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Discussion Starter #1
I'm having some odd issues with my Allison as of late.

A few weeks ago it started to not want to shift into 3rd or 4th when I went from reverse to going forward. At times I'd hit a bump and I'd lose all gears. Went under truck and manually shifted it into N and did the same for the shifter. Started truck. Put in gear and made it home no issues. Shifted fine for 200 miles.
Researched and most said the NSBU switch. Picked up a new one from GM. Installed it and all seemed fine for the next 700 miles.

Took the truck out today. Lots of rough gravel roads and it felt like it had a surge twice. Thought it was nothing as all seemed ok.

Then went over some railway tracks and it felt like it slipped out of gear. Revved right up and wouldn't go anywhere. It also popped a SES light after this.
Got under truck as I was thinking maybe the bump loosened up one of the connectors on the NSBU. All seems ok. Then cycled it on off on. And all gears were back. Drove for maybe 7 miles. Got up to 65mph and that surge was back. Dropped down and it would go away.
Drove for another 30 miles and it just went out of gear again. Pulled over. Put it in park and back to drive. All was fine.
Got back into city. Drove into a friends driveway to drop him off. Backed out. No reverse at all. Tried to cycle it again. Nothing. But for word gears worked still. So pushed it back and drove home.
Now while driving home it felt like a transmission was slipping when I went from a stop to going forward. Once moving it was ok.
Got home. Shut it off. Went to take a leak. Came out checked the fluids again. All full. Got in truck. Now all gears are back again and strong.

Any ideas what it could be? Or where to start. I don't want to head to the trans shop as they will just say pull it and charge me a fortune.

The truck is never abused. Always highway miles.
 

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Go to your favorite parts store to have your code read for free and report back. All of our parts stores around where I live have a reader that is capable to read TCM codes. There are garages that do this and also our dealer reads codes for free.
 

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There are thousands of posts about transfer case "pump rub" to read, then you can look for the pinhole where all the atf leaked out of the transfer case. There are several after-market options to prevent it in the future. Be sure a new or rebuilt t-case has something to avoid it happening again in the future. GM provided a "solution" but it's not as robust as the after-market options.

On Edit: See sticky on pump rub in the Drivetrain - 2001-2010 section.
 

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Pump rub may not have had anything to do with this problem. Transfer case fluid is know to vaporize. I did a big post about this years ago here. Do a search.
 

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Discussion Starter #9
Well back to what it was doing Sunday. Was fine all week. Pulling a trailer and everything. Then tonight driving down Hwy and hit a bump and it popped out of OD. Then slowly started its slipping feeling. Stopped. Reset it. Everything perfect again.

On the way home was fine. Hit a bump again same thing. Stopped and reset it and perfect.

Any ideas what's going on now ?
 

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Discussion Starter #11
Ok solved the tow / haul

Broken wire in column. Was hoping it was the cause and making a short. No luck.

All I have left is the TCM. Which a lot that I have read points to it. This trans only has 90k miles on it from being new and never tuned. Never abused. Heck I didn't even light up the tires. It was just for Hwy towing my skid steer. So for it to be burnt up I find odd.

All the fluids are clear ( red clear ). No burnt smells either. Plus the trans works perfect once you reset it. I'd think if it was toast it would t work perfect. And I'm not just losing OD. It starts the cascade into limo. But I lose reverse at times. All for word gears at times. Sometimes I will only lose OD and all the rest are fine. It's hit or miss tbh.

So I found a used trans for $1000. Going to pick it up. Pull the TCM out of it. Stuff it in mine. If it works and solves the issue. Bonus. If not. Then still a win as I have the trans to stuff in.
A new TCM here in Canada from GM is $1400. My mechanics price is still $850. So I figure the $1000 trans is a bargain and I will have a trans and TCM.
 

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you MIGHT need to get the tcm reflashed, as it might be vin-locked
 

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Discussion Starter #13
you MIGHT need to get the tcm reflashed, as it might be vin-locked
So what's the diff if I just went and changed the entire trans vs the TCM ? Same trans. Same year. Are these TCM actually vin locked to the transmission they are currently in ? Never heard of anyone just swapping transmissions themselves having to get a reflash
 

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I believe the TCM is external to the transmission itself, so just swapping the transmission isn't a big deal,as you just keep using the same tcm. But likely GM wants the tcm flashed for the specific application (likely specific to engine & chassis).

But I don't know, for sure, if you do need to do it.
 

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Discussion Starter #15
I believe the TCM is external to the transmission itself, so just swapping the transmission isn't a big deal,as you just keep using the same tcm. But likely GM wants the tcm flashed for the specific application (likely specific to engine & chassis).

But I don't know, for sure, if you do need to do it.
I think you are correct on that. Just looked it up and seems like I'd need to get the TCM flashed again to my vin. Always something with GM lol.
Stock for stock though annoys me. My car I had to. It that was because it was aftermarket tune. But I could still put in any GM stock tuned one in amd it would work.

Wonder of the TCM reflash is only if it's aftermarket ?
 
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