How to Identify Functions for Each Wire on a Trailer

Updated 08/24/2016 | Published 08/23/2016

Question:

How do I connect the wires? When I got the trailer the black wire was not connected to anything. Hock up a 4 wire green , yellow,brown , white. To a 6 wire black white green yellow brown red

asked by: Camilla

Expert Reply:

Trailer wiring often uses specific color wires for a particular lighting function but there is no guarantee that your trailer uses the common wire colors. This just means that you'll test the wires on the trailer to see what each one does; then you will know how to connect it.

You can refer to the linked article on trailer wiring that shows the usual configurations of 4-pole, 5-pole, 6-pole and 7-way wiring. Photos are included to show you the usual wiring colors for each function.

In a 4-pole setup the wire colors/functions are typically:
Brown wire is running lights
Yellow wire is LEFT turn and stop light
Green wire is RIGHT turn and stop light
White wire is ground

For all wires on the trailer you can simply apply 12V power from a car battery to see what function each wire activates. Ground the battery negative terminal to the trailer frame and then one by one apply +12V power to each wire to see what each wire connects to. The 5th and 6th wires on the trailer are likely to be for a 12V power feed to help maintain a trailer-mounted battery and a brake signal circuit to activate electric trailer brakes.

expert reply by: Adam R