Troubleshooting Trailer Lights Working Until Trucks Running Lights Turned On

Question:

All trailer lights work until you turn on your truck lights

asked by: David A

Expert Reply:

If I understand correctly it sounds like the signal lights (brake and turn) on the trailer work until turning on the truck's running lights. If this is the case then the most likely culprit is a weak or bad ground on the trailer.

A weak ground usually won't present itself until using a lighting function that puts more load onto the circuits (such as when turning on the running lights while using turn/brake signals). There is enough power going back on your trailer for just the turn/brake lights to work by themselves but not enough for the running lights as well.

First check the main trailer harness ground to make sure it's securely connected to a clean, bare metal surface that is free of any debris or corrosion. Then do the same for the individual light assemblies themselves, whether they ground through a dedicated wire or through their mounting hardware. It might be a good idea to just remove all the grounds, sand down the trailer frame surfaces where they're mounted, and reattach them just to be certain of a good connection.

This should resolve the issue you're having but please let me know if not and we can continue to troubleshoot.

expert reply by: Chris R