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Troubleshooting Trailer Lights That Only Work When Tow Vehicle Head Lights Are On  


All the lights on my tow vehicle and trailer do not work when headlight on tow vehicle is off. Everything works fine as it should IF headlight/running lights is on on tow vehicle. I confirmed that trailer is working properly. By hooking it up to friends tow vehicle. So I am almost 100 sure its the tow vehicle. But where do I start the troubleshoot. I already started by looking at the t-connector. Which I confirm is working properly. From there. I am not sure where to go. I trace/follow the wiring on the tow vehicle to mid-truck. But found nothing out of ordinary. Any advice would be appreciated!


Expert Reply:

I would start by checking the ground wire on the vehicle to make sure that it is firmly connected to a dirt and debris free spot on the vehicle's frame. Without knowing which harness you've got connected or what the tow vehicle is that would be the first place I'd recommend trying to look. Typically most wiring issues, especially with plug and play type harnesses, are caused by faulty ground wires.

expert reply by:
Erik B

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