P2 Brake Controller Shows nc Not Connected when a Trailer is Connected

Question:

I would like to have ts hits and information enabling me to troubleshoot the system, first i have 40 plus years of Aircraft elctrical expierence. Just no clue how the system functions and outputs from the controler and inputs back to the controler. A complete dc eldctrical failure occurred on the RV due to a shorted battery celldc lights flashed on and off miltiple times. The controler does not show C, nc appears when either beske is applied via pedal or manually by trailer brake knob.

asked by: Bill D

Expert Reply:

When NC shows up on the P2 brake controller, # 90885, it means that the blue wire is not sensing the brake magnets. I suppose it is also possible that some sort of power surge could have fried something inside the controller.

The functions of the wires on a brake controller are as follows:

White is ground
Black is 12-volt power in
Blue is output to the trailer brakes
Red is input from the brake switch

There are ways to test the controller. In the vehicle cab you will need to sever the blue wire several inches from the back of the controller. Then use a circuit tester such as # PTW2993 to test the end of the blue wire coming out of the controller. It should only have power when you apply the manual override. 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 vehicle side trailer connector. Many times, dirt or corrosion can cause a short in a trailer connector. Make sure the vehicle 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 vehicle side checks out fine, then it could be the wiring to the brake magnets or the magnets themselves though based on what you have explained this is a long shot. 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.