Troubleshooting Rear Running Lights on Re-Wired Utility Trailer

Updated 03/06/2018 | Published 03/04/2018

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Question:

On my utility trailor I installed new wiring. The side running lights work but the rear ones do not. The brakes lights and both blinkers work

asked by: Patrick B

Expert Reply:

In standard trailer wiring all clearance lights and running lights operate from the same circuit. This means that if some of the lights are working but a few do not, then those non-functioning lights may have a ground issue or other wiring problem isolated just to them.

Different lights can use different ways of grounding. For example, if your side clearance lights that do work use a wired ground, but the rear lights use the light mounting studs or posts to make the ground then it is possible that paint, primer, rust or other material at those lights' mounting points could be preventing a good ground. This could easily case them to not operate. Since all of your other lights work I suggest you focus on the grounds and wiring for just the rear running lights.

If the problem lights use plugs like # STL178RFPB then make sure there is no corrosion on either the trailer plug or the light socket. Once you get everything working you can protect the connections with dielectric grease # 11755.

expert reply by: Adam R