Dang it man that is not good news. From what I have read these are some of the most complex transmissions ever built for a passenger vehicle and only around 30,000 were ever manufactured.
I saw one post where a fella had spent $5,000.00 for a replacement from a dealer. I just did a partial fluid change on mine today. It has a drain plug in the trans bottom cover and I pulled it and it drained out 8.31 quarts of Dexron VI. I refilled with fresh fluid the same amount. I used an electronic scale to weigh it. The fluid looks pretty good, it still is red with a slight brown color. I was afraid to do a flush since I do not know when or how many times it has been changed over its 200,000 mile life. If you find a good source that can rebuild these for a reasonable fee please post the information. As these vehicles age there will be many more people needing that service....

 

I pulled the pan on mine yesterday, fluid looked great, normal amount of fuzz on the magnet, no large chunks. Pulled TECMS (the solenoid/electrical half of the valve body with the TCM), cleaned and inspected, looked fine.

Looking at the parts breakdown it doesn't look anymore complicated to rebuild than any other 4spd transmission. The 4th gear is bolted onto the back of the trans and looks like it could be removed with the trans still in the vehicle.

Things holding me back:
Parts supply- where do I get parts, I think I've found the ATRA/Transtar parts rebuild kit number which means any trans rebuild supply shop should be able to order a rebuild kit. Will have to make some calls on Monday and see if its available and price.
Pulling the whole trans myself isn't really feasible as its apparently a very heavy unit and I don't have a trans jack.
Not 100% sure the problem is the 4th gear clutch, could be the control circuit not applying the correct apply pressure. If it is really an electrical problem then I could truly be screwed as a replacement would need to be programed. I can assign VIN numbers to new modules but thats it.

At this point I'm thinking my best bet is just to drop the t-case and remove the rear 4th gear housing and physically see if the clutch is toast. If it is, great! Replace and go. If it looks ok then I'd be back to square 1.

Link to a trans rebuild catalogue that lists rebuild parts and pics:
https://www.transtar1.com/TranstarIn...ndex.html#p=36

A GM service repair book that I'm contemplating buying:
https://members.atra.com/TMG-2ML70

 

You seem to be a "Can Do" type person. Most people at this point would just throw up their hands. I had read somewhere that the 2ML70 is an "Allison design" based upon HD bus hybrid application which has been known to run stop and go 24 hour a day 7 days a week for 100s of thousands of miles trouble free. Please let the forum know what you end up doing on this, Maybe you will find that your issue is not even trans related and the code is leading you in the wrong direction? Either way I applaud your tenacity.

 

I'm a HD mechanic by trade so ripping stuff apart doesn't scare me and at this point and I don't have much to loose. I cann't in good conciseness sell it to anyone so the only option are to fix it, scrap it, or send it to auction. I've been curious about hybrids for awhile now so this is a good learning experience for me. Just frustrating trying to find info on them. Usually stuff I work of my own there are good online forums of info and self help tech, but these seem to be pretty rare and not many people wrenching on them themselves.

More technical reading:
http://www.atraonline.com/gears/2009...5/2009_5_4.pdf

Skip to pg 47:
https://www.slideshare.net/ThangTong...seminar-manual

 

 

No is does not, I wish it did. It does have the old style PRND21 selector. Next time I'm out with it I will put it into 2 and see what happens. Unfortunately the problem is with the 3/4 shift.