Troubleshooting 7-Way Trailer Connection and 6-Way Adapter on a 2005 GMC Sierra

Question:

Hello. I am using a 6 pin to 7 pin adapter. I plug it into one vehicle that only has flat four wiring to the 7 pin plug on the vehicle and all the lights on the trailer work fine. The trailer brakes do not, but that is because I dont have that wired at the vehicle. The second vehicle has 7 pin plug with trailer brakes and brake controller. I plug in the 6 pin to 7 pin adapter and then plug it to the vehicle and the brakes immediately lock up. Also, zero lights work blinker, brake, tail None work. I have read that trailers can be wired with the center and brake being one of two ways. So we switched the center pin and the brake pin inside the trailer wire itself. assuming that would be the same as switching those inside the adapter. Then nothing we plugged in the adapter and nothing happened, trailer brakes didnt lock and still no lights. The vehicle that with the problem is 2005 GMC Sierra. The other vehicle works fine with plugged up, but again, doesnt have the wiring for 12v or brakes...just lights. Why is nothing working when plugged into the GMC? Thank you.

asked by: David C

Expert Reply:

It sounds like the 2005 GMC Sierra may have a bad trailer plug. I recommend the Curt T-Connector Vehicle Wiring Harness for Factory Tow Package - 7-Way and 4-Pole Flat Connectors # C55774 if you have tested the functions and they are not working properly at the trailer connector on your truck. To test each of your 7-way functions on your truck, I recommend using a circuit tester like # PTW2993. I've attached an article to assist with 7-way function locations as well as the 6 way function locations. If the connector is working properly, then you will need to replace your adapter.

For a new adapter, I recommend the Hopkins Endurance Multi-Tow Trailer Connector Adapter - 7-Way to 6-Way, 5-Way or 4-Way # HM47570 to adapt to any other trailer. The 6-way can be switched as you mentioned for function.

expert reply by: Jason S