The monovalve controls the water flow through the heater core to regulate the heat.

If you leave it unplugged, you will have full heat all the time. In the coldest position, the temp mixing flap (if there is one on the W123) will be to full cold, and the AC will run, so the air will be colder.

If it doesn't work properly connected, check the valve itself -- the rubber bellows goes bad and leaks, and if its' really bad it will corrode the armature of the solenoid and it will intermittantly stick open or closed randomly. Replacment cures it -- if the armature is corroded, you need to replace the solenoid coil as well.

If the heat won't regulate properly, there are other things to check -- the foam hose to the interior temp sensor, one of the temp sensors bad, bad pushbutton control unit, etc. that will have to be chased down.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!