Foam tube was in bad shape. Replaced it, but as suspected, that didn’t fix the problem. Based on the climate control circuit diagram I thought it had to be either the pushbutton climate control unit (N22/2) or the Temperature Control Unit (N22/1 little box below the glove box). Had access to a good climate control unit. Installed the good unit. Problem persisted. Installed a rebuilt Temperature Control Unit. Problem fixed.

ps: Monovalve was ruled out because of the following diagnostic test suggested by Programa (the rebuilder of the climate control electronic units):
Referencing Left (L) and Right (R) terminals connecting to the pushbutton control unit (N22/2). Resistance between (L) pin 12 and (R) pin 4 should be 11 to 19 ohms. If not, replace Monovalve. When I performed the test, resistance was within range.

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Riadh
'89 560SL Rauchsilber
And what is good, Phaedrus, and what is not good -- Need we ask anyone to tell us these things?