Be aware that your monovalve will not work properly unless your auxiliary water pump is running. The monovalve requires a certain amount of water pressure to operate. The solenoid valve controls a small bleed that uses the water pressure to move the main valve, and if the aux. water pump is bad, the monovalve operation will be erratic, especially at lower engine rpms. I replaced the (good) diaghram in my monovalve twice before discovering this fact. Now I have replaced both the monovalve and auxiliary water pump with a vacuum controlled heater valve ($17.00 from NAPA, 1974 Ford) and a switchover valve ($38.00 from Performance Products, 1990 Mercedes 300D). This setup works at all engine speeds, cuts the heater water flow completely off at all speeds, and gives excellent modulation all through the temp wheel range. It also looks very much like the original setup.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C