Right Side Trailer Lights are Not Working But Vehicle Side Trailer Connector is Working

Question:

I have a harness on my trailer looks like a 4lead but, it looks like the green and yellow wires are paired with with a brown wire going into the connector. There is a white wire by itself then a yellow/brown then a green/brown. The problem Im having is I have no lites , directional, or turn signal on the right side of the trailer when looking from the rear. The plug on the truck checks out to be good. The wiring on the trailer appears to be good. I think the problem is in the trailer plug.

asked by: Russ S

Expert Reply:

You have what is called a wishbone harness. A wishbone harness has two running light wires, one for each side of the trailer, but they are tied to the same pin on the trailer connector. The yellow/brown is running lights for the left side and the green/brown is running lights for the right side. Solid yellow is left turn signal/brake lights and solid green is right turn signal and brake lights. The white wire is the ground wire.

Usually if some or none of the lights are working it is 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 it is not limited to this scenario.

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.

expert reply by: Michael H