Trailer Wiring Harness is Funtional Except Trailer Brakes

Question:

I HAVE A 7 ROUND PRONG PLUG FOR A RACE CAR TRAILER. EVERYTHING WORKS BUT THE TRAILER BRAKES. I FOUND OUT THAT THE BRAKES DO WORK BUT ONLY WHEN THE TEST LIGHT IS HOOKED UP TO A GROUND AND THE TESTER IS CONTACTED WITH THE TRAILER BRAKE WIRE....HOPE I HAVENT LOST YOU?? PLEASE LET ME KNOW WHAT YOU THINK?? I TRIED JUST PUTTING A JUMPER FROM A GROUND TO THE WIRE BUT IT SHOWS UP OL ON THE BRAKE CONTROLLER....NOT SURE WHAT I NEED TO DO TO FIX THIS? THANKS

asked by: EDDIE

Expert Reply:

Since every other function works, my guess is the brake magnet wires have a problem. One wire from each assembly magnet should be wired to the brake controller wire. The other should be grounded. Since they work when you provide an alternate ground via the tester, I assume the ground is the problem. The first thing I would do is check these grounds. The closer the ground connection is the the brake assembly the better.

There may be a break or a pinch on the brake wire going from the trailer connector back to the brakes, so you could check that wire.

Finally the magnets themselves may need to be replaced. You can pull the wheel and hub off and if you can see the copper wire windings on the magnet, the surface has been worn and the magnet is past the useful life point. I have included a video on replacing trailer brake magnets.

expert reply by: Patrick B