Did this system work good last week and now doesn't? Have you had this problem for a long time? Did you just do some other repair and now the climate control is acting up? I would check the mono valve for having power to it. No power supplied, I believe it remains open allowing coolant to flow into the dash. Report back

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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.