Troubleshooting Trailer Lamps That Receive Power But Won't Illuminate

Updated 06/17/2015 | Published 06/15/2015

Question:

I have installed a brand new trailer light kit. The connection to the car operates, there is power to both trailer lights, but the bulbs will not work. I have tested the ground and used a probe in the light sockets. both the turn signal pin and the tail light pin operated do the peanut bulb socket on the side but the bulbs will not light. I replaced all four bulbs with new ones and they dont work either.

asked by: R. M

Expert Reply:

If you're showing power on the black wire feeding the lamp, and the bulbs are known to be good, the only thing that could cause the lamp not to illuminate is a poor ground connection. You can test this theory by getting a long length of wire and attaching one end to the negative battery terminal of the tow vehicle and touching the other end to the ground wire of the lamp while the running lights are turned on. If the lamp lights, you've confirmed that there's an interruption in the ground circuit somewhere on the trailer. Make sure that the ground wire from the lamps is connected directly to the trailer brake, or a metal piece that's in contact with the trailer frame.

It's pretty unlikely that two lamps could be faulty. I'll link you to our trailer wiring troubleshooting FAQ article.

expert reply by: Mike L