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Trailer and Tow Vehicle Brake Lights Not Working when 7-Pin Trailer is Connected to 4-Pin Vehicle

Question:

Weve been manually wiring up our trailer 7pin to our 4 pin vehicle and we have the right and left blinkers and running lights working. When we apply the brakes, the blinkers stop working on both the car and the trailer. Any ideas? It seems like we cant have both the blinkers and the brake lights working at once, but weve gotten them both working separately.

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Expert Reply:

On both a 7-Way and 4-Way trailer connector the brake lights and turn signals are on the same circuit so it is strange that the brake lights aren't working. But, brake lights draw more constant power than the running lights or the turn signals so it could be a situation where the trailer lights are causing to much power draw.

The first thing I recommend doing is using an adapter to connect the 7-Way on the trailer to the 4-Way on the tow vehicle. You can use # 37185. You will plug the 4-Way into this adapter, ground the white wire to the vehicle frame, and that will provide the light functions only to the 7-Way receptacle.

If you do this and still have the same problem then it is likely that the trailer is drawing more power than the tow vehicle can handle. Switching the trailer to LED lights will solve that problem. Also make sure that the grounds on the vehicle and trailer side are attached to clean and corrosion free bare metal surfaces. A weak ground can also cause this issue.

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Michael H

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