2001 GMC Truck with Flatbed Trailer Lights Going Dim when Lights Are On and Brake Pedal is Pressed

Expert Reply:

You have started in the right direction. My knee jerk reaction at the beginning of your question was that there is a ground issue. It still may be a ground issue but since it seems you have eliminated all of the ground connections on the trailer and the trailer worked well on the previous Ford vehicle then it would have to be related to the ground on the GMC which may be a bit harder to run down. ABS systems do not connect in any way with the taillight system. This is done to isolate the ABS from any wiring problems that may occur. If the brake controller is attached to the wrong taillight circuit then this may be where the problem lies. Use a circuit tester, item # 3808, and test each wire on the brake controller to be sure that the correct connections have been made. The black wire should have 12 volts coming from the battery, the blue wire should have voltage when the brake is pressed or the manual slide is activated, and the red wire should only have current when the brake pedal is pressed. The white wire should be attached to a clean ground connection preferably the negative battery post. If you have used a factory style connector, like the Universal Brake Control Wiring Adapter 1999-02 Chevrolet, GMC, Cadillac, item # 20263, under the dash these connections should be correct. If testing the brake controller does not show any issues then the truck is most likely the issue. I would attach a test light to the brake circuit on the trailer and a good ground. With the lights on press the brake and if the test light dims you know the problem is on the vehicle side. The ground for the 7-Way is located along the drivers side frame rail and there are ground straps that you can check to the bed and inside the engine compartment.

expert reply by: Bob G