Taken from TSD4200: Basic ACU operation for Series II models:
Within the limitations of the system, the air conditioning unit (ACU) will automatically maintain any pre-selected in-car temperature in both the upper and lower part of the car. The required temperatures for both the upper and lower part of the car are independently selected by two dials and the in-car temperatures are measured by sensors located in the upper and lower part of the car.
Sensors also measure the outside air temperature. The required temperature and actual temperature are compared by a servo module which signal actuators and fans to operate accordingly.
Air entering the ACU can either be fresh from outside the car or recirculated from inside the car dependent upon the position of two flaps which are controlled by a servo. All air, fresh or recirculated, is dried and cooled by blowing it through a refrigeraton evaporator. A proportion of the cooled air passes through a heater matrix and is subsequently mixed with the remaining cool air which has by-passed the heater matrix. The proportion is determined by two infinitely variable temperature flaps which are controlled by two servos, one for the upper system and one for the lower system air. The upper air passes to either the facia or the windscreen depending on the position of a mode flap. The lower air passes to front and rear footwells when the lower control flap is in open position. An interlock system prevents the ACU from operating until the car engine is running and an inhibit system prevents the fans operating under certain circumstances.