My '88 190E 2.6 has a vacuum operated heater coolant flow control valve that allows or blocks coolant flow to the heater core, but I would expect there is an electric solenoid buried somewhere in the HVAC system that allows/disallows vacuum to the valve. I've always thought that other Mercs of similar vintage have the same vacuum operated valve.

I know I've heard the term "monovalve" to describe what I've always thought of as a heater core flow control valve, but I didn't know that some models are electric. Is the S-class heater control valve electric?

Maybe the 201/124 is different because they were designed a few years later.

Does anyone with a 201 or 124 have an answer to my original question.

Duke