Part 2
Control Components
Air Distribution Control
Two air distribution doors are used to direct air into the cabin. The doors are operated by stepper motors, which are controlled by the ATC (automatic temperature control) module using LIN (local interconnect network) bus messages.
When the A/C (air conditioning) system is in automatic mode, the ATC (automatic temperature control) module automatically controls air distribution into the cabin in line with its 'comfort' algorithm. Automatic control is overridden if any of the TSD air distribution soft buttons are selected. Air distribution in the cabin will remain as selected until the 'Auto' switch is pressed or a different manual selection is made.
Programmed Defrost
When the 'DEF' button is pressed, the ATC (automatic temperature control) module instigates the programmed defrost function. When selected, the ATC (automatic temperature control) module configures the system as follows:
- Automatic mode off
- Selected temperature unchanged
- Air inlet set to fresh air
- Air distribution set to windshield
- Blower speed set to level 6
- Windshield and rear screen heaters on.
The programmed defrost function can be cancelled by one of the following:
- Selecting any air distribution switch on the TSD
- Pressing the 'AUTO' switch below the TSD
- A second press of the 'DEF' button
- Switching the ignition OFF
NOTE:
The blower speed can be adjusted without terminating the programmed defrost function.
A/C Compressor Control
When A/C (air conditioning) is selected ('Econ' off) the ATC (automatic temperature control) module maintains the evaporator at an operating temperature that varies with the cabin cooling requirements. If the requirement for cooled air decreases, the ATC (automatic temperature control) module raises the evaporator operating temperature by reducing the flow of refrigerant provided by the A/C (air conditioning) compressor.
NOTE:
The ATC (automatic temperature control) module closely controls the rate of temperature increase to avoid introducing moisture into the cabin.
If the requirement for cooled air increases, the ATC (automatic temperature control) module lowers the evaporator operating temperature by increasing the flow of refrigerant provided by the A/C (air conditioning) compressor.
When economy mode is selected by pressing the 'Econ' TSD soft button, the compressor current signal supplied by the ATC (automatic temperature control) module holds the A/C (air conditioning) compressor solenoid valve in the minimum flow position, effectively switching off the A/C (air conditioning) function.
The ATC (automatic temperature control) module incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high pressure limit the compressor current signal is progressively reduced until the system pressure decreases. When the system pressure falls below the low pressure limit the compressor current signal is held at its lowest setting so that the A/C (air conditioning) compressor is maintained at its minimum stroke. This avoids depletion of the lubricant from the A/C (air conditioning) compressor.
A/C Compressor Torque
The ATC (automatic temperature control) module transmits refrigerant pressure and A/C (air conditioning) compressor current values to the ECM (engine control module) over the medium speed then high speed CAN (controller area network) bus, using the instrument cluster as a gateway. The ECM (engine control module) uses these values to calculate the torque being used to drive the A/C (air conditioning) compressor. The ECM (engine control module) compares the calculated value with its allowable value and if necessary forces the ATC (automatic temperature control) module to inhibit the A/C (air conditioning) compressor by transmitting the 'ACClutchInhibit' CAN (controller area network) message. This forces the
ATC (automatic temperature control) module to reduce the drive current to the A/C (air conditioning) compressor solenoid valve, which reduces refrigerant flow. This in turn reduces the torque required to drive the A/C (air conditioning) compressor.
By reducing the maximum A/C (air conditioning) compressor torque, the ECM (engine control module) is able to reduce the load on the engine when it needs to maintain vehicle performance or cooling system integrity.
Cooling Fan Control
The ATC (automatic temperature control) module determines the amount of condenser cooling required from the refrigerant pressure sensor, since there is a direct relationship between the temperature and pressure of the refrigerant. The cooling requirement is broadcast to the ECM (engine control module) on the medium speed CAN (controller area network) bus. The ECM (engine control module) then controls the temperature of the condenser using the cooling fan. For additional information, refer to Electronic Engine Controls
Windshield Heater
The ATC (automatic temperature control) module controls operation of the windshield heater using 2 relays in the power distribution box. When a request is made for windshield heater operation, the ATC (automatic temperature control) module broadcasts a message to the CJB (central junction box) on the medium speed CAN (controller area network) bus. On receipt of this message, the CJB (central junction box) energizes the relays by providing a ground path for both relay coils. This allows a battery feed to flow across the relays to power the left and right heater elements.
NOTE:
Windshield heater operation is only enabled when the engine is running.
There are 2 modes of windshield heater operation; manual and automatic.
Manual operation is activated by pressing the windshield heater control switch. When the switch is pressed, the ATC (automatic temperature control) module illuminates a LED (light emitting diode) in the switch and powers the heater elements for 6.5 minutes.
The switch LED (light emitting diode) remains illuminated until the windshield heater switch is pressed a second time (to switch the system off), the heating phase is completed, or the engine stops. If the engine is re-started within 30 seconds the windshield heater resumes the previous heating phase.
There are 2 variants of automatic operation; automatic operation at the start of a journey and automatic operation during a journey.
