Trailer Brake Lights and Turn Signals Not Working When Running Lights are On.

Published 09/01/2020

Question:

I am trying to diagnose a problem with the trailer lights on a Boston Whaler boat trailer. The first problem is that no ligs connecthts work unless the the trailer tongue ied to the hitch ball. When the tongue is connected to the ball and the trailer is being towed, the lights flicker on and off. Lastly, when the trailer tongue is connected to the ball and the running lights are on, the brake lights and turn signals dont work. when the running lights are off, the turn signals and brake lights work fine. Any help with this would be greatly appreciated. Thank you.

asked by: Josh

Expert Reply:

This is most likely a trailer grounding problem as indicated by your report that (all) trailer lights sometimes flicker and do not work at all if the trailer is not hitched (grounded) to something.

First check the ground point at the tongue of the trailer. From the trailer electrical connector you will find a white wire. Follow that to the nearby end where you will find a ring connector crimped to the end and then screwed to the trailer frame. I would ensure that the ring connector is clean, and if not use a wire brush. I would also ensure that ground point is clean bare metal - not painted.

If this does not resolve your issue. then I would thoroughly clean all connections on your trailer and truck connectors to ensure there is no corrosion there. If that does not work I would check for grounding points at the lights. They are likely ground by being screwed to the frame or other grounded metal.

You might also check the white grounding wire that is behind your tow vehicle connector. Like the trailer, the vehicle ground will be screwed to a metal part and it requires a clean connection.

If you are still having issues after this you will need to inspect the remainder of the trailer wiring.

expert reply by: Les D