Trailer Lights Don't Work When Headlights are On

Updated 09/08/2025 | Published 07/06/2015

Question:

when headlights are on trailer signals will not work,but work fine when lights off. i checked grounds and connections. all seem good ?

asked by: Greg

Expert Reply:

The problem you are having is a classic ground issue. If the ground wire to the trailer frame or the tail light grounds are not connected securely to clean, rust and paint free surfaces then when the headlights are turned the extra amperage draw for the running lights will cause all sorts of lighting issues.

I would start by removing the ground wire at the trailer coupler and cleaning up the connection point and re-securing it to the trailer frame. Next I would check the ground connection of the trailer lights. Usually they are grounded through the mounting bolts for the lights. Make sure that the connection is clean and free of paint, rust and debris and, if there is a plastic license plate holder on the drivers side, make sure that it is not inhibiting the lights ground. Finally if the trailer light mounting brackets are bolted to the trailer frame you should remove them clean up that connection point also.

These simple steps should get you going and ensure that your trailer lights will work properly.

expert reply by: Jameson C

Patrick

5/25/2024

I have the issue that the left turn signal on the trailer does not work when head lights are on. I checked continuity with multimeter- all good. What else could this be?

Jameson C.

5/27/2024

@Patrick It's gotta be a weak ground on the trailer wiring. See the above response for more info.

Tony

10/28/2024

And of that doesn't fix the issue, then what? I have the same problem. Brand new pole lights. Brand new wire harness. Brand new harness adapter.Everything is wired and grounded correctly. Also, all of the bulbs in the new pole light fixtures are not illuminated.

Jameson C.

11/1/2024

@Tony have you tried using a circuit tester to check for voltage on all of the circuits first at the 4-way and then back to each light to see if you can see where it stops and then go from there.

Rose L.

9/6/2025

We are having this same issue with a brand new trailer when hooked up to our 7 pin plug on our 2021 F150. Tested trailer with another truck and lights work perfectly. When we turn running lights on, the left blinker and break light no longer work and we get a fault message. Any idea what could be wrong on the truck side?

Jameson C.

9/8/2025

@RoseL Try cleaning all of the pins of the 7-way on your truck and see if that helps. It sounds like a weak ground which corrosion on the 7-way notoriously causes this.