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Troubleshooting Running Lights on Trailer


all the lights work on my trailer except the tail lights and marker lights.

Expert Reply:

The first step to troubleshoot your problem would be to determine if the wiring problem exists on the vehicle or on the trailer. Turn on the vehicle headlights, and use a circuit tester like part # 3808 on the pin on the trailer connector that feeds the running light circuit. If your vehicle has a 4-pole connector, test the pin being fed by the brown wire. If you are dealing with a 7-way connector, you would test the pin in the 11:00 position.

If the tester indicates voltage on the pin you are testing, the problem is with the trailer wiring. Start with the connector on the trailer. Make sure it is not damaged or corroded. 4-pole connectors are known for having the wire insulation crack right behind the connector after a few years. Once the insulation cracks, moisture can cause the copper conductor to corrode, and current will not pass through.

Trace the running lamp circuit back from the connector (usually a brown wire) Since none of your running lights are functioning, chances are that the problem will be between the trailer connector and the first marker light. Look for cracked or worn insulation. Make sure all connections at each marker light are tight, and that each lamp is bolted directly to clean metal. If each lamp has a separate ground lead, make sure those are securely connected to metal on the trailer frame.

If you are not getting a signal on the running lamp pin, how to proceed will depend on the year, make and model of the vehicle, as well as the type of trailer connector on the vehicle and if the vehicle is equipped with the factory tow package.

If you would respond with that information, I can provide more specifics. I've linked you to some trailer wiring FAQ articles that I hope you'll find helpful.

expert reply by:
Michael L

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