The Monovalve is not part of a 1980 or earlier cabin heating system. On those 123's & 126's the 'Dreaded Servo' takes care of this function when it is working.

At any rate the monovalve operates in an on-off cycle related to the heat required and regulates the hot water flow to the heater coil in the cabin. It is not a variable opening valve, just has two positions.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150