Trailer Lights Malfunctioning behind a GMC Truck with a Factory 7-Way and a 7 to 4-Way Adapter  

Question:

Expert Reply:

If the 7-Way blade socket on your GMC is factory, then the brake and turn signals are combined. There are a few possible sources that come to mind. It could be some problem with the wiring on the trailer, a bad ground somewhere, the adapter may be mis-wired internally or if it has been exposed to moisture, it may be corroded inside and causing faulty connections.

I am not sure how brake lights would cause the trailer lights to go on and also turn all lights off.

Use a circuit tester like part # PTW2993 to test the 7-Way on the truck. I have added a link to a wiring help page that shows how the 7-Way should be functioning. Make sure you get the functions in the proper location. Mainly, as you face the connector at your truck bumper, the pin at the 3:00 position should provide right blinker and brake. The pin at the 9:00 position should be left turn and brake. The pin in the 11:00 position should be parking/running lights. The pin in the 7:00 position should be the ground pin.

If everything checks out on the truck, I would then check the adapter. Make sure it is clean and the wires are going to the right spots to make the proper connections. If everything checks out here, I would then suspect ground connections or the trailer. Check all the ground connections on the trailer making sure they have good metal to metal contact and that the ground pin on the trailer connector is solidly attached to the trailer frame with good metal contact. Paint on the frame can insulate a connection.

Write back and let me know what you find and we can continue from there.

expert reply by: Patrick B

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