Troubleshooting Trailer Lights Turning off When Brakes are Applied

Updated 01/15/2013 | Published 01/14/2013

Question:

2002 Ford Explorer Sport Trac Bought a new set of trailer lights today. I rewired and connected the new wiring harness, ran the trailer light wires and installed the lights according to the instructions. Tested new harness with voltmeter everything read good. Next I connected the harness, and turned on my running lights, both the running lights and marker lights worked, next tried the blinkers and they worked. Lastly, I pressed the brake and every light on the trailer went off. When I let off the brake pedal all the running lights came back on. I turned the running lights off, and pressed the brake again and nothing. ANy suggestions? Thanks.

asked by: Dave

Expert Reply:

What you described sounds like a textbook ground problem. It sounds like the ground is just good enough so that most of the taillight functions work, but when you apply the brakes the ground is not strong enough for that and all of the lights shut off.

I would first check the condition of the tail light wiring checking for exposed wire or corrosion and fix as necessary. You should also make sure that if your new tail lights ground through the mounting hardware that the surface they attach to is a clean metal surface. You should also make sure that the metal is not aluminum. It may be necessary to run a wire from the mounting hardware to the frame of the trailer. Next, I would check the trailer connector on your vehicle and trailer and look for any problem areas there too.

Finally, you should check the wiring between the connector and the tail lights. I attached an FAQ on trailer wiring that you should find helpful also.

expert reply by: Jameson C