Trailer Lights Show Very Little Voltage But Used to Work with No Problems

Published 10/07/2011

Products Featured in This Question

Tekonsha Prodigy P2 Trailer Brake Controller - 1 to 4 Axles - Proportional

(2698)

$137.45

Hook Probe Circuit Tester

(56)

$11.88

Question:

I have low voltage to NO voltage at my trailer plag. With lights on I get 1/2 a volt on two terminals. Checked the only fuses marked trailer I could find. They used to works fine.

asked by: Wayne P

Expert Reply:

This sounds like a bad ground. A bad ground usually shows up when the system is under load. Start by checking the trailer and vehicle side connectors. Clean out any dirt or corrosion built up inside and in the backs of the connectors.

Next check the ground on the vehicle side connector. It should be attached to a clean, bare metal surface on the vehicle frame. Also check the trailer side main ground. It should be attached to a clean, bare metal surface on the trailer frame.

From there, check the grounds on the trailer lights. If there is not an obvious ground wire on the light, it probably grounds through the mounting studs. In either case the lights, too, will need to be attached to a clean, bare metal surface.

With all of the grounds checked, you can then look to see that the wiring is physically intact. First looking for any obvious brakes or nicks in the wiring and repair as needed. You can then check the entire length of the trailer wiring by periodically testing the wires with a circuit tester, # PTW2993, to locate breaks you cannot see. You can also do this on the vehicle side if you determine that power is not making it to the trailer.

Your vehicle owner's manual should tell you the location of any fuses or relays related to the trailer lights. I have included links to some videos that might help you.

expert reply by: Michael H