When placed in automatic mode, the electronic control unit uses the last operator desired cabin setting. For this example 24°C [or 75.2°F]. The electronic control unit checks the temperature of the outside air - called the ‘ambient air’ temperature. This is registered by the ambient air temperature sensor at 28°C [or 82.4°F].
The car is parked in bright sunlight so the radiation from the sun has heated the cabin air temperature to 45°C [or 113°F]. The sun load sensor registers that the vehicle is in bright sunlight.
When the engine has started and the climate control system is functioning, the electronic control unit switches on the air-conditioning compressor and increases the speed of the interior ventilation fan.
Air is drawn into the fan either from outside the vehicle, or from within the cabin. The air then flows through the air-conditioning evaporator core where heat is removed.
The cold air-conditioned air is directed by flaps and ducts throughout the cabin of the vehicle, absorbing heat from the vehicle cabin and occupants, and thus lowering the temperature.
The cabin air temperature is continually measured by the cabin air temperature sensor. When the cabin air temperature has been lowered to within approximately 2°C [or 35.6°F] of the desired selection, the fan speed is gradually reduced.
To achieve final control the heater circuit may be opened and a blend door may be moved to direct some of the cold air-conditioned air through the heater core to provide fine temperature control. Both the fan speed and blend door openings are variable.