Wiring Trailer Light When Color Coding of Trailer Wiring and Lamp Wiring is Unknown

Question:

The lights have 2 black wires and 1 red wire. The wiring harness has a Black, green, brown and yellow wire. What wires go to what?

asked by: William R

Expert Reply:

You can test the wiring harness for function by using a circuit tester like part # PTW2993. Usually, green is right turn/brake, yellow is left turn/brake, and brown is running lights. The black wire is most likely the ground. Ground your tester to the vehicle and activate each lighting function one by one as you test each wire.

You can determine the function of wiring from the lamps by using a battery or other 12v power source and applying voltage to each wire. The wire that makes the lamp very bright is the turn signal/brake lamp circuit. The wire that makes the lamp less bright is the running light circuit.

expert reply by: Mike L