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Troubleshooting Failing Brakes on Trailer Being Towed by a 2017 GMC Sierra 1500

Question:

So I borrowed a friends trailer to bring a load of lumber up to a cottage and the brakes worked when I picked it up but only briefly. I drove it off his driveway and tested them with the factory brake controller override and they slowed down the tuck/trailer. I drove to the lumber yard and on the way the info Center told me to check trailer wiring. I assumed it was a poor ground connection and his trailer grounds through the hitch. I loaded up the lumber 4,600 lbs of it and then realized it wasn’t a bad ground, lights all work great but no brakes. Tested the connection and couldn’t get it to work so I brought it to my GM desert. They tested the truck and all checked out good they figure something on the trailer has a bad wire or magnet and the 2017 Sierra are “too smart” as in they can determine hydraulic vs electrcial brakes and if the load coming back isn’t quite right it defaults to no brakes. So I drove it home and parked it, thought I’d test it again myself and I connected only the 7 pin wireing harness, not trailer physically connected to the trucks hitch or Saftey chains connected. Go the the cab and try the brake controller, all works as it should. I can adjust the gain and the display shows as if the brakes are engaging as they should when I use the over ride or the brake pedal. However as soon as I the trailer onto the receiver in the hitch it has the error message and the brake seem to not work again. Of course I do not know if the brakes were actually engaging or not because the trailer was not moving but according the the display it all seemed to work until the trailer is physically connected to the truck. As soon as I crank the stand up and there’s not metal trailer to truck connection , only the wires connected the brakes appear to work again. Any ideas ? Is there a ground / brake wired backwards on the 7 pin trailer side ? GM tested and said all good on the trucks end...

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Expert Reply:

If the issue occurs only when there is metal-to-metal contact between your 2017 GMC Sierra 1500 and the trailer then it is definitely a ground issue. Since you have a GM shop check out your truck I would venture to say that your trailer is the issue.

Start off by checking the 7-Way connector on your trailer and making sure that it is clean and free of corrosion. If that looks good move on to the ground connections on your trailer. Since the issue is with your brakes check your main ground and then the ground connection point for all of your trailer brakes. Be sure that they are connected to a clean, bare-metal surface as even some paint can cause issues.

We do have replacement parts for your trailer such as a 7-Way with a Junction Box # 277-000141, Ring Terminals # 44-5310A, and Butt Connectors # DW05745-10.

If this doesn't fix your issue let me know and I can help you troubleshoot further.

expert reply by:
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Jon G

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