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Troubleshooting Trailer Wiring That Stops Working When Headlights are On

Question:

When I hook up my trailer, when the head lights are turned off the trailer lights function normal, brake lights and R/L turn signals and 4 way hazards. But when I turn the headlights on the 4 way hazards stop and go out, if I turn the headlights back off, the 4 way hazards will start blinking again. Everything use to work normally 2019 a couple of times. With a 3 light tester plugged into the factory 4 way plug, same thing happens. It has been at the dealer for a month. New battery New Flasher New Multi function switch I even tried one of the kits that plug into the back brake/tail lights like it didnt have a factory trailer harness, same thing....

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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.

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Jameson C

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