Troubleshooting Trailer Brake Lights that Don't Function With Headlights or Running Lights On

Question:

I have a 16foot landscape trailer that recently started having an issue. When the running lights are on and you hit the brakes the lights completely go out. When the running lights are off and you hit the brakes the lights go on but very dim. The running lights also seem pretty dim when they are on. Now, what I have done thus far: I put a new 7 pin connector on the trailer, I installed new wiring to see if that fixed the problem and it was the same. I cleaned the light mounting brackets where the studs bolt onto, didnt change a thing. I have tried it on two vehicles and it does the same thing, so its not the vehicle. I put a light tester on the wiring where the lights connect roughly 6 inches from the light and it shows no power on either wire when both are activated. I took a battery and connected to both lights in the back and they were nice and bright with the battery grounded to the mounting studs. So I am competely stumped on what could be the issue here. Any help would be greatly appreciated. Thank You

asked by: Dillon

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.

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. I suspect that your grounds might be sufficient for activating one of the lights at a time like when you put the battery on the lights individually, but just not enough for all of the lights at the same time. This is why I suspect that the problem is the main ground for the trailer that runs to the trailer frame.

expert reply by: Jameson C