During my digging into my own W126 transmission problem (no reverse gear engagement - since solved via B3 reverse clutch repair), I Googled the keywords 722.3, Mercedes, transmission, and repair, and found the following .pdf file:

http://w126.pp.ru/akp722.pdf

The web site it is on is some form of Russian W126 site (with cyrillic text), but the document itself is in English. It covers the transmissions used in 1995 and 1996 models, but several models of these years use 722.3xx transmissions, and so the info on diagnosis is reasonably valid for W126 models. The document covers 722.3 - 733.5 transmissions. I wouldn't want to use these for actual repair procedures, unless you had exactly the same transmission, but it certainly can help to understand the possible causes of most symptoms.

I have no idea if the web site that this is from had permission to post this document - it just came up via Google.

Bob Schleicher
1991 420SEL - 171,000 miles.