Troubleshooting Running Light Circuit on Trailer  

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Expert Reply:

The first step in troubleshooting your trailer taillights would be to determine if the problem is on the vehicle, or in the trailer wiring. Start first by using a circuit tester like part # 40376. Turn on the vehicle headlights, and use the tester to check the pin on the trailer connector that carries the running light circuit. If you have a 4-pole connector, test the pin fed by the brown wire. If you have a 7-way, check the pin in the 11:00 position. If you get a signal on that pin, the problem exists in the trailer wiring. If you do not get a signal, there are several things that could be causing the problem.

If the vehicle has an OEM tow package, it could be a blown fuse. Typically, vehicles will have multiple fuse boxes, be sure you are checking all of them. The lid of the fuse box typically has a legend showing which fuse protects which circuit. If the vehicle does not have a tow package, and the trailer connector is aftermarket, the converter on the trailer wiring harness might be bad and in need of replacement. Without knowing the year, make and model, I could not speculate further.

If the problem lies in the trailer wiring, start by examining the trailer connector, checking for corrosion or damage and clean/replace/repair as necessary. Find the the circuit that feeds the running lights, and trace the wiring back to the lamps. Look for loose or broken connections and pinched or broken wires. Check the wiring insulation, making sure it is not cracked or worn through.

We have some excellent trailer wiring and trailer wiring troubleshooting articles. I have linked you to both of them.

expert reply by: Mike L

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