I may stand corrected, but look around the monovalve for an elect. wire connection. If it's removed, the system defaults to "constant heat". See if there's an elect. conn. in this area that's been disconnected. If so, reconnect and see if this cools things off.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"