Troubleshooting Trailer Running Lights That Do Not Work When All Other Trailer Lights Work

Published 02/09/2017

Question:

I have gone through and wired up my trailer with two truck lite 60 two tail lights in the rear and a couple amber turn signals on the sides. Using a four wire harness, dielectric grease, auto connectors, shrink wrap, etc. Everything works as far as turn signals, brakes, and hazards. However, the taillights arent lit when the lights are turned on on the truck. I dont have an idea where to begin on this.

asked by: Jonathon M

Expert Reply:

When all trailer lighting functions work correctly except for the running lights this usually means a problem with grounding. Light functions such as turn signals and brake lights do not remain on continuously so they pull less current. Running lights on the other hand remain on continuously and so they pull more current. if you main trailer ground is not strong enough there may not be enough power for the taillights.

Check your main trailer ground to ensure that the white ground wire is affixed to a clean bare metal spot on the trailer frame. This grounding point should be free of primer, paint, grease and dirt to allow good metal-to-metal contact. You can apply dielectric grease to protect this connection.

If the trailer frame is aluminum, which is not a great electrical conductor, you may need to run separate ground jumper wires from each light directly back to the trailer's main ground wire.

If the running lights use one of their mounting studs as their ground connection you may need to sand/clean that mounting point on the trailer to ensure a good ground.

expert reply by: Adam R