What are the Functions of the Factory Trailer Brake Control Wiring Harness on a 2005 Ford F-150

Question:

My 2005 F150 came with a wiring harness for a brake controller. There are a total of 5 leads Black, blue, red, green, and white. I was considering buying the Tekonsha P3 but noticed in the photos it only uses four leads My wiring harness that came with the vehicle has five. Is this common? Do I need all five leads or will it work with four? I am concerned that if I do not use all the leads that the device may not funtion properly.

asked by: Ted P

Helpful Expert Reply:

It is common to see a factory harness with 5 wires. These factory harnesses usually have what is called an illumination circuit that the vast majority of aftermarket brake controllers do not use. But that is fine because they do not need that circuit to work.

On your 2005 Ford F-150 and factory harness the red wire should provide 12 volt power. Use a circuit tester like # PTW2979 to confirm. It would attach to the black brake controller wire.

On the factory harness the green wire is most likely input from the brake switch. It should show power on the circuit tester only when the brake pedal is applied. This wire will attach to the brake controller red wire.

The white wire on the harness and on the controller should be ground and they would connect to each other.

The blue wire on the factory harness should be for output to the trailer brakes. To test this what you can do is apply 12 volt power to that wire (using a separate 12 volt battery) and then use the circuit tester to test the brake output pin on the trailer connector ( 5 oclock position if looking at the pins on a 7-Way). The blue wire will connect to the brake controller blue wire.

The black or brown wire you see on the harness is most likely that illumination circuit I mentioned. It is not used so you can tape it up and stow it out of the way.

I have included a link to our brake controller FAQ article for you. The P3 is an excellent controller and I have also included a link to its instructions for you to view.