Testing to Determine Lighting Function on Each Trailer Wire for Light Installation

Helpful Expert Reply:

I can help you sort out your trailer wiring so that you can correctly install your new LED trailer lights.

The color of the wire on a trailer often can tell you what function it serves but since this is not always the case it is best to test all of the wires first to confirm what lighting signal each one handles. You can use your vehicle battery or a well-charged spare if you have one to apply ground and power to each trailer wire, one at a time, to see what function is activated. If you do not have a wiring guide for your new lights you can repeat this test on the light's wires so you know what light function is activated by each wire.

Most often a white wire is ground, a yellow wire provides left side turn signals, a green wire provides right side signals and a red wire will provide stop light signals. The 4-pole wire you referenced in your question, part # 16-4B-1, is configured for the North American trailer wiring format which is as follows: WHITE is ground, BROWN is running/tail lights, YELLOW is left stop and turn and GREEN is right stop and turn. I linked a photo showing a 4-pole and the usual pin/wire/function assignments.

It sounds like you have a wiring setup with a separate stop light circuit that is not combined with the left and right turn signals the way it is with standard 4-pole wiring.

The linked article on trailer wiring show you all of the different wiring and connector formats in use in North America.

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