Testing to Find Which Trailer Wire Carries Trailer Brake Circuit

Question:

I have a 2007 outback made by keystone my question I have blue wire on truck outlet suppose to brake system well what color wire is hook on trailer side

asked by: Randy

Expert Reply:

On the standard RV-type of 7-way socket and plug, parts like # 37185 and # H20046, the trailer brake signal is carried on the socket pin (as you look at the back end of the truck) that is at the 5-o-clock position. Please refer to the linked photo.

On the trailer plug side the mating pin takes in the brake controller's signal for the trailer's brake. The color of wire usually used for a brake circuit is blue, but there is no guarantee that your trailer is wired with blue wire for the brake circuit. It could be but there is no absolute standard for wire colors. For this reason it is best to test.

You can confirm which wire on your trailer activates the brake circuit by applying 12V power from a spare battery (or the vehicle battery) directly to the trailer plug's circuits until you find the one that causes the brakes to engage.

expert reply by: Adam R