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Trailer Tail Lights and Running Lights Do Not Work When Connected to 1994 Ford Ranger

Question:

With the connector installed, the vehicle lights work properly. The brake lights on the trailer work, the tail lights and turn signals do not work. Do you have a suggestion to fix my problem?

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Expert Reply:

This is a strange situation because on a trailer, the brake lights and turn signals are on the same circuit. Unless your trailer has turn signals that are separate, it should be a standard 2 wire system. What may be occurring is a bad ground, a alternative wiring situation on the trailer, or a bad converter box.

First, testing the converter is simple. Start by disconnecting the trailer. Next, use a circuit tester like # 40376 to first test the signals going into the converter and then coming out of the converter. If the signals are correct going in but not coming out then the problem is ion the converter and it will need to be replaced.

Checking grounds is the next easiest thing to check. Start at the trailer side connector and make sure that the main ground is attached to a clean and corrosion free metal surface. Work your way back to the tail lights and make sure each of them is also attached to a clean and corrosion free bare metal surface.

If the trailer has a plastic license plate holder, this can cause ground problems. If the trailer connectors have corrosion, clean them out because corrosion can cause a bad ground or connection as well.

Determine how the trailer is wired. It should have one wire going to each tail light that carries the brake and turn signals, and another wire going to each light that carriers the running lights. On the harness, the white wire is ground, the green wire is right turn and brake, the yellow is left turn and brake, and the brown wire is running lights.

These are the most likely issues that could be causing the problem. I have included a link to our FAQ article on wiring that does include some information on troubleshooting for you.

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