How to Determine Wiring Functions on Trailer to Allow Light Installation


I would like to purchase that, however, my trailer light middle bar led light with stop, and turn signals has 4 wires coming out of it. How would I attach your product to the electrical wiring? Thanks,.


Expert Reply:

It sounds like your existing trailer light may handle more functions than the 3-function (stop/turn/tail) Optronics LED light # STL79RB. This light has a white ground wire and two leads for power; the red wire handles stop/turn signals and the black wire accepts the tail light signal.

Thank you for your included photo. In a typical 4-pole wiring configuration the green and yellow wires carry combined stop/turn signals, the white wire is ground, and the brown wire carries taillights. Please refer to the linked article.

You can confirm what each wire on the trailer does by testing your wiring while a helper activates the lighting functions on the vehicle. Use a tester like # 40376 to test each wire on the trailer while your helper engages lights, brakes and turn signals. You can also test by applying 12V power from a battery directly to the trailer connector. Once you know the function on each wire you can select a light that supports those functions.

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expert reply by:
Adam R

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