Troubleshooting Trailer Running Lights Not Working Using a 7-Way to 5-Way Adapter, 2008 GMC Yukon

Question:

I have a 2008 GMC Yukon XL. I recently purchased a 7 round to 5 flat Trailer Wiring Adapter. I plugged the adapter into the Yukon and plugged in the 5 flat from the boat trailer into the adapter. All lights work accept the trailer running lights.

asked by: Adam R

Expert Reply:

You will want to make sure that the 5-Way portion of the adapter is pinned out to match the 5-Way on the trailer. You will need a circuit tester like the Quickee # 3808. Test the adapter when it is plugged into the 7-Way on your 2008 GMC Yukon to determine which pin is for the running lights. Then make sure the corresponding pin on the trailer side connector matches. If not, you will need to re-pin either the trailer side connector or the adapter to match.

If there is no signal at the adapter, test the 7-Way without the adapter plugged in. If there is a signal then the running light circuit pin on the truck 7-Way or the adapter may need to be moved. If there is no signal for running lights at the 7-Way without the adapter plugged in then you may need to add or replace a fuse or relay. Check the vehicle owners manual to determine the fuse and relay locations.

If everything on the truck side checks out then the problem must be on the trailer side. If the trailer has a lot of running lights, side marker lights, or other lights that use the running light circuit, it may be too much power draw fo the system, causing the lights to not work.

You will also want to make sure the trailer connector ground is properly connected to a clean, corrosion free bare metal surface on the trailer frame. You can also check the grounds for each of the lights.

expert reply by: Michael H