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Trailer Right Turn Signal Only Works when the Brake Lights are On

Question:

1986 f-150 right rear blinker only works when brake is aplied

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Expert Reply:

This sounds like a ground issue and/or a short. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time. A short could be caused by corrosion or loose or damaged wiring connections.

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.

Trailer lights have the turn signal and brake lights combined on the same circuit. So if the brake light is working but the turn signal is not then it would have to be a ground or short. Usually it is the turn signal that works and the brake light that doesn't but if there is a weak ground the higher draw of the brake light might be able to overcome it. Ground issues make lights do weird things.

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