Troubleshooting Trailer Wiring When Some Lights Fail To Work

Question:

Lights worked well until last outing when I noticed the brake lights and turn signals were not working. I started working on the lights by checking ground connections at three different points on the trailer and at the marker lights. I took them off and cleaned them. I then changed the plug from the trailer to the vehicle. At that point, I had brake lights and signal lights working, but no trailer lights when the headlights in the vehicle were on. That was working with the trailer plugged into a Buick Enclave. I then tried my nearly 400,000 mile Mercury Mountaineer and all of the trailer lights went on with the headlights. The only thing that didnt work were the left turn signal on the trailer nor did it work when the brakes were applied. Went back to the Enclave and had no lights, but brakes and turn signals worked. I am thinking its still the trailer wiring...

asked by: Greg

Helpful Expert Reply:

It sounds like a ground problem on your trailer. You can check everything out by using a circuit tester like part # PTW2993. First check the output form your vehicle to make sure a signal is getting through the connector.

If that checks out move to the trailer side. Use a 12V battery with the circuit tester and start at the trailer connector. Look for corrosion or broken wires and correct as needed.

If your trailer is aluminum or a tilt trailer the ground is known to be an issue. If so, you may need to run jumpers from the grounds on the lights to the main trailer connector ground.

expert reply by: Jeffrey L