How to Test Lights on a Trailer With a Meter

Plug your truck's wiring harness into your trailer's wiring harness and have an assistant get into the cab of your truck to test the lights. Test brake lights, left and right turn signals, hazards and reverse lights.

Take note of which bulbs are not working.

Remove the lens cover on the light or lights that are not working with a screwdriver.

Remove the bulb or bulbs that are not working by pushing them in and twisting them to the left. Inspect the bulb at first to see if it appears to be burned out. If you have a working bulb handy you can plug it in to see if that solves your problem. If the new bulb doesn't work, proceed to next step.

Turn on your voltmeter and place the voltage setting at 20 volts.

Place the negative (black) meter tester lead on the frame of your trailer to establish a ground.

Place the positive (red) test lead against the bottom of the bulb socket. Do not allow the test lead to touch the bottom and side of the bulb socket at the same time because this can cause a short.

Note the reading on your meter. It should read 12 volts. If it does not read 12 volts it means this socket or its wiring is bad. Remove the wire from the bulb socket. Place one the negative test lead on the trailer frame and the positive lead on the end of the trailer wire. If the wire tests okay, it means the the wire works, but the socket is bad. If the wire is bad, both it and the bulb should be changed out.