Malfunctioning Brake Lights on a Trailer Behind a 2004 Ford Expedition  

Question:

Expert Reply:

The Ford Expedition should already be grounded properly, but we will check that. Start with the vehicle and work back to the trailer. We can eliminate the vehicle as the source of the problem pretty easily.

Disconnect the trailer and have someone sit in the driver seat. Take a circuit tester, like part # PTW2993, and test the Expedition connector. Clip the alligator clip onto the exposed post of the connector, this is the white wire and should be your ground. Have someone turn on the headlights, the brown wire should be active. Test the yellow wire, it should come on when the brake is pressed, and the left blinker is activated. Test the green wire, it should come on with the brake and right blinker. If everything checks out here, the problem is not with the Expedition but the trailer.

I am fairly sure that the vehicle is not the problem since you have everything but brake light on the trailer, so start with the the tail and brake lights on the trailer. If the lights are grounded through the mounting points, make sure the area is clear of corrosion and paint. Often times, the studs on the lights are the ground points and if they do not have good metal to metal contact with the trailer, you will not have a good ground.

Check your wire connections and make sure they are good and free of corrosion. Finally, check the trailer connector. Follow the white wire back to the trailer frame. Again check for corrosion and paint at the connection point and make sure you have good metal to metal contact.

If none of these fix your problem, please write back and we will go from there till we find a solution.

expert reply by: Patrick B

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