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Trailer Brake Lights and Turn Signals Work but Running Lights Do Not Work

Question:

the turning lights and break lights work but the tail lights do not work and I checked the trailer wireing.

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Expert Reply:

Since you have checked the trailer wiring the issue must be on the vehicle side. What you can do is use a circuit tester such as # 40376 to check the running light pin on the vehicle side trailer connector.

If you are not getting any power on the vehicle side running light pin then the easiest thing to check is the fuse related to the tow package running light circuit. If the fuse is good then check the owner's manual because there might be a relay that is also associated with the tow package running light circuit.

If the fuse and relay (if present) are fine then the issue is somewhere between the connector and where the wires terminate further forward on the truck. Make sure the trailer and vehicle side connectors are clean and free of corrosion inside where the wires attach and outside on the pins.

From there you can test the running light wire from behind the connector forward as far as you can to check for a break or damaged wire.

If you do have power on the vehicle side running light pin then the issue is on the trailer. Likely a bad ground. The light grounds should be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

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