This may sound obvious to the point of offending you but...I had a mercedes mechanic do some work to my 88 560 sel including the mono valve a few months ago. I had no heat and with the help of this forum purchased the valve. The mechanic was good enough to install for me. Upon return of the vehicle I had nothing but heat - could not regulate at all. I inspected the mono valve install and found they had not hooked up the electrical connection - all was well after I completed the "job". Could yours be related to a similar problem? My .02.....

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Woody