Trailer Running Lights Not Working w/Wiring Harness Installed on 2018 Jeep Wrangler Unlimited JL

Updated 04/30/2018 | Published 04/29/2018

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Question:

I purchased this wiring harness for my 2018 JL wrangler unlimited. Once i installed it everything seems to work except the park lights. Turn signals and brake lights all work. Is this a problem with the harness or did i miss something?

asked by: Kevin S

Expert Reply:

To determine if the issue is on the vehicle side or the trailer side you can test wiring harness # 118786 on your 2018 Jeep Wrangler Unlimited JL. Use a meter or a circuit tester such as # PTW2993 to test the pin on the 4-Way that is right next to the ground pin. The ground pin is the only one that is exposed on the 4-Way.

If that pin has power when you test it and the Jeep lights are on, then the issue is on the trailer side. If it does not have power, then use the meter or tester to test the brown input wire that goes into the black converter box.

If that wire has power when it should then there is a problem inside the converter box and the wiring harness will need to be replaced. If there is no power on that brown wire when there should be then something is off on the vehicle side such as a blown running light fuse.

If you find the issue is on the trailer side, then the first and easiest thing to check is the trailer's connector. Make sure the mating running light pin is clean and free of corrosion. It could also be a ground issue. 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.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also 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.

If the trailer has a lot of running lights it could be overdrawing the converter. Switching to LED lights on the trailer will solve that problem.