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Troubleshooting Brake Controller Not Working When Brakes Are Applied but Works With Override Switch

Question:

My trailer brakes will not work when appying the truck brakes but will work when you push the test button on the controller box! Does this mean my box is bad or what?

Expert Reply:

This sounds like the trailer brakes are not receiving the signal from the brake controller when you apply the brakes. I believe what is happening is that you may have a loose, broken, or inaccurate connection on the red wire from the brake controller.

The red wire needs to attach to the stop light switch above the brake pedal. It connects to the wire on this switch that carries a signal ONLY when the brake pedal is pressed. It should not have a signal at any other time. Use a circuit tester, like the Quickee # 3808, to test the wires. If your brake controller red wire is not connected to the correct wire, or is loose or broken, this would cause the problem you are experiencing.

When you use the manual override, it bypasses the need for a brake signal on the vehicle and applies power to the trailer brakes. That is why it works when you hit that button.

If you find that the red wire is connected in the right place and is not loose or broken, then this is probably a problem with the internal circuitry of the brake controller and it will need to be replaced.

I have included an FAQ article on brake controllers and a video on testing a brake controller that might be helpful as well.

expert reply by:
Michael H

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