Trailer Turn Signals Work but Brake Lights and Running Lights are Not Working  

Question:

Expert Reply:

On a trailer the brake lights and turn signals are on the same circuits so when the turn signals work but not the brake lights it is a little strange. There are some common things that can cause that though.

First it is possible that there is a trailer brake light fuse on the tow vehicle that is blown or missing. It could also be a stop light or CHMSL fuse. You can test the vehicle side connector without the trailer connected using a circuit tester such as # PTW2993 or a meter if you have one. Have someone sit in the vehicle and apply the brakes while you test the connector. If there is no power on any of the pins when doing this test, then the problem is on the vehicle side. If there is power, then the problem is on the trailer side.

It could be the same thing for the running light circuit. You can perform the same test only with the vehicle head lights turned on. If there is no power, then the issue is on the vehicle side, and you can check for trailer running light/parking light fuses. If there is power, then the problem is on the trailer side.

If the trailer side wiring is all intact and both the vehicle and trailer side connectors are clean and free of corrosion inside and out, then it could be a ground problem. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

The light grounds (and all grounds) should be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Michael H

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