Trailer Running Lights Turn Off When Truck Is Started, Turn On when Truck Is Turned Off

Updated 11/08/2011 | Published 11/07/2011

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Question:

Our running lights were dim when we started out on our trip. After a brief stop our running lights went out on us. We turned the engine off to diagnose and the lights came on. Started the engine and the lights went out on us. Have NO idea what the issue is. Thanks for any help.

asked by: Corinne L

Expert Reply:

It sounds like a ground and/or short circuit problem. What you will want to do is disconnect the trailer from the vehicle. use a circuit tester like # PTW2993, to test the 4-Way on the vehicle side. If the signals are all coming through like they should, then you know the problem is on the trailer side.

If the signals are not coming through correctly, the converter box could be bad. To test the converter, test the signals going into the converter from the vehicle, if those are correct but coming out, they are wrong then you know the converter is bad. If the signals are wrong going into the box, then there is an issue with the vehicle wiring. You may need to check if a fuse is blown, or a relay has gone bad that is related to the tow package wiring.

If everything checks out on the vehicle and all signs point to the trailer, start by checking the vehicle and trailer side connectors for dirt and corrosion and clean them out. Then you can make sure that the main ground for the trailer and the grounds for the lights are all connected to clean and corrosion free bare metal surfaces.

expert reply by: Michael H