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Brake Lights and Tail Lights Do Not Work on Trailer With 4 Wire System

Question:

The turn signals work on my trailer but the brake lights and running lights dont work. All the lights work on my pickup. Wiring is 4 wire.

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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 brake lights are not working when the turn signals are. There could be a few different issues going on here, so I recommend starting the troubleshooting process from the beginning.

First, with the trailer not connected, check the connector on the tow vehicle for signal using a circuit tester like # PTW2993. This is to make sure that a signal is coming through to the vehicle side trailer connector. I would also recommend checking that there is no corrosion or debris on vehicle side or trailer side connectors.

If the brake or running light signal is not coming through, then there must be a problem with the vehicle wiring. Depending on the vehicle, it could need additional fuses or relays to make the wiring fully functional if this is the first time you are trying to tow with your vehicle. I would recommend checking all of the connections for the wiring harness on your vehicle. 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 the lights will not work.

Next, 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 failure.

If your trailer has 5 wires coming from the back of the plug rather than 4, then you have 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, that could be a problem as well.

Also check the ground connections on the trailer. The main ground should be attached to a clean, bare metal surface on the trailer frame. The lights should also be making contact with clean and corrosion free metal surfaces. I have attached an article on trailer wiring that you may find helpful when troubleshooting.

If the issue persists after following these steps, then let me know what you come up with and we can do further troubleshooting.

expert reply by:
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John H

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