Troubleshooting 4-Pole Wiring Harness on 2007 GMC Sierra

Question:

I have the wiring all connected correctly, testing a new set of lights and all wire is new, not on the trailer yet... I connect the ground to both lights they are dim, disconnect one and the right side is bright and the left doesnt work. its plugged into the connection of my 2007 GMC Serria pick up. Keep in mind this is a new set of lights . Could there be a problem with the connection at the rear of my truck. All truck lights work as they should. What could be the problem?

asked by: Rex

Expert Reply:

One of the most common causes for a 4-pole wiring harness to have one function not operate (or to operate weakly) is a bad ground connection. Since this is easy to check I suggest that you make this your first troubleshooting step. Check that the harness white ground wire's ring terminal is firmly affixed to a clean bare metal spot on the vehicle.

If there is corrosion on the surface, grease or a build-up of road grime then there may not be a good enough ground connection to support all the harness circuits. Clean and/or sand off the grounding point (or choose a new one with a good clean surface) to establish a solid ground. You can also protect that new connection with an application of dielectric grease like # 11755 which will help protect the new connection from moisture and contaminants.

If this does not remedy the problem then next check your plug-in connectors at the truck taillights to ensure they are fully seated together. You can also check the connector contacts for any signs of corrosion or other issue such as a bent pin. The linked article on troubleshooting 4-pole installations will help you.

expert reply by: Adam R