Troubleshooting Running Light Function on Trailer with 2008 GMC 1500

Updated 04/17/2019 | Published 04/16/2019

Question:

Hey guys, First of all, I appreciate all that you do for us trailer fanatics. Your website is the best one out there for all things trailer related! I have a 2008 GMC Sierra 1500 SLT, All-terrain, z71. I recently bought a 16 foot trailer on which the owner had cut all the wiring and wasnt using the lights or single-axle brakes. So i rewired the trailer. The rear lights had 3 wires coming out of each one: one side had 2 green and 1 gray, the other had 2 yellow and 1 gray. I know when wiring trailer lights, function matters more than color. So i wired the colors with my new wiring to coordinate with my 7-pin connector on my truck. After hooking everything up, my stop lights work, my turn signals work, and the side markers on the rear lights work with my running lights on. When i wired it, i ran RH/stop to one wire, running lights to another wire, and a single ground to the gray wire. HOWEVER, the running lights for the main bulb for each rear light dont work. On previous trailer usage, ive used a 7 way to 4 pin and never had this issue. So far ive checked the trailer running light fuse, its good. The trailer has good ground because i drilled and sanded to bare metal on the trailer for the ground wires I ran to the lights. Whats next? Checking the running light pin on my 7 way from the truck? What are other things that could cause this? It appears that the bulbs in the rear lights are 2 filament bulbs so they should have running light functionality. Did i hook something up incorrectly? These issues are hard to track down sometimes and they will drive you bonkers until you find them! LOL

asked by: Scott

Expert Reply:

For your 2008 GMC Sierra 1500, I do recommend checking the running lights pin on your 7-way. That will let you know if there is a trailer issue or a truck issue. This pin is located at the 11 o'clock position. There is a trailer park lamp fuse #25 in your underhood fuse block.

If that checks out, then you will want to check the function of your lights you can simply connect a jump wire to each wire for function. It is possible that your 3 wire lights could use that grey wire as a running light and be grounded through the mounting hardware on the light which isn't uncommon at all. You are correct most three wire lights have a ground wire, a running light wire, and then brake/turn wire, but there are some that will function differently and without knowing what lights you have you will need to test all the possibilities for wiring.

expert reply by: Jason S