I have a 1985 300D with a malfunctioning climate control system. The blower will only come on before engine start when the ignition key is in the on position. After engine start the blower will almost always shut off (while controls are still requesting the blower to run). If the blower continues to run after engine start the heater will blow warm air as long as the engine remains at low RPM. Once the engine RPM increases above 2000 RPM the heater will blow cold air. What is the problem?

Thanks

Pilot