What year is your W123? I have the Electrical Troubleshooting Manual for the 107 from 1983-89, which covers ACC generation III. Unfortunately most of the schematics for the ACC unit are block diagrams.

There is a separate manual that covers the system in detail. While this link connects you to a source for the R107 ACC, it may likely cover the same ground or you may be able to find a source for a specific year of the W123.

Mercedes Benz 107 Climate Control Heat AC Air Condition Manuals

Then there's the Encyclopedia Germanica Volume 107, here:

https://www.benzworld.org/threads/the-encyclopaedia-germanica-volume-107-aka-egv107.1450550/

Scroll down to the section on ACC. There are a variety of articles on problems and repairs. Yes, it's for the 107, but I suspect all of MB's ACC units are the same or similar enough.