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Troubleshooting Tail Lights on Trailer Only Work When Running Lights Are Turned Off  


My brake lights and turn signals on my trailer only work when the running lights are turned off. When I turn the running lights on they work but the brake lights and turn signals no longer work. Everything is new from the 4 wire plug all the way to the lights. Thanks


Expert Reply:

I tend to think it's a ground issue or potential short on your vehicle side wiring. Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector ground should be attached to clean and corrosion free bare metal surfaces. Also, I would check the grounds for each of your tail lights. If the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

Another potential issue is your harness is overloaded when your tail lights are engaged because your trailer has too many running lights. Check your harness for a maximum amperage rating and determine the draw of the trailer lights. Ensure they are not drawing more than the harness can handle. You could replace your trailer lighting with LED versions because they draw much less power.

I included a couple informative articles for you too.

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