Automatic operation at the start of a journey is initiated if the ambient air temperature is below 5 °C (41 °F) at the start of a journey. In this instance, the ATC (automatic temperature control) module will automatically power the windshield heater elements and illuminate the switch LED (light emitting diode) for 6.5 minutes. The windshield heater can be switched off during this period by pressing the control switch or stopping the engine.
Automatic operation during a journey is initiated when low ambient air temperatures are experienced and the vehicle has been travelling for a set period of time above a threshold speed. In this instance, no feedback is given to the driver to inform him the windshield heater is operational (the switch LED (light emitting diode) is not illuminated) and the duration of operation is variable depending upon the ambient air temperature, vehicle speed and the amount of time the vehicle has been travelling.
Rear Window Heater
The ATC (automatic temperature control) module controls operation of the rear window heater using a relay in the auxiliary junction box. When a request is made for rear window heater operation, the ATC (automatic temperature control) module broadcasts a message to the auxiliary junction box on the medium speed CAN (controller area network) bus. On receipt of this message, the auxiliary junction box energizes the relay by providing a ground path for the relay coil. This allows a battery feed to flow across the relay to power the rear window heater element.
NOTE:
Rear window heater operation is only enabled when the engine is running.
There are 2 modes of rear window heater operation; manual and automatic.
Manual operation is activated by pressing the rear window heater control switch. When the switch is pressed, the ATC (automatic temperature control) module illuminates a LED (light emitting diode) in the switch and powers the heater element for 21 minutes.
The switch LED (light emitting diode) remains illuminated until the rear window heater switch is pressed a second time (to switch the system off), the heating phase is completed, or the engine stops. If the engine is re-started within 30 seconds the rear window heater resumes the previous heating phase.
There are 2 variants of automatic operation; automatic operation at the start of a journey and automatic operation during a journey.
Automatic operation at the start of a journey is initiated if the ambient air temperature is below 5 °C (41 °F) at the start of a journey. In this instance, the ATC (automatic temperature control) module will automatically power the rear window heater element and illuminate the switch LED (light emitting diode) for 21 minutes. The rear window heater can be switched off during this period by pressing the control switch or stopping the engine.
Automatic operation during a journey is initiated when low ambient air temperatures are experienced and the vehicle has been travelling for a set period of time above a threshold speed. In this instance, no feedback is given to the driver to inform him the rear window heater is operational (the switch LED (light emitting diode) is not illuminated) and the duration of operation is variable depending upon the ambient air temperature, vehicle speed and the amount of time the vehicle has been travelling.
Exterior Mirror Heaters
Exterior mirror heater operation is determined by ambient air temperature and windshield wiper status. When ambient air temperature reaches a pre-determined level, the ATC (automatic temperature control) module broadcasts an exterior mirror heating request to the door modules over the medium speed CAN (controller area network) bus. On receipt of this message, the door modules provide feed and ground connections to both exterior mirror heater elements.
NOTE:
Operation of the exterior mirror heaters is fully automatic and not controllable by the driver.
The amount of time the exterior mirror heaters are operational increases if the windshield wipers are switched on. This ensures the mirrors remain mist free in damp and wet conditions, where there is an increased risk of misting.
Seat Heaters
There are 4 seat heater settings available; Off, 1, 2 or 3, which can be set through the TSD. The heat setting is relayed to the vehicle occupants through a graduated display on the TSD.
Operation of the heated seats is controlled by the ATC (automatic temperature control) module. When the ATC (automatic temperature control) module receives a heating request from the TSD, it broadcasts a message to the CJB (central junction box) over the medium speed CAN (controller area network) bus. The CJB (central junction box) then provides a hardwired 12 V supply to the 3 heater elements contained within each seat. The 3 heater elements, 2 in the seat cushion and 1 in the seat squab, are wired in series.
The ATC (automatic temperature control) module monitors seat temperature using a temperature sensor located in each seat cushion. The CJB (central junction box) provides the temperature sensors with a 5 V supply. The level of the returned voltage back to the CJB (central junction box) is proportional to the seat temperature. The value of the return signal is broadcast to the ATC (automatic temperature control) module over the medium speed CAN (controller area network) bus which it then converts into a temperature value to allow it to control seat temperature to the required level.
The ATC (automatic temperature control) module will suspend or disable operation of the seat heaters if any of the following occur:
- Battery voltage exceeds 16.5 ± 0.3 V for more than 5 seconds. Seat heating is re-enabled when battery voltage decreases to 16.2 ± 0.3 V
- If a short or open circuit is detected
- If the seat heat temperature rises significantly above the target temperature setting.
The graduated display on the TSD remains illuminated until the seat heaters are turned off or the engine stops. If the engine is restarted within 30 seconds the seat heater resumes the previous heating level.
Steering Wheel Heater
The steering wheel heater has 1 heat setting and can be turned on and off by pressing the soft button located on the seat heater TSD screen. The on/off status of the steering wheel heater is relayed to the vehicle occupants through the TSD. When the ignition is switched off, the heater will reset to off.
Power for the heater element is supplied by the CJB (central junction box) on receipt of a request from the ATC (automatic temperature control) module over the medium speed CAN (controller area network) bus. Temperature control for the heater element is provided by the steering wheel heater control module which receives a temperature feedback signal from a NTC (negative temperature coefficient) thermistor located within the steering wheel.