Fred,Now for the "rest of the story"...
With the harness disconnected (i.e., 0 VDC) at the monovalve, the cabin floods with heat. In checking the output voltage from the automatic temp control module, it is a constant 12VDC, regardless of the setting at the temperature "wheel". As noted by John350, a constant 12VDC keeps the monovalve closed.
Except for the constant 12VDC output to the monovalve, all other aspects of the ATC module appear to work as designed (AC is modulated properly, diverter doors' operation is correct, etc.).
This, to me, suggests that the fix is repair/replacement of the ATC module. If that is a valid conclusion, 'would welcome experience of the owner community, if any, with ATC disassembly, visual inspection of electronics, etc. before ordering a replacement part. The OEM part appears to be available at mboemparts.com,but at a cost of about $600. There are many sources for rebuilt ATCs starting at around $150 but absent favorable experience in the owner community with rebuilts, I will seek the lowest cost of an OEM replacement.
1990-1991 Mercedes-Benz Dash Control Unit 126-830-09-85 | MB OEM Parts1990-1991 Mercedes-Benz part # 126-830-09-85 - Dash Control Unittinyurl.com
Fred (AKA pengineer)
Late last year I sold my 91 560sec. When I turned over the keys, manual, all maintenance dealer purchased parts receipts I also gave him a used ATC/PBU (push button unit) that I had repaired by flowing a deteriorated solder joint. The AC in that car was functioning normally. The car had absolutely zero problems but, being he was a young kid who flew in from Minneapolis I thought it would be a good gesture.
Instead of letting some parts changer replace the PBU just repair yourself. I feel you are confident and capable to carry out this repair. Engineers rock!!
P. S. I could text him and see if he will send that repaired unit to you.
i know he has it and it’s repaired by me with care.
Dave. (AKA) Gogi