Trailer Right Tail Light and Turn Signal Do Not Work with Running Lights On  

Question:

Expert Reply:

This sounds like either a bad ground connection or there is too much amperage draw from the trailer for the tow package wiring on the vehicle. Bad grounds usually show up when there is more load on a system. The bad ground can be anywhere in the system, not just at the light.

The first step is to isolate the issue by disconnecting the trailer and testing the vehicle side trailer connector using a circuit tester such as # PTW2992. Have someone sit in the vehicle and run through the lighting functions while you test the trailer connector.

If the same issue is present on the vehicle side then make sure the connector is clean and free of corrosion inside and out and that it is grounded to a clean and corrosion free bare metal surface. Some vehicles have fuses or relays related to the tow package wiring if it is a factory harness. If any lighting functions is not working at the connector you can use the vehicle owners manual to locate the fuses/relays associated with that function and replace as needed.

If you are using an adapter like # 37385 that you referenced if could be an issue inside the adapter. After testing the vehicle side connector by itself, plug in the adapter and test again. If the signals do not come through as they should then there is something wrong inside the adapter and it will need to be replaced. When testing the vehicle side connector make sure that you are testing with and without the running lights on.

If everything on the vehicle is working properly then the issue must be on the trailer. Make sure the trailer side connector is clean and free of corrosion inside and out. Then make sure the main ground is attached to a clean and corrosion free bare metal surface. And then just doublecheck that the light in question is also grounded properly.

If you recently added a plastic license plate frame between the light ground and the trailer, that can interrupt the ground. If the trailer has an aluminum frame, tilt bed, or folding tongue, those, too, can interrupt the ground. Adding jumper wires from the light grounds to the main trailer connector ground can help.

The easiest way to check for too much draw from the trailer would be to disconnect some of the lights on the same circuit as the light in question. If that light then starts working you know that all the lights were too much for the system. Switching everything over to LEDs can solve that problem.

I have included a couple of links that should also be helpful.

expert reply by: Michael H

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