How To Convert A Wishbone Trailer Wiring Harness To A Regular 4-Flat Trailer Wiring Harness

Updated 07/17/2017 | Published 07/14/2017

Question:

The lights I bought for my trailer have a 4 flat connector with 5 wires, a yellow brown and green brown connected to the second terminal next to the white wired terminal. The 4 flat connector from my van only has 4 wires. For whatever reason, the left trailer light will not light up. I returned the first set of lights and hooked up a second and have the same issue. I didnt notice the 5 wires to the 4 flat plug the first time and thought it was a faulty set of lights. I have a set of magnetic lights to a 4 flat plug with 4 wires and it works fine. Is there some sort of adapter that will convert a 4 flat 4 wire to a 4 flat 5 wire? Im not finding anything on the site, and am confused why it doesnt work since the two brown wires terminate on the same terminal as the single brown wire.

asked by: Jon G

Expert Reply:

It sounds like you have a wishbone wiring harness install on your trailer. A regular 4-way wiring harness has 4 wires from the back of the connector, 1 for left turn/brake typically (yellow), 1 for the right turn/brake typically (green), 1 for the tail lights typically (brown), and 1 for the ground typically (white). A wishbone 4-way connector has 5 wires, 1 for left turn/brake typically (yellow), 1 for the right turn/brake typically (green), 2 for the tail lights typically (brown w/ yellow or green stripe), and 1 for the ground typically (white). The difference between a regular 4-way harness and a wishbone 4-way harness is the extra running light wire typically (brown w/ yellow or green stripe).

In order to troubleshoot the left turn/brake light, you will need to first check the bulb. If it is blown, then this could be the problem. If the bulb is still good, you will want to trace the ground wire from the light to ensure it is attached to a bare metal surface on the trailers frame.If both the bulb and ground are good, you will need to test each of the brown wires to see where the left wire is losing power.

The other option you have would involve installing a new wishbone wiring harness such as the Wishbone 4-Way Trailer Wiring Harness part # A35W42B. I have attached an installation video of this wiring harness for you.

expert reply by: Conner L