How to: voltage drop testing

Your question presents a great opportunity to share a useful diagnostic method: voltage drop testing.

Voltage is a lot like water pressure, and a vehicle’s bulbs, motors, pumps, injectors and other devices that perform work need a proper supply delivered to them. Your charging system and battery create and store this pressure, and the vehicle’s circuits — containing fuses, wires, switches and connectors — deliver it.

Your dim taillight is like a hose-end spray nozzle that dribbles because of a kinked hose or clogged-up faucet.

Before we get our hands dirty, let’s see if the left brake light and other lights near the vehicle’s left-rear side might also be dim — this would point to a faulty common ground, a wire likely screwed into nearby body metal.

To check delivered pressure, you’ll need to access the lamp socket and perform a few easy measurements using an inexpensive multimeter and a couple of T-pins.

Inspect the taillight connector and socket for excessive corrosion (dirty-looking grease in the socket is OK). Try wiggling the bulb in the socket to see if brightness might change. If so, that would be the problem.

Next, look at the color of the wires leading to the various companion bulbs. You’ll likely find one color (possibly

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black) that leads to all of the bulbs. This is the ground circuit.

Next, turn on the headlight switch to just the first click: parking lights on. Take a measurement across the battery and record the number. With the parking lights still on, push two T-pins into the back of the taillight socket, sliding them in right next to the wires and circuits just mentioned.

We’ll use the T-pins as access points to measure voltage delivered to the bulb, with it in place and operating. A 1-volt difference is the most that’s acceptable.

In your case, we might see 2 or 3 volts less at the bulb. This would indicate a circuit fault rather than a problem with the device. (Bulbs work or don’t at all, unlike motors, horns and most other devices that can be kludgy even with good voltage.)

Next, we’ll take our difference meter — our voltmeter — and place the red lead to the ground T-pin and the black to some bare body metal. With the parking lights still on, you want to see less than 0.3 volts difference. A higher reading indicates a corroded or loose ground circuit connection.

Then, using a length of wire, measure the difference between battery positive and the parking light positive Tpin. We’ll allow up to 0.7 volts difference here because of the fuse, switch, connectors and length of circuit.

Because your right-side light is normal, we can rule out parts of the circuit that are shared (fuse, switch, some wiring). A wiring diagram with connector locations would be needed to repeat these tests at other locations. You will be looking to see where you have normal voltage and where you don’t. The problem, likely a loose or corroded connector somewhere, is the culprit!