How do you perform a voltage drop test on a branch circuit such as a headlight circuit?

• A. With the load on, measure voltage at power source and at the load terminals, calculate voltage drop; compare to acceptable spec.
• B. With the load on, measure current draw through the headlight circuit and compare to spec.
• C. With the load off, measure voltage at the battery terminals only.
• D. Only measure the voltage at the battery's positive terminal.

Answer: (A)

A voltage drop test on a branch circuit checks how much voltage is lost between the power source and the load while current is flowing through the path. For a headlight circuit, you want to see what voltage the headlight actually gets when it’s drawing current.

Do it by leaving the headlight on (the circuit is under normal load) and measuring two points: the source side where power enters the circuit (near the battery or the fuse/feed point) and the load side at the headlight’s input. Use a digital multimeter set to DC volts, keeping the same ground reference for both measurements. Subtract the load-side voltage from the source-side voltage to get the voltage drop. Compare that drop to the vehicle’s specified acceptable range. If the drop is within spec, the wiring, connectors, and terminations are likely good; if it’s too large, check for loose or corroded connections, damaged wiring, or high-resistance terminations causing extra loss.

Measuring current alone won’t tell you how much voltage is actually reaching the headlight, and taking measurements with the load off misses the effect of resistance under operation. Likewise, measuring only at the battery positive terminal or only at one point doesn’t reveal how much voltage is lost along the path to the headlight.