How to Determine Function of Trailer Wiring Based When Wire Colors aren't Labeled/Known

Question:

Im installing an older kit that Ive had for some time. It is 4 pin flat with wishbone wiring. My problem is the lights. They have 2 black wires and 1 red wire and the instructions dont give any details on hooking up the electrical connections to the lights. What is the approach.

asked by: Ray P

Expert Reply:

Three wires on the lights indicates most likely that you have a ground wire and two power wires for the two brightness intensities (difference between running light and stop/turn). Since there are two black wires I'd assume those are the power wires and the red wire is the ground but those are non-standard colors. Your best option is going to test the wiring to determine what functions each wire covers. Pop the hood of a nearby vehicle (preferably one of yours haha) and then touch the wires to the positive and negative posts of the battery in various combinations till it becomes obvious which wire is for each circuit.

If you see a spark you've touched the ground to the positive so maybe lightly touch them at first.

expert reply by: Jameson C