Troubleshooting Trailer Lights That Won't Work When Vehicle Headlights Are On

Question:

All trailer lights work perfectly when I do not have lights on ...I have trailer with three side markers and three underneath markers. New brake/turn and brake signals have been installed. When I turn on lights on vehicle the left tail light on trailer does not work in any function....everything else works except that location under that scenario..

asked by: Burke R

Expert Reply:

When strange things happen to trailer lighting very often the cause is simply a weak ground connection at the vehicle wiring harness or at the trailer's main wiring ground. A weak ground will allow certain light functions to work properly while others will not. Usually the problem shows up when you turn on the tow vehicle headlights; this activates the trailer running lights which tend to draw the most current (since they remain on continuously).

Check the white ground wire on your vehicle's wiring harness to make sure it is solidly affixed to a clean bare metal surface. If you need to you can sand off and clean that grounding spot if it is rusty or greasy. You need bare metal on bare metal for the best ground. Repeat this ground wire check on the trailer.

Dielectric grease # 11755 works well to protect those clean solid connections from corrosion.

expert reply by: Adam R