Trailer Brake Lights Don't Work when Running Lights are On

Question:

So I just had to rewire my whole trailer the plug is a 6 wire but I wired it with only 4, the ground has been sanded down to bare metal an is secured to the frame of the trailer. The lights are 2 wires plus side marker lights, I have read the instructions and everything is wire correctly. My problem is when the running lights are off the blinkers and brake lights work, I turn the running lights on all the lights come on, both turns work but when I hit the brakes the the brake lights go out until I release them. Can someone please help me out

asked by: Erin T

Expert Reply:

This sounds like a ground issue. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time. It is also possible that the lights overdraw what the tow vehicle wiring can handle (unless they are LED lights).

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Michael H