Trailer Brake Lights Do Not Work when Running Lights are On

Updated 02/23/2015 | Published 02/22/2015

Question:

I have a 2011 Ford Ranger with a 4 prong wiring harness. My boat has a 4 prong on it but 5 wires. All my turn signals work with running lights on and off. The brakes work with running lights off but not when the running lights are on

asked by: Casey

Helpful Expert Reply:

First, the 4-Way on your trailer is what is called a wishbone harness. It just has 2 wires for the running light circuit to go to each side of the trailer so that you do not have to run a jumper from one side of the trailer to the other.

The issue here sounds like a ground problem. 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.

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