I'm all for repairing the monovalve, but it might be worthwhile to test it.

The valve works by closing to shut off the flow of hot water to the heater. The higher the temp wheel is set, the less frequently it will close. There are two wires attached to the valve. One is red/brown and is hot when the ignition is on. The other is brown/black and is a ground. The ground is switched by the control unit. When the valve gets ground it closes and shuts off the hot water.

So the test is to remove the connector from the top of the valve, and ground one pin. Then (carefully) touch 12V to the other pin. The valve should click as it closes. If it doesn't, replace it.

It is very possible that the valve is fine, but is getting a constant ground signal from the control unit. This can be caused by a bad control unit, or a bad sensor which makes the control unit think that the cabin has reached the desired temperature and thus tells the valve to stay closed (by sending ground signal).

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe