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Running Light Pin On Vehicle Has Power But No Running Lights On Trailer


2005 silverado and a fifth wheel. 7 pin trailer connector. When I test the plug on the truck, the running lights work on the tester. When I connect a stand alone battery to the running lights wire on the trailer, all the lights work. When I test the running lights wire at the end of the cable that connects to the trailer, it has power. When I connect the running light wire from the truck, which is hot I tested it and I connect it to the wire on the fifth wheel, nothing happens. I test the connection and its dead. I disconnect the wire and test again and there is power from the truck to the end of the cable. When I connected the wire from the truck that normally connects to the electric brakes and instead connect it to the running light wire on the trailer, it works, all the light light up. Its a heavier wire, 10 guage verses running light wire from the truck which I think 14 or 16 guage. Does that make a difference


Expert Reply:

The running light wire is generally a 14-16 gauge as you mentioned but this is common. I recommend you take a multi-meter and test for voltage both at the running light pin on the vehicle side trailer connector and at the wire just before it reaches the tail light while you have a hot lead hooked up to that circuit at the trailer side trailer connector. This will then let use know if we are receiving any voltage drop from a frayed wire. That said, the running light circuit can experience issues from improper grounds so make sure to re-secure these as well to new bare metal spots on the frame.

If you don't have any voltage drop and the grounds are good then I would be willing to bet the issue is inside the trailer connector either on the vehicle or trailer side. Therefore, best option would be to replace these with part # HM40975 for the vehicle and part # H20043 for the trailer. The pin inside these connectors can become bent or corroded which would then cause the issue you are having.

expert reply by:
Conner L

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