Troublesooting No Left Turn Single at 7-Way Plug on 2013 GMC Sierra with Factory Tow Package

Question:

Hi, I have a 2013 GMC Sierra with a tow package. I plugged in my tow lights that I use when I tow another trailer that I dont know if the wires are good or not. It just saves me time and gives me piece of mind. Any way, everything works, but my left turn light. I have checked and have no power to the plug. Can you give me a idea on where to look for where the trailer plugs into the truck harness? The wire harness is all stock factory. Thanks.

asked by: Dave G

Expert Reply:

First, I would apply a circuit tester like # PTW2992 to the truck's 7-way socket. Ground the tester and apply the probe to the pin for the left-turn signal and brake. If no turn signal is present on the pin at the 9-o-clock position (refer to the photo and video, (I have linked for you) then next have a helper step on the brake while you test the same pin. If you detect a brake signal, then you know the wiring works and it is likely to just be a turn signal fuse. Look for any fuses labeled trailer/towing and of course for the left turn signal fuse. Replace as needed.

You should also inspect the truck 7-way for any signs of corrosion, front and rear. Even slight greenish or whitish discoloration can indicate mild corrosion that is enough to interfere with signal conduction. Clean contact surfaces with a spray-on electrical contact cleaner (available at auto parts and electronics stores) as needed, to remove the corrosion as well as any dirt/salt that may have accumulated. If you find you need to change out your truck's 7-way you can use the link provided at right to select a replacement.

Once you find the correct signals on the Sierra's 7-way pins you can re-connect the trailer. If you still have the left turn malfunction, I would apply 12V battery power directly to the trailer's plug, to test the trailer light functions independent of the tow vehicle. You may need to trace the wire for the left turn (usually this is a yellow wire) all the way from the 7-way plug back to the light assembly, checking for any signs of wear or damage to the insulation that could be causing a short, and replace as needed.

expert reply by: Adam R