Update - The tranny is completely back together and on the car. The entire project was a success. Here is the story.
I had the transmission on my 91 420 fail suddenly. I took it to a reputable shop to have it rebuilt. He quoted $3000 to put in a factory rebuild and $2000 to rebuild the one I had. I didnt have much choice since my primary vehicle was out of commission. I let him rebuild the thing and it ended up costing $2600 by the time it was over. So I said to myself "this is nuts". My price on a factory unit was $1800 so I didnt understand why it took another $1200 to put it in. He would not explain it either. So after rebuilding my own, I understand why he wanted to rebuild the one that was in the car. Its because it is easy and very profitable. Assuming that no hard parts are needed and B2 is reusable, the parts cost about $200 - $300 for the complete unit.
So I decided that I was going to learn how to do it myself. I located a junker tranny for $200. I figured the most I could do was f__k it up. I accumulated as much information as I possibly could. Then I proceeded to open the thing up. It turns out that this tranny had water in it so it was a complete rebuild. The seal kit was $100 and the friction parts added up to $250 because the B1 & B2 bands had to be replaced. Normally B2 never needs to be replaced. I proceeded to follow the ATSG manual for the teardown proceedure. I found it to be complete and accurate. However it did not detail how to replace the hidden O rings as is done at
www.transonline.com. And the O rings were not in the kit either. I had located a rebuilder near me
www.jie.com and he had all the parts to do the hidden O rings. Screws, taps - everything. I did have unused parts in the gasket kit. So after I finished replacing parts, I took it to him for a dyno test. The dyno test cost $80 and it proved that a novice can rebuild a MB tranny right the first time. He checked and set the pressures and I was on my way. I have been driving the car and it shifts like it should. The best part is that it dosent leak any fluids now. I replaced the rear main engine seal while I was in there. I have the tranny that was in the car rebuilt as well. It was much easier and less mysterious the second time around.
The moral of the story is; If you have the time and inclination to atempt such a task, I say - go for it. The information at transonline is not complete and does not detail the tear down & assembly proceedures. But the ATSG manual does. So both sets of information are needed. Also the ATSG manual details the valve body which is necessary if you should happen to misplace some of the check valve balls or other small parts. But the hidden O ring information is not included in the ATSG manual SO there you have it. You need both. And it helps to have an expert standing by.
Overall it was a worthwile investment for me. I am well equiped with tools and I have doing my own auto work since I was 16. I always wanted to understand how automatic transmissions work. I certainly have achieved that goal and I have a spare tranny that can be used in either of my 420's at a minimal investment. I have found that everything associated with MB cars are designed to be serviced. Even the electronics is very repairable. They are great cars.
Good luck, let me know if I can be of assistance.