New Trailer Taillights Malfunction When Running Lights are Turned On

Question:

Hello, I recently installed new tail lights on a trailer. standard red lens LED brake, signal, hazards. Wiring appears to be the same as previous ones. I tow with a 2018 F250. When I turn the truck running lights on the trailer running light turn on but the tail lights do not function. When I turn the running lights off however, the tail lights function properly. Can you tell me where I should start to troubleshoot next? Thanks

asked by: Chris G

Expert Reply:

That's the classic symptom of a poor/insufficient ground connection. Everything works fine until you run maximum amperage through the system by turning on the running lamps. With the maximum amperage running through the system, if there's a ground problem, that's when it'll show up.

Begin on the vehicle side. Make sure the ground for the trailer connector is securely attached to the frame or chassis of the vehicle, and that it isn't corroded. Next, on to the trailer itself. Find the trailer main ground. Trace the white wire from the trailer connector, usually it's grounded to the trailer tongue. Once again, make sure the ground connection is tight and isn't corroded. Finally, each individual lamp will have its own ground connection to the trailer frame. Check and repair those as needed.

For an foolproof ground connection, remove the ground screw. Sand down a dime sized area around the screw hole down to bare metal. Reinstall the ground screw with a star washer between the ring terminal for the ground wire and the trailer frame so it bites into the metal when the screw is tightened. Once it's back in place, give the connection a quick shot of Rust-Oleum to protect it. At that point, you should be good to go.

I've linked to an article on trailer wiring that you might find helpful.

expert reply by: Mike L