Troubleshooting Turn & Brake Lights Not Working Once Headlights Are On

Question:

Turn truck lights on and trailer lights only clearance lights work, no turn or brakes. Turn off lights and the turn and brake lights work fine

asked by: Daniel F

Expert Reply:

The wiring issue you have is common and easily solved.

The cause for your running lights to create an issue with the brake and turn lights is simply a weak ground. All you need to do is make better ground connections on the truck and trailer wiring harnesses and you should see this issue go away.

On both the vehicle and trailer find the main ground wires attached to the frames. Usually these are white wires with a ring terminal. Remove the ring terminals and sand off those grounding points to expose clean shiny bare metal. Apply dielectric grease # 11755 and then re-attach the ring terminals.

expert reply by: Adam R