Trailer Marker Lights are Not Working But All Other Lights Work

Question:

Trailer marker lights are out. The 4 ways and the turn signals do. I have a 7 way plug. I checked the truck fuses and they seem to be fine. I would appreciate any ideas you may have.

Expert Reply:

To determine where the problem is first disconnect the trailer from the tow vehicle. Use a circuit tester such as # ALL640595 or a meter if you have one to test the vehicle side trailer connector while the head lights are on. If there is power on the pin in the 11 o'clock position then the issue is on the trailer. If there is no power during this test the issue is on the tow vehicle. Besides fuses some vehicle tow package wiring also use relays. The various fuse panel lids under the hood, and in the cab on some vehicles, will list the fuse and relay functions so you can find any relays related to the tow package wiring.

If there is power when testing then the issue is on the trailer side. It could be something simple just as a corroded trailer connector. Make sure the trailer side (and the vehicle side while you're at it) is clean and free of corrosion inside, where the wires attach, and outside on the pins.

If you are still having problems inspect the wiring from the connector back and fix any pinched or damaged wires.

It could also be a ground issue. the lights should be grounded 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.

Another possibility is that if the trailer has a lot of running lights the combined power draw may be too much for the tow package wiring to handle. To test this you can disconnect some of the lights that use the running light circuit and if the others start working then that was indeed the issue. Switching to LED lights would solve that problem.

expert reply by:
Michael H

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