P2 Trailer Brake Controller Only Shows 4 Volts On Blue Wire No Matter what Gain is Set To  

Question:

Expert Reply:

It does sound like there is a problem with the controller. But there are some things you can do to test it. In the truck cab you will need to sever the blue wire several inches from the back of the controller. Then use your meter 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 on certain controllers but not the P2). 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

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