My 1983 300TD passenger compartment heating system leaves something to be desired now that cool weather has arrived. I get warm air in the "EC" mode only for a short time, at which time the unit starts blowing cool air. If I turn the blower to the "low" setting, I get warm air again but only for a few minutes before it reverts to cool air. If I turn the temperature wheel to the highest setting (past the "click") I get continuous warm (not hot) air but only with the fan set at "low". This effect is particularly noticeable at highway speeds.

My ACC pushbutton assembly is relatively new (rebuilt) and I replaced the thermostat so the engine runs at a normal operating temperature (80+). What steps should I take to trouble shoot this problem before it gets REALLY cold outside? I am not the only one with this problem and we are both currently STUMPED!

Andrew
1983 300TD
252K miles
Andrew