Installing 4-Way Flat Connector on Trailer with 3 Wires

Updated 07/25/2018 | Published 07/24/2018

Question:

I am trying to connect a 4 way flat plug to my trailer. I have 3 wires coming from the trailer. White, green/brown, and yellow/brown. The trailer plug has a green, white, yellow, and brown. How do I connect the wires?

asked by: Bobby

Expert Reply:

If your trailer is wired for both brake/turn signals and running lights, I'm wondering if the two multi-color wires you found (the green/brown and yellow/brown) are actually each 2 wires sealed together and they simply need to be pulled apart. If this is the case, then you actually have what is called a wishbone harness on your trailer, which uses a separate brown running light wire for each side of the trailer, along with the green wire for right side turn/brake and the yellow wire for left side turn/brake. The white wire should be for ground.

Both brown wires will just go to the running lights pin on the 4-Way.

If this is not the case, then I recommend applying 12V power to the trailer wires to see what each of them activates, as a 3-wire trailer is not common. This might indicate that the white wire is not actually for ground and that the trailer somehow grounds through the frame, or that trailer is simply not wired for running lights.

expert reply by: Chris R