Why Wouldn't Trailer Wiring Work with Vehicle Headlights On

Question:

All the lights on my tow vehicle and trailer do not work when headlight on tow vehicle is off. Everything works fine as it should IF headlight/running lights is on on tow vehicle. I confirmed that trailer is working properly. By hooking it up to friends tow vehicle. So I am almost 100 sure its the tow vehicle. But where do I start the troubleshoot. I already started by looking at the t-connector. Which I confirm is working properly. From there. I am not sure where to go. I trace/follow the wiring on the tow vehicle to mid-truck. But found nothing out of ordinary. Any advice would be appreciated!

asked by: Bum H

Expert Reply:

Your main problem is an insufficient ground for the trailer wiring. Try improving the connection of the main ground wire of the trailer wiring harness. Make sure it is attached to a clean metal surface. Or you can run all of the trailer wires back to the main ground. This is actually the ideal solution. But you can also check the current grounds for the lights to make sure they are making a solid connection to a clean metal surface.

What you described is the classic insufficient ground scenario so finding a way to improve the grounds will fix the problem. The issue is that when your headlights are on the maximum amount of amperage draw is put on the setup which if there's an insufficient ground this would be when this would show up.

expert reply by: Jameson C