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Troubleshooting Running Lights Not Working on an Enclosed Trailer

Question:

REPLACED WIRE HARNESS ON TRUCK AND TRAILOR STILL RUNNING LIGHTS NOT WORKING HAVE TURN SIGNALS AND BRAKE LIGHTS

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Expert Reply:

The first thing I would check is the running light circuit on the truck side trailer connector to make sure a signal is getting through. Using a circuit tester like # 40376, and test the pin next to the covered ground pin. If the signal is getting through for the running lights then you know the problem is on the trailer side. if it is not, then you know the problem is on the truck side.

If the problem is on the truck side, check to see if there is a trailer package running light fuse or relays that needs to be installed or replaced. You can also check for any broken or loose connection along the wiring to the trailer connector. Finally, make sure the ground is connected to a clean and corrosion free bare metal surface.

If the problem is on the trailer side you can start by checking for and cleaning out any dirt or corrosion in the trailer side connector. Next, move on to checking the main ground from the plug. It should be attached to a clean and corrosion free bare metal surface on the trailer frame. You can also check to verify that the pin on the trailer side for the tail lights matches up with the pin for running lights on the truck.

Next, you can check the grounds at each of the lights. Trailer lights usually ground through the mounting studs and they should be attached to a clean and corrosion free bare metal surface. If the trailer has extra running lights, it may be too much of a power draw on the system and this could cause the lights not to work. You will need to either remove some clearance lights or replace them with LED versions if this is the case.

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