Turn Signals and Brake Lights Do Not Work on Trailer Towed with Ford F-350

Question:

Good afternoon. I have a f350 truck and when the left signal, right signal and stop lights are not working but the other lights do. It is a round plug and I have determined it is not the trailer but something with the wiring harness of the truck. Any suggestions? Thank you.

asked by: Kellen G

Expert Reply:

The best way to determine if the issue is indeed on the truck is to use a circuit tester like part # 3808 and test each individual pin on the connector as its function is activated.

First you will want to clean the connector and make sure there is no corrosion on the pins. You may also need to remove the wires from the connector as corrosion can build up in the connector. If all of the connection points are corrosion free, then begin testing the pins or the wires themselves.

If you find that you are not receiving a signal on some of the pins, then I recommend tracing those wires back until you find a point where the signal is active. It may be possible that connection points between the wiring harness and the vehicle wiring are bad.

I would also recommend checking for any blown fuses if you have the tow package. I have attached a wiring article for you to review while troubleshooting your wiring harness.

I would also recommend checking the ground connections on trailer for the lights and trailer connector. Faulty ground connections can cause different issues with trailer lighting.

expert reply by: John H