Troubleshooting Trailer Brakes that Lock Up When Headlights are Turned on 2006 GMC Sierra 1500

Question:

2006 GMC Sierra 1500, 5.4, Z71, towing package. New travel trailer. Used the fuse box in driver side to plug in brake controller. All lights work fine, turn indicator, brake lights, and running lights. Brakes on trailer work when manual controller is pushed but not when brakes are applied. Also, brakes on trailer go on when bright lights are manually turned on. Coming home this afternoon the brakes came on when it got dark enough for the headlights to come on. Does not happen when daytime running lights are on all day but no brakes. Thanks,

asked by: Stephen S

Helpful Expert Reply:

The first thing I would do is make sure you have plugged into the correct port in the electrical junction box under the dash of your 2006 GMC Sierra 1500. You need to be using the second slot from the left on the top row of the box. Check out the picture I attached that shows which slot to use. It is the slot highlighted by the red square.

The next thing you need to do is make sure all the correct connections are made under the hood of your truck. For more details on that check out Step 2 of the FAQ article I attached that covers this.

Next, you will want to determine if the problem is on the truck side of the setup or the trailer. I would recommend you test the connector on your truck with a circuit tester like our part # PTW2993. I attached a video on how this is done. I also attached a picture that shows how the 7-way should be wired.

There is a possibility that have the wrong adapter from your controller to the port under your dash. The correct harness is the # 3015-P. If the red wire of the harness is next to the blue wire then you have the wrong harness. An easy fix for this would be to remove the red wire and move it to the pin in the opposite corner. So that it is between the black and white wire.

expert reply by: Jameson C