How To Troubleshoot 4-Pole Trailer Wiring When Your Light Functions Don't Properly Operate  

Question:

Expert Reply:

If light functions aren't properly matching up on the trailer either the 4-pole connector on the truck or on the trailer is wired incorrectly. To test the truck side you can use a circuit tester # PTW2993 on the connector and test each function (press the brake pedal, turn on either turn signal, turn on running lights) to ensure that the signal is coming through. Then you can test the trailer side to see if the functions are wired correctly to match up to the vehicle connector.

Unfortunately, manufacturers don't all follow one standard color code so you can't rely on the color to ensure it's wired correctly. You'll have to connect to the vehicle to add power to the trailer connector and then test each function to see which wire is gaining power and what light function turns on.

For example, if you pressed on the brake pedal and let's say there's a green wire in the harness that gets power and your right turn signal comes on then you know the green wire is your right turn and you an take some painters tape and label it. You'll just go through each function until you know what each wire color's function is and then can go to the junction box (if you have one) or just cut the connector and and swap the wires to match up to colors to the correct functions. You could also disconnect the wires at the lights if the trailer is open enough to get to those connections. Basically just do whatever is easiest for you.

It's also very important that you have a good ground, otherwise your light functions will not properly function. I've attached a help article below that explains all this in greater detail if you'd like to check it out as well.

expert reply by: Kevin C

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