Troubleshooting Non-Functioning Trailer Brakes

Question:

I RUN MY OWN SHOP FOR OVER YRS. I KNOW THE PROBLEM IS IN TRUCK. SEVERAL YEARS AGO I REPLACED THE MODULE ON THE REAR OF TRUCK. WORKED FINE FOR A WHILE. IT WAS A GM PART. THE TRAILER HAS BEEN REBUILT BRAKE UNITS. HAVE NOT TESTED YOUR SUGGESTION YET. this is my personal vech. THANKS

asked by: TOMMY H

Expert Reply:

I suggest testing the trailer brakes even though you're fairly certain the issue is on the truck side.

Disconnect the trailer plug from the vehicle and use the vehicle battery (or a spare that is fully charged) to directly apply +12V power to the trailer plug's brake circuit. I linked a photo that shows where the battery ground and hot connections need to be made. With full battery power directly applied to the plug contacts the brakes should fully energize. You should be able to hear them humming gently. Since this test is so easy to do it just makes sense to confirm the trailer is okay before moving on to the truck.

If you have a circuit tester like # PTW2993you can test the socket on the truck (without the trailer connected) to see if signals from your trailer brake controller are making it to the 7-way. Apply the grounded tester to the socket contacts for the ground and brake circuit (the 7:00 and 5:00 positions, respectively) while a helper activates the manual override on your trailer brake controller. If you do not get brake power then you know for sure there is a problem on the vehicle side.

Next, cut the blue wire at the back of the controller a few inches from it (so you can re-connect that wire after testing). Repeat the same test on the blue wire while applying the manual lever. If you get signal there (but you didn't at the 7-way) then you know the problem is somewhere in the vehicle wiring. If you do NOT get a signal on the blue wire right at the controller then the controller is bad.

You can also refer to the two linked articles.

expert reply by: Adam R