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Troubleshooting Weak Ground Connection That Causes Lighting Malfunction

Question:

Lights on chassis...Marker lights are working...3 small lights in the middle if chassis in the rear is working...I have brake lights and turn signals...No tail lights...Everything works on some chassis...Some chassis same problem...I change my 7way plug on back of truck an pigtail...Same problem...I need to run a jumper wire to get me through the morning....what wires do I need to tap into...to get tail lights....4o ft chassis container...

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Expert Reply:

What you describe sounds like a classic grounding problem. It often happens that some light functions on a trailer will work under some conditions, but stop working when other lights are activated. Tail lights are especially likely to cause this issue because they draw more current. Tail lights draw more current because they stay on, unlike brake and turn lights that are generally on for only brief moments.

The fact that your jumper wire corrects for this problem pretty much confirms that you have a weak ground.

Try cleaning all contacts on your 7-way vehicle socket and trailer plug, but especially the grounds. Please refer to the linked photo to see which pin on the socket is ground; the mating one on the plug should also be cleaned.

You can also check the main ground wire on the trailer. This main ground should connect solidly to a clean bare metal surface. Sand off the grounding point to remove paint, primer or rust that could be interfering with a good ground. Once you get the ground correct you can protect it from corrosion with dielectric grease like # 11755.

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