Troubleshooting No Power on Vehicle Running Light Circuit

Question:

i have no power at the plug on the truck for the marker light, everything else works fine. cant find a fuse thats out. please help.

asked by: Mark J

Expert Reply:

Marker/clearance lights on a trailer are powered by the vehicle headlight/running light circuit. In 4-pole wiring this signal is carried on the brown wire, which is the next to the white ground wire/pin. In 7-way wiring the running light signal is carried on the contact at the 11-o-clock position as you view the trailer socket; please see the linked photo.

You can use a circuit tester like # PTW2993 to test the socket contact, and from there move upstream on the vehicle wiring for that circuit until you find the point where the signal is lost. If you find that the running light signal is present on the wiring ahead of the trailer connector you could have a corrosion issue inside the connector. You can also check the fuse for the vehicle headlight circuit (if you have not done so already).

You can test the trailer too, by disconnecting it from the vehicle and directly connecting a 12V power source (like your vehicle's battery) to the ground and running light circuit contact on the trailer's connector. If the trailer lights work in this test then you know to focus on the vehicle wiring.

It is not uncommon for a 7-way hookup to develop internal corrosion that can be tough to spot. The outside of the 7-way may look perfect even though there could be corrosive build-up inside the socket and/or at the plug that connects to the socket.

expert reply by: Adam R