Troubleshooting Trailer Lights Where Brake & Turn Lights Work But Running Lights Do Not Work

Question:

I have a boat trailer and put new wiring on, the brake lights, and turn signals work but when I turn on the truck lights the trailer lights do not light up. Is it a grounding issue.

asked by: Anthony

Expert Reply:

Your wiring problem does sound like a grounding issue. When some trailer light functions, such as brake lights and turn signals, work but running lights do not, this usually turns out to be related to a weak ground either at the vehicle wiring harness or on the trailer harness. Both ground wires must make a solid connection to a clean bare metal surface. You can clean/sand the grounding point (to remove rust, grease, primer, etc.) or you can re-locate the ground wires to new clean spots on the frames.

If you have a circuit tester like # PTW2993 you can test the vehicle 4-pole to see if it has the correct running light signals on the pin for the brown wire. Have a helper turn on the headlights while you test that pin. If you get a signal, then the issue is in the trailer. You can also apply 12V power directly from a battery to the trailer's lighting circuit to test them independent of the vehicle wiring.

If that does not fix the issue, then carefully inspect both the vehicle and trailer connectors for any signs of corrosion and clean as needed. The linked article on troubleshooting will give you additional guidance.

expert reply by: Adam R