How to Test Trailer Wiring to Determine Function on Each Circuit

Question:

I have a 1980’s horse trailer that I’m rewiring. The color of the wire coming out of the brakes is red and black. I have a 7-way connector and a junction box. Everything I’ve read online has blue wire as the brakes and says nothing about red or black wiring for brakes. I have a break away switch that has a ground coming out and connecting into the junction box as well. Not sure if the red or black wire should go into the junction box or if the black is the ground for the brakes and the red is the live wire. Please help! Thanks so much!

asked by: John M

Expert Reply:

If you are referring to the wires on each electric brake assembly, these are non-polarized and are usually white. On electric brakes like 12 x 2-inch # AKEBRK-7-SA either wire can be used for ground or hot since they are non-polarized.

Wire colors on a trailer are not a reliable way to know what function they carry. On an older trailer like your 80s horse trailer there could have been several re-wiring efforts already put into it so there is no telling which wire does what unless you trace/test, but you are correct that the usual function assignment for a blue wire on a trailer is the electric brake circuit. You can figure what any given wire does by using a spare 12V battery (or the one in your vehicle) to apply power so you can see which function is activated.

To help you sort things out I have linked both a general trailer wiring article as well as two articles concerning breakaway kit installation and testing. If you do re-wire the brake circuit use 10-gauge duplex wire # 10-2-1.

I also linked a photo showing the standard function/pin assignments on a 7-way trailer plug.

expert reply by: Adam R