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Trailer Lights Not Lighting Up Properly on Harbor Freight Trailer

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Hi, I have a haulmaster 4 way wiring harness from Harbor Freight and 2 seperate lights. One is from Harbor Freight and is submersible and the other is from West Marine and is also submersible. The left light HF has a yellow and brown wire and white wire coming out of it and the right light WM has a green and a brown wire coming out of it. The wiring harness has 5 wires, white, brown and red from 2nd port, yellow, and green. I have attached ground up front by coupler and ran the rest of the wires all the way to the back of the trailer in a flexible split loom. The white wire initially ran all the way to the rear with the other wires but I cut it and attached it by the coupler instead. I attached green and brown to the right light and red to brown and yellow to yellow on left light. I attached the red to the brown because the yellow and red wires were together and it seemed like they should run that way. Finally got everything hooked up and grounded the left light to the frame. Turned on truck lights and there are no running lights showing on the trailer. When I put on my right turn signal both lights flash dimly. When I put on left turn signal the left light flashes bright with no light on right. Any idea how I can isolate the problem here? Not 100 sure about the red wire connecting to brown, but it seemed non sensical to have to run brown all the way to right light and run a jumper to left light. I can send some pictures if needed. Thanks in advance!

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Helpful Expert Reply:

It sounds like the wiring harness you have is what is called a wishbone harness. These will have 2 wires that connect to the single running light pin so you don't have to run a jumper wire from one side of the trailer to the other.

It also sounds like you have everything hooked up correctly. Yellow is usually left turn and brake lights, green is usually right turn and brake lights, brown is usually running lights, and white is the ground. You can test this by connecting the trailer to the truck and testing the wiring on the trailer side using a circuit tester such as # 40376 or a meter if you have one. If the wire colors/functions are different you will be able to identify which wire does what and attach it to the lights accordingly.

You could also have a ground issue. This sounds like a ground issue. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help. I have also included some additional links that will help.

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