Air conditioning (Operation): Description and operation

Operation

General

While the system is on, the ATC ECU operates the refrigerant system and the inlet air, blower speed, air temperature and air distribution functions to produce the conditions requested on the control panel. When the system is first switched on, the ATC ECU resumes the control outputs in use when the system was last switched off. If conditions have changed, or a different mode is selected to switch the system on, the control outputs are then changed to produce the required new settings.

The system operates in automatic, economy and defrost modes, with manual overrides of the inlet air source, blower speed and air distribution. The air temperature is automatically controlled in all operating modes.

In the automatic mode, the ATC ECU operates the system to warm-up or cool down the cabin to establish and maintain the temperature selections on the control panel, while directing the air to those outlets most comfortable for the occupant(s). If a difference between the LH and RH temperature selections causes a conflict of the required inlet air source, blower speed or air distribution settings, priority is given to achieving the temperature requested on the driver's side of the control panel.

The ATC ECU enters the economy mode when the refrigerant compressor is selected off while the system is in the automatic mode, which reduces the load on the engine. Economy mode operation is similar to the automatic mode, but without the ability to cool the cabin if the ambient temperature is higher than the temperature selections made on the control panel, or to dehumidify the air in the cabin.

In the defrost mode, the ATC ECU sets the inlet air source to fresh air, the blower to maximum speed, the air distribution to windscreen and side windows, and outputs signals to the BCU to operate the rear window heater and (where fitted) the windscreen heater. The BCU starts or, if the heaters are already on, resets the heater timers and energises the rear window and windscreen heaters for a complete on cycle.

Air temperature control

To determine the amount of heat or cooling required by the cabin, the ATC ECU uses the sensor inputs and the temperatures selected on the control panel to calculate target air outlet temperatures for the driver's and the front passenger's side of the heater assembly. The ATC ECU then signals the servo motors controlling the respective blend flaps in the heater assembly to move to the flaps to the appropriate position. The target temperatures are constantly updated and, in the automatic mode, also used in further calculations to determine the inlet air source, the blower speed and the air distribution.

Inlet air control

The inlet air source is automatically controlled while the system is off or on. While the system is on, the inlet air source can also be manually controlled to give timed recirculated air or latched recirculated air.

While the system is off, the ATC ECU uses vehicle speed to determine the inlet air source. With the vehicle at rest, the inlet air source is set to recirculated air. When vehicle speed reaches 17.5 mph (28 km/h), the inlet air source changes to fresh air. The inlet air source then remains at fresh air until the vehicle speed decreases to 5 mph (8 km/ h), when it returns to recirculated air.

While the system is on, the ATC ECU uses the LH and RH temperature selections, vehicle speed, ambient air temperature and coolant temperature to determine the inlet air source. In the automatic mode: l If one temperature selection is set to LO and one is set to a specific temperature or HI, the inlet air is set to recirculated air. l If one temperature selection is set to HI and one is set to a specific temperature or HI, the inlet air is set to fresh air. l When specific LH and RH temperature selections are set, the inlet air source remains at fresh air except when the air distribution function is set to face level only or face level and footwell outlets. If the air distribution function is set to face level only or face level and footwell outlets, at 56 mph (90 km/h) the inlet air source changes to recirculated air (to exclude ram effect, which becomes excessive at speed). When the vehicle speed decreases to 37.5 mph (60 km/h), the inlet air source returns to fresh air.