Trailer Brake Lights and Running Lights Work but Turn Signals Do Not  

Question:

Expert Reply:

On a trailer that uses a 4-Way wiring harness, the brake lights and turn signals are on the same circuit. So, it is strange that the signals are not working when the brake lights are. First, with the trailer not connected, check the connector on the tow vehicle using a circuit tester like # PTW2993. This is to make sure that a signal is coming through to the vehicle side trailer connector.

If a turn signal is not coming through there must be a problem with the vehicle wiring. Depending on the vehicle, it may need fuses or relays to make the wiring fully functional. Also, it may show a signal at the connector but when connected to the trailer, the power draw is too great and without those fuses or relays not all the lights will work.

Next, you can connect the trailer and test the wiring right behind the trailer side connector. If you are getting a signal at this point, work your way back towards the tail lights, testing the wiring along the way. You may find a break in the wiring that is causing intermittent failure.

If your trailer has 5 wires coming from the back of the plug rather than 4, you have what is called a wishbone harness. On this type of harness, the running light signal is split at the trailer connector. One wire goes to the left running light circuit, the other goes to the right. There should still be 1 wire for turn/brake for each side. If these wires were somehow attached to the lights incorrectly, then that could be the problem.

Also check the ground connection on the trailer. The main ground should be attached to a clean metal surface on the trailer frame. The lights should also be making contact with clean and corrosion free metal surfaces.

expert reply by: Michael H

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