Troubleshooting Trailer Lights Not Working with T-One 7-Way Connector Install on GMC Acadia

Updated 07/26/2019 | Published 07/25/2019

Question:

The 2012 and a half year model without the tow package has the factory plug for the brake controller, but I had to run power from the battery and a ground wire to the controller. It also has the T-connecter behind the rear passenger wheel well for a 7 pin trailer harness. I connected a trailer to the seven pin harness part 118271. My brake controller showed a connection so the factory blue works. I am having a problem with the left side running lights and blinker. I can plug the curt seven pole tester into the harness and it show that everything works. However, when I plug a trailer in the left side does not work. I tried two different trailers so it is not the trailer. I dont think it is a fuse because the seven pole tester shows the blinkers working. What can I do to trace the problem? Do you have the amperage for DC current to left blinker that I can test with a multimeter?

asked by: Michael F

Expert Reply:

That is really interesting that you are having the same problem with the left running and turn signals on your trailer since you are seeing power on that pin for the T-One # 118271. If you check out the linked installation instructions you can see that the stop/turn and tail (running) light circuits have a max rating of 10 amps.

What you might want to try doing is testing both your trailers independently of your GMC Acadia. It would make sense that the turn signal wouldn't work on one side as both sides are on different circuits, but the running lights are all on the same circuit so if they are working properly on one side that is a good indicator that your trailer wiring on the left side is damaged or you have bad grounds. It is rare that the same issue would be happening on 2 different trailers but it isn't completely unheard of.

Try applying 12V power directly to your running and left turn pin for your trailer to see if they work that way. You can use a vehicle battery by grounding the negative terminal and then using the positive terminal to to apply power.

Let me know if you are still having issues after testing this and I can help you troubleshoot further.

expert reply by: Jon G