Trailer Lights Work when Vehicle Is Off, But Do Not Work When Vehicle is On

Question:

I tested the vehicle 4 prong harness when vehicle is off by turning on hazards. Im getting voltage correctly the meter goes back and forth with the lights. However when I start vehicle and have hazards on and test plug, I get a constant voltage reading. I had a hitch and harness put on by a trailer shop. Is this a sign of the vehicle harness not grounded properly? Thanks for any advise you may. have

asked by: Ray

Expert Reply:

That is interesting. What about testing the 4-Way without the hazards on. Instead, have a friend sit in the vehicle and operate each light function with the vehicle off and with the vehicle on. It could be an issue with just the hazards on.

I have included a link to a help article that explains how you can go about troubleshooting the trailer wiring.

If it is a bad ground, 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