Trailer Lights Come On when Tow Vehicle is Turned Off and the Trailer Is Connected

Updated 04/06/2020 | Published 10/12/2014

Question:

Upon initial installation, my trailer lights worked fine. The trailer has sat all summer without being connected to the tow vehicle. I connected the trailer to move it and the lights came on and stay on even when the tow vehicle is turned off. What happened? The connection uses a 4 wire flat plug.

asked by: Joe

Expert Reply:

Since the trailer sat for so long it could be corrosion build up in its trailer connector. The same could be said for the vehicle side. Clean out corrosion with a wire brush and see if that helps.

If it does not then you could be looking at a bad ground. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light.

The main connector grounds should be attached to clean and corrosion free bare metal surfaces. This includes the vehicle side harness ground. You can try moving the vehicle side ground to any clean bare metal surface.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

I have included a link to the right that should help.

expert reply by: Michael H

Chad

4/4/2020

So I’m having wiring issues I have a 7 pin camper plug I have had the camper cleared of any issues but when I plug it to the tow vehicle it works fine until I turn my headlights on then the turn signals quit working and no brake lights also with all the lights off and I open the door it puts power to the running lights so basically it turns the lights on when I open the door please help apparently this is a one of a kind issue

Chris R.

4/6/2020

That's quite an interesting problem - especially the part about the door. The issues with the running lights sound like the result of a weak/bad ground. This could also be leading to the open door thing (bad grounds can cause all kinds of weird things to happen). Make sure the main harness ground on your tow vehicle is securely attached to clean, bare metal, and do the same thing on the trailer. Check the individual grounds for each of the tail light assemblies as well (whether they ground through a dedicated wire or through their mounting hardware). Let me know what you find and we'll go from there.