Trailer Running Lights Not Working when Connected to One Vehicle But Work Fine w/Another Vehicle

Updated 06/06/2018 | Published 06/04/2018

Question:

what causes your trailer running lights not too work, but brake and turn signals work on the trailer. When trailer connected to another vehicle the lights work.

asked by: Dean H

Expert Reply:

This sounds like 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. Some vehicles can overcome a weak ground where others cannot, which could be why it works fine on a different vehicle.

It also could be a blown tow package fuse. You can test the running light circuit on the vehicle side by turning on the headlights and testing the pin in the 1 o'clock position on a 7-Way or the pin next to the ground pin on a 4-Way using a circuit tester such as # PTW2993. If there is no power on that pin, then it is likely a blown fuse or a wiring issue with the connector or the wiring to the connector.

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.

expert reply by: Michael H