Compressor stays engaged in all positions except econ. or off. The system mixes cold and warm air to stabilize in car temp to what you dialed up on the wheel. When the diaphram tears in the MONO valve, the heater might produce warm air at idle, but will go colder as driven. Verify that the diaphram is intact within the valve. If not, replace it. Then report back. If intact, you might have air in the cooling system or the aux. water pump might be inoperative which would keep the heater from getting very hot.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.