Trailer Turn Signals and Brake Lights do Not Work when Running Lights are On  

Question:

Helpful Expert Reply:

This sounds like a ground problem or possibly a short on the vehicle side connector (on the trailer the vehicle is not working with). The first thing to check is the trailer connector on the suspect vehicle. Make sure that it is clean and free of corrosion and that the connector is properly grounded to a clean, bare metal surface.

You can then use a circuit tester such as # PTW2992 to test the vehicle side connector. Have someone sit in the vehicle and go through the light functions, with and then without the running lights on, to see if the issue remains. If it does, then there is an issue with the vehicle side wiring that needs to be addressed.

If the vehicle checks out fine then move on to the trailer. Sometimes lights will work with one vehicle but not with another because of the differences in the amperage ratings between the vehicles. And some trailers might not work with certain v3ehicles for many reasons. Make sure that the trailer side connector is clean and free of corrosion, which probably is since you changed it out.

Then make sure the ground wire is connected to a clean and corrosion free bare metal surface. Do the same with each of the lights. Then you can trace the wires from the connector to the lights looking for any breaks in the wiring and fix as needed.

If the problem trailer has a lot of lights on it, then they could be drawing more power than the vehicle trailer wiring can handle. If you disconnect some of the running lights and the issue stops then this is likely what is going on. Changing all the lights to LEDs will solve that issue because LEDs draw a fraction of the power that incandescent bulbs draw.

I have included a couple of links on trailer wiring and testing vehicle side wiring for you.

expert reply by: Michael H

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