Going from somewhat cool to hi heat could be a torn diaphram in the MONO valve. Or no voltage at the MONO valve to activate it. This valve opens and shuts off coolant to the heater core depending on the temp you've set on the dash temp wheel.

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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.