I hate to disagree, but there is another symptom of a malfunctioning monovalve. I too was unable to shut off the heat on my '84 300SD. I replaced the pushbutton unit but still had heat. I then decided to take the monovalve apart and found out that it had become stuck in the full open position. after significant banging I was able to free the piston and now the ACC works great. I understand from others that this is a problem on the older Benzes with the monovalve, and is an easy fix. If only I could say the same about the servo unit.......