Testing an Electric Trailer Brake Controller

Updated 06/25/2014 | Published 06/23/2014

Question:

I have a 2012 GMC Denali 2500 with a factory installed brake controller and 7-way male plug adapter at the bumper. Im trying to test the male adapter on the truck to see if Im getting power for the brakes at that point. I grounded a test light to the tailpipe for a good ground and then to the brake male terminal in the plug. All I got was a quick flash of light. I had a second person pressing on the the brake pedal as well as holding in the brake control switch with and without pressing on the brake pedal. I know I have a good ground tailpipe because the hot + terminal lights up the tester good and bright. Am I performing this test properly? As always, thank for all your help.

asked by: Ray S

Expert Reply:

The ground may be sufficient but it wouldn't hurt to test it using an alternate ground location as well.

Perform the test using just the manual override and not the brake pedal. Make sure the controller is set to the maximum output level and that it is mounted within the range specified by the instructions. Also make sure that it is not tilted or turned to the side. If the controller you are using has a leveler knob, you will need to make sure it is properly adjusted according to the instructions.

There are some common ways to test a brake controller. In the truck cab you will need to sever the blue wire several inches from the back of the controller. Then use your tester to test the end of the blue wire coming out of the controller. It should only have power when you apply the manual override or brake pedal. If it shows power at idle then there is something wrong with the controller and it will need to be replaced.

If it tests fine then reconnect the blue wire. Then move on to the truck side trailer connector. Many times, dirt or corrosion can cause a short in a trailer connector. Make sure the truck and the trailer side connectors are clean and free of corrosion inside and out.

You can test the brake output pin and it should only show power with the manual override. If it shows power any other time then there is a short somewhere between the connector and the brake controller you will need to track down.

If the truck side checks out fine then it could be the wiring to the brake magnets or the magnets themselves. Inspect the power wires to the magnets and look for pinched, frayed, or damaged wires and fix as needed. Make sure the ground wires are attached to clean and corrosion free bare metal surfaces.

You can visually inspect the magnets and if you can see the copper windings coming through the magnets will need to be replaced. To further test the magnets you can follow the help article I have linked. I have also included some help articles on testing and troubleshooting brake controllers for you.

expert reply by: Michael H