You can get a decent one for about the price of a couple of pepperoni pizzas.
Meeting With Resistance
Back to your dim headlight. There’s resistance in the circuit, reducing the voltage available at the headlamp. You can use the DMM’s ohmmeter scale to find the extra resistance, right?
Wrong. We’re chasing very small resistances, often smaller than a single ohm. The resistance (ohm) scale on your 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. Let’s dig in.
Start by turning on the offending circuit–in this case the headlight low beams. Now we’ll measure the battery voltage. We need to know the exact number you see when metering across the battery posts. And I mean the lead posts themselves, not the clamps. It should be around 12.5 to 12.8 volts if the battery is fully charged.
Back-probe the connector on the dim headlight. The black lead on your DMM should go to a good ground–preferably to the battery negative post. The voltage you meter at the low-beam lug, as it turns out, is about 11 volts. That’s lower than our system voltage at about 12.5–but not low enough to explain the severe dim-out. Now probe the ground lug at the bulb connector. Surprise! The meter reads nearly 4 volts–it should read zero. This indicates a resistance in the ground side of the wiring, leaving only 7 volts for the filament.
First lesson: Electricity always runs in a circle, and the ground side is just as important as the hot side.
Second lesson: Use a little systems analysis. Only one headlamp is dim, so you can skip troubleshooting any part of the circuit that’s shared with the one that’s working.
As you’re metering the ground side, suddenly the voltage on the meter jumps up. And it doesn’t jump to the 11 volts we saw before–it jumps right up to 12.5 volts, exactly what we can meter at the battery. The bulb goes out at the same instant. Now what?
You’re metering full battery voltage. That means there is lack of continuity–an “open” in the circuit somewhere 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 for sure. What used to be a resistance, around 1 ohm, in that ground circuit has suddenly become an open, with essentially infinite resistance. Culprit? 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–eventually causing the wire to fail.
Which brings up another lesson: Never poke a hole into a wire to check a circuit. So, you replace the wire. Problem solved; at least until you go around front to check the lights. Now they’re both the same color. Perking up the dim one suddenly makes you realize they’re both less than brilliant–which is what I’d expect when I meter 11 volts at the bulb socket instead of the 14 I’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 is 15 amps or more. Any resistance between the clamp and the 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 nearly a volt. Popping in a new relay puts that reading down 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 we can calculate that it has a resistance of about 3 ohms. Our cheapo trouble light has a resistance of 10 to 12 ohms, meaning that if we poke the trouble-light probe into a circuit, it becomes part of the circuit, changing the values we’re trying to diagnose. Our DMM has a resistance of over 10 million ohms, eliminating the possibility that attaching the meter probe will change the voltage in a circuit. It’s important to do this testing with the circuit turned on and operating when you’re troubleshooting. Imagine that our corroded wire was in the positive side of the headlamp circuit, not the ground side. And the battery is a little low, so you just pop the connector off the bulb and meter the socket. If the wiring is fine, you’ll see full system voltage on the meter, so everything must be peachy, right? But there’s our damaged wire in there, with an internal resistance of an ohm or three. You’d expect the meter to show reduced voltage, and you’d be wrong. 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.
I’m not happy with more than a few hundred millivolts of drop across any connector. 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.






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