Why Won't Trailer Lights Work When Running Lights are On
Updated 11/26/2025 | Published 11/25/2025
Question:
I just installed new led trailer lights with a 4-pin connector. All circuits work individually with power applied but turn/brake lights will not come on if running lights are also powered. When running lights are on, only the side lights work on the taillight. The rear only illuminates with the brake and turn circuits. Ive made sure all ground connections are shiny and tight. I just dont understand why power to running lights renders all other circuits inoperative and dont turn on the dimmer brake light. There were no fuses in the new harness and brake lights work on other trailers that the vehicle tows. Thanks for your help!
asked by: Malachi H
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Expert Reply:
Hey Malachi, what you described does sound like an insufficient ground even though you did say you checked all of them. I would try running a ground circuit back from each light all the way back to the main ground of the trailer wiring and see if that fixes the issue. Let me know if that helps.
Product Page this Question was Asked From
Pollak Metal 7-Pole, RV-Style Trailer Socket - Vehicle End
- Custom Fit Vehicle Wiring
- Trailer Hitch Wiring
- Vehicle End Connector
- 7 Blade
- Plug Only
- Pollak
more information >
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Malachi H.
11/26/2025
That fixed it! I measured 1.5k ohms through the trailer from the tongue ground to tail light ground so I guess the trailer is corroded at the frame bolts and I’ll have to run individual grounds