General

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:
^ 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.
^ 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.
^ 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.

In the defrost mode, the inlet air source is set to fresh air except at low ambient air and coolant temperatures. If, within 5 minutes of the ignition being switched on, the vehicle speed is less than 5 mph (8 km/h) while the external air temperature is - 16 °C (3 °F) or less and the heater coolant temperature is - 10 °C (14 °F) or less, then the inlet air source is automatically set to the timed recirculated air mode. The timed recirculated air mode is cancelled immediately the vehicle speed reaches 8 km/h or more.

Timed recirculated air
The timed recirculated air mode sets the inlet air source to recirculated air for 5±1 minutes, after which it automatically reverts to fresh air. Timed recirculated air can be manually selected:
^ In the automatic mode, by pressing the fresh/recirculated air switch for 1.5 seconds or more: the audible warning sounds twice.
^ In the economy or defrost modes, by pressing the fresh/recirculated air switch for less than 1.5 seconds; the audible warning sounds once.

Latched recirculated air
The inlet air source can be latched to recirculated air:
^ In the automatic mode, by pressing the fresh/recirculated air switch for less than 1.5 seconds; the audible warning sounds once.
^ In the economy or defrost modes, by pressing the fresh/recirculated air switch for 1.5 seconds or more; the audible warning sounds twice.

Blower control
The ATC ECU operates a blower relay, power transistor and power relay to run the blower at one of 31 stepped speeds. All speed steps are available in the automatic modes of blower control. In the manual mode, speed steps 3, 10, 16, 22 and 31 are used to provide slow, three intermediate and fast blower speeds. The ATC ECU energises the blower relay and modulates the power transistor to operate the blower for speed steps 1 to 30. For speed step 31, the ATC ECU energises the power relay, which switches the earth side of the blower motor direct to earth, bypassing the power transistor.

In the automatic, economy and defrost modes, blower speed is corrected for vehicle speed to compensate for the increase in ram effect on the inlet air as the vehicle speed increases. Correction begins at approximately 50 km/h, when blower speed is progressively decreased as vehicle speed increases, until a maximum decrease of 13 steps occurs at 123 km/h. Similarly, blower speed increases as vehicle speed decreases down to approximately 50 km/h.

In the automatic and economy modes, if the LH or RH temperature is set to LO or HI, the blower runs at maximum speed with correction only for vehicle speed. If both the LH and RH outlet air temperatures are set to a specific temperature, blower speed corrections are added to compensate for the heater coolant temperature, external air temperature, and the solar load acting on the vehicle:
^ During warm-up, the blower speed is set to 3 while the heater coolant temperature is below approximately 20 °C (68 °F). From approximately 20 °C (68 °F), the blower speed is progressively increased as the coolant temperature increases, until maximum speed is set at approximately 50 °C (122 °F).
^ During cool down, blower speed is set to 3, for 5 seconds after the system is switched on. Over the following 6 seconds, the blower speed is progressively increased up to maximum speed.
^ As the temperature in the cabin approaches the selected temperatures, blower speed is progressively reduced until, once the selected temperatures have been established, blower speed stabilises at approximately 6.
^ Solar heating correction is employed when air distribution is set to face level or to face and footwells. The correction progressively increases the blower speed, up to a maximum of 9 steps, with increasing values of solar heating.

Air distribution control
To control the air distribution within the cabin the ATC ECU signals the servo motor controlling the distribution flaps in the heater assembly to move to the flaps to the appropriate position.

In the automatic and economy modes, if the LH or RH temperature selections are set to LO or HI, air distribution is fixed as follows:
^ If one is set to LO and one is set to a specific temperature, to face level only.
^ If one is set to HI and one is set to a specific temperature, to footwells only.
^ If one is set to LO and one is set to HI, to face level and footwells.

When specific LH and RH temperature selections are set, air distribution is determined from the target air outlet temperatures. For higher target air outlet temperatures, air distribution is set to footwells only. For lower target air outlet temperatures, air distribution is set to face level only. For intermediate target air outlet temperatures, air distribution is set to face level and footwells. When the air distribution is set to face level and footwells, the ATC ECU varies the bias between the footwells and the face level outlets, in three stages, to provide a gradual transition of air distribution from footwells only to face level only. The three stages of bias are also employed when the air distribution is manually selected to face level and footwells.

During warm-up, the air distribution changes to face level and footwells for a period, then reverts to footwells only. The period of air distribution at face level and footwells can be cancelled by pressing and holding the on/off and defrost mode switches, then turning the ignition switch from off to on. Pressing and holding the AUTO and defrost switches, then turning the ignition switch from off to on, restores the period of air distribution at face level and footwells.

Compressor control
To engage the compressor clutch, the ATC ECU outputs a compressor clutch request to the ECM, which then energises the A/C compressor clutch relay. Compressor operation is governed by the evaporator outlet air temperature, at one of two settings, dependent on the amount of cooling required. When more cooling is required, the compressor clutch request is output if evaporator outlet air temperature increases to 4 °C (39 °F) and cancelled when it decreases to 3 °C (37 °F). When less cooling is required, the compressor clutch request is output if evaporator outlet air temperature increases to 11 °C (52 °F) and cancelled when it decreases to 10 °C (50 °F).

Engine cooling fan control
While the A/C system is on, operation of the electric engine cooling fan, to assist refrigerant condenser operation, is determined by a combination of vehicle speed and external air temperature. When cooling fan operation is required, the ATC ECU outputs a cooling fan request to the ECM, which then energises the cooling fan relay. The cooling fan request is output if vehicle speed is 80 km/h or less while the external air temperature is 28 °C (82 °F) or more. The request is cancelled, and the cooling fan switched off, if either the vehicle speed increases to 100 km/h, or the external air temperature decreases to 25 °C (77 °F).

Default settings
If the battery power supply to the ATC ECU is disrupted for any reason, e.g. battery disconnected, the system reverts to default settings when the battery power supply is restored. Default settings are:
^ Temperature indications in °C (in some markets a conversion connector is fitted to the ATC ECU to change the default temperature scale to °F).
^ LH and RH outlet temperatures of 22 °C (72 °F).
^ audible warning switched on.
^ Warm-up air distribution (to face level and footwells) function switched on.
^ If the system is first switched on using the A/C on/off switch, the automatic mode is engaged, regardless of the settings in use when the battery was disconnected.

Diagnostics
The ATC ECU performs a diagnostic check each time the ignition is switched on. To avoid nuisance fault indications at low light levels, the sunlight sensor is omitted from the diagnostic check. If a fault is detected, the audible warning sounds three times and the AUTO window on the control panel display flashes for 20 seconds. The ATC ECU then reverts to normal control but uses a default value or strategy for the detected fault. Faults are identified by performing a manual diagnostic check of the system.

A manual diagnostic check includes a check of the sunlight sensor, and is initiated by pressing and holding the AUTO switch and the air distribution switch, then turning the ignition switch from off to on. The audible warning sounds once and the indications on the control panel display illuminate. FC is shown in the LH temperature window and the results of the check are shown as a two digit fault code in the RH temperature window. If a fault is detected, the audible warning sounds three times and the AUTO window on the display flashes on and off for 20 seconds. If more than one fault is detected, the fault codes cycle in numerical order, at 1 Hz. The audible warning sounds as each fault code is shown. In low light conditions, to avoid false sunlight sensor fault indications, the sunlight sensor should be illuminated with a strong light source.

Diagnostic Fault Codes And Fault Descriptions: