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Troubleshooting Lighting Functions on 7-Way on 2002 GMC Sierra

Question:

On a 2002 GMC Sierra pickup, a 7 circle with a 4 flat was installed. It works fine with just the running lights, but if the headlights are turned on, the system goes crazy. For example, the turn signals do not work if I apply the brakes, but both turn signals on the dash glow slightly. Emergency flashers do not work if I push on the brake pedal. Back up lights are real pale. If I do not push on the brakes, but put on the flasher, the license plate lights flash. Please help!

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Expert Reply:

The lighting problems you describe sound like the classic examples of issues caused by a weak ground. If a wiring system's ground connection is not 100-percent tight to a clean bare metal surface the system may be able to deliver enough current for some circuits to work but not enough for all of them to work at one time.

In trailer wiring harness installations what usually happens is that everything works normally until the headlights/running lights are turned on. Since these lights remain on continuously they draw more current and will usually be the trigger that causes the grounding issue to show up. Both the vehicle harness ground and the trailer harness ground must be solidly connected to clean bare metal.

The other most likely cause for this issue is corrosion on and/or inside the trailer connector and its attached wiring. Disconnect and inspect all trailer-related wiring looking for corrosion or even just discoloration on the contact surfaces. Even slight greenish or whitish discoloration can indicate corrosion that can interfere with signals. Once connections are clean you can apply dielectric grease like # 11755 to keep those connections free of moisture and other contaminants.

You can also test each circuit on the truck by applying a tester such as # 40376. This will let you test each individual lighting circuit (without a trailer connected) to see if the correct signals are present. Please refer to the linked articles for more on testing.

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