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Troubleshooting Turn and Brake Signals That Only Come On with Running Lights  


Hey I have a 6x10 utility single axle trailer with a 4 pin connector. My problem is the lights only are illuminated when the truck headlights are on. Turn signal doesn’t work, brakes don’t illuminate they only work when I turn on headlights, and they are constantly on. Any advice would be much appreciated.


Expert Reply:

The first thing I recommend doing is using a circuit tester like # PTW2979 to test for power on the tow vehicle side 4-Way connector. Have a buddy sit in the vehicle to go through the different lighting functions while you test the corresponding pin on the 4-Way for power. You will want to make sure that you are getting power at the correct pin and that you aren't also getting power with the other pins.

If you test the connector and the results are similar to the issue's you're seeing (no power with turn signals or brake lights but power at all the pins other than ground when the running lights are on) then there is likely some kind of short or a crossed wire on the vehicle.

If this is the case then I recommend inspecting the 4-Way itself for any debris or corrosion inside the pins. Sometimes corrosion can build up inside the connector and bridge multiple circuits together, which would explain why everything turns on with a single function (running lights). You can clean the pins out if you're able or replace it with a new one. If you can tell me what vehicle you have I will be happy to make a recommendation.

Also check just behind the connector to see if you can find any pinched or exposed wires that need to be replaced.

If when testing the vehicle's 4-Way you found that everything gets powered as it should, then the issue is on the trailer side. Inspect the trailer side 4-Way for debris/corrosion and any wiring you can gain access to for damage.

If you are still unable to locate and resolve the issue please let me know and we can continue to troubleshoot. I have also attached a wiring troubleshooting article that you might find helpful.

expert reply by:
Chris R

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