Locating Malfunctioning Running Light Circuit on Boat Trailer Towed by 2015 GMC Sierra

Updated 05/23/2016 | Published 05/17/2016

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Question:

I have a 2015 GMC quad cab. It has the trailer pkg.but no trailer braking system. At first all my trailer lights worked until I turned on my headlights. Then all I had were brake lights on constantly and nothing else. Turns out that the lights on the trailer were wired incorrectly. I corrected that and added a second ground to the trailer as the original one was no good. Know all the lights work fine, But when I turn on my headlights I have No running lights. Everything else but no running Lights? Can you Please tell me where I went wrong?

asked by: Barry O

Expert Reply:

Begin by testing the running light pin on your trailer connector with a circuit tester like part # PTW2993. If you get a signal on that pin when the truck headlights (not daytime running lights) are turned on, then the problem exists on the trailer, not the truck.

With the trailer plugged in and the truck headlights switched on, check the running light circuit wire on the trailer (typically a brown wire) and trace back to the running lights. Test the wire with the tester, making sure the circuit is getting power.

Check your connections at the trailer connector on the trailer, making sure the connection for the trailer running lights is secure. I'm guessing there's a loose or broken wire somewhere in your running light circuit.

I'll link you to our Trailer Wiring and Trailer Wiring Troubleshooting FAQ articles.

expert reply by: Mike L