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All Trailer Lights are Working Except for Brake Lights

Question:

Running lights working properly. Turn signal lights working but no brake lights. Using a 4 wire harness.

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Helpful Expert Reply:

On a trailer the turn signals and brake lights are on the same circuit. Since the brake lights are not working I suspect a bad ground connection. Bad grounds can show up when there is more draw on a system. The brake lights light up the brightest and would draw more constant power than the turn signals.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or 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.

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