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Discussion Starter #1
So after about 5 months of dealing with the stupid "service trailer brake system" message on the display every time the truck was started as well as random points while driving with and without a trailer, I decided to dig into it. After reading other posts and stories of dealer guess work, I was given a good head start. This may not be able to fix everyone's problems, but the few that it does help, this is for you.

After checking the easiest first, the related fuse(s) were still good, but that's where I would start. First when I got under the truck, I removed and cleaned all trailer connections as well as the TBCM connectors to find a ton of dirt and such inside them. I cleaned with an air hose and applied minor dielectric grease to the main trailer harness, checked the connections, and plugged back in...still nothing. So my next option was to go for the relay that is on the driver's frame rail by the rear axle (for the LMM atleast, I believe the older models you have to drop the spare and it is sitting above near the axle). I was going to make the "$33 bet" and purchased the part via amazon. [AC Delco pt#20904439]. With doubts, I replaced the relay assembly and low and behold, the message has not been seen since. I have been driving it for several days and have not seen it once. In the past months, every single time I started the truck it was there as well as multiple times every drive.
I hope this helps anyone with this issue. Attached is a picture of the relay for a visual.
 

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Thanks for letting us know what your solution turned out to be. Seems the relays are a common failure point. Fortunately they are neither expensive nor difficult to replace.
 

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When I bought my 2012 it had same message. After reading here I decided to check. Mine was MIA gone. Had to get a new one. Now I need the bolts/ screw that holds it in place. Anyone know the size and thread?
 

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Did you try a parts lookup from one of the dealer websites? That's where I would start.
 

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Discussion Starter #5
Im not sure of the thread or bolt size. Im in Texas and when it was 110 degrees trying to get to the bolts, I said screw it and used zipties to hold her up there. Ill remove the old one when it cools down...
 

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Thanks THerb,

I've been staring at that "Service trailer Brakes" message all winter - getting ready to pull my trailer this weekend, so I started troubleshooting - took the $35 gamble and its fixed... Funny - it reset and then came back on at next key cycle! I removed plug at bumper and sprayed with WD40 then compressed air to clean and dry... Then checked underhood fuse box... really dusty, so sprayed down with WD40 and dried with compressed air. removed fuses one at a time and cleaned. then finally removed knee panel in cab, disconnected brake controller wire from socket and reconnected... Message is gone - don't know which action solved it but I'm glad its gone.
 

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I ended up having to replace my brake switch on the brake pedal to solve my trouble.

No Service Trailer Connection issue, but it kept kicking out of cruise while I was on the bumpy I-10. Every bump the cruise would kick off.

I replaced the brake switch, problem solved.
 

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Check Trailer Brake system, Cruise control, back up camera, heated seats not working

So I just installed the Fleece Lift pump last weekend, which I had to add a fuse jumper and replace the 10a fuse in position 40 with the jumper to the fuse block under the hood. I had some issues getting the remote to switch the new relay for the lift pump. Finally got is to squirt fuel out and put the bed back on the truck. Now I have the start of this threads issue along with a couple other. My Rig is a 2011 Denali LML pretty stock. List of issues to follow; "Check Trailer Brake System', cruise control not working, back up camera not displaying, heated seats not working. I did see a thread that this chap installed a Fass lift pump and searched for a remote on the under hood fuse block and had many of my issues, he stated he had to replace that fuse block. Problem solved.
 

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So I think I solved my issues. I started trouble shooting the install of the loft pump by checking to see if taking it out of the fuse block would show results. The fuse jumper that was advised to replace the 10a fuse was installed with a 5a. I replaced with just the 10a and stuff started tti week again. Si then I tried the jumper with 10a and all is working again. So it was interesting that it was advised tti replace the 10a with three 5a in the jumper, pretty limiting. Must have been enough tti not allow some sirens to not work.
 
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