Now that I vaguely grasp how the AC system works, how does the temp get controlled in the cabin? I know that there is a temp sensor near the rear view mirror. When I dial 70F into the temp wheel, and the push button is on auto what happens?
Thanks,
Anziani

Temperature regulation in the cabin is accomplished primarily through control of the fan speed. Higher fan speeds deliver more cooling, lower fan speeds deliver less cooling. As the temperature of the cabin approaches the setpoint, the fan speed is reduced. As the fan speed is reduced, the TXV will reduce refrigerant flow through the evaporator. Eventually the evaporator temperature will approach the freezing point of water. The car will then begin cycling the compressor off&on to avoid freezing condensate on the evaporator. This control is performed by the CCU, using input from an evaporator temperature sensor in the HVAC case.

A secondary method of controlling cabin temperature is to start pulsing the monovalve to warm up the heater core. Generally this only happens at night, when there is no radiant heat from the sun and minimum fan speed still supplies too much cooling. In this situation the air is cooled - too much for comfort - by the evap, then warmed back up a little bit by the heater core. This sounds wasteful, but actually supplies the best comfort. Cooling the air as much as possible reduces the humidity to a comfortable level, then reheating prevents the cabin from becoming uncomfortably cold.

Hope this helps.

Thanks so much. Now I have a handle on it. This car has always throttled right down and worked perfectly. However, last weekend I sensed that it was not backing off and it was getting right cold in the cabin. Will have to pay more attention next time and do a little testing.
Anziani
'93 300CE 188K
'95 E420 118K