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Allison OEM Discuss repairing & maintaining an OEM Allison automatic transmission.

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Old 04-09-2017, 10:46 PM   #1 (permalink)
Crimson72
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2002 Allison issues

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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Old 04-10-2017, 08:12 AM   #2 (permalink)
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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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Old 04-10-2017, 10:42 AM   #3 (permalink)
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Old 04-10-2017, 01:48 PM   #4 (permalink)
Mike L.
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Possible transfer case problem
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Old 04-10-2017, 02:28 PM   #5 (permalink)
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Pulled the plug on TCase. Basically dry. 2 little ounces came out.
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Old 04-10-2017, 07:42 PM   #6 (permalink)
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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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Old 04-10-2017, 10:44 PM   #7 (permalink)
Mike L.
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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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Old 04-10-2017, 11:06 PM   #8 (permalink)
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Filled it up. So far so good.
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Old 04-13-2017, 12:14 AM   #9 (permalink)
Crimson72
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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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Old 04-13-2017, 12:23 AM   #10 (permalink)
Crimson72
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I also don't have TOW/ HAUL mode anymore.

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