Operation

SYSTEM OPERATION

A/C Compressor Control

The variable displacement A/C compressor is permanently driven by the engine. The flow of refrigerant through the A/C compressor, and the resultant system pressure and evaporator operating temperature, is regulated by the refrigerant solenoid valve. Operation of the refrigerant solenoid valve is controlled by the ATCM using a 400 Hz Pulse Width Modulated (PWM) signal. The duty cycle of the PWM signal is calculated using the following parameters:
^ A/C compressor torque.
^ A/C compressor torque maximum.
^ A/C cooling status.
^ A/C demand.
^ A/C refrigerant pressure.
^ Ambient air temperature.
^ Blower speed.
^ Engine cranking status.
^ Evaporator temperature.
^ Transmission gear status.

When A/C is selected, the ATCM maintains the evaporator at an operating temperature that varies with the in-vehicle cooling requirement. The ATCM increases the evaporator operating temperature, by reducing the refrigerant flow, as the requirement for air cooling decreases, and vice versa. During an increase of evaporator operating temperature, to avoid compromising the dehumidification function, the ATCM controls the rate of temperature increase, which keeps the cabin humidity at a comfortable level.

When the economy mode is selected, the PWM signal holds the refrigerant solenoid valve in the minimum flow position, effectively switching off the A/C function.

The ATCM incorporates limits for the operating pressure of the refrigerant system. When the system approaches the high pressure limit, the duty cycle of the PWM signal is progressively reduced until the system pressure decreases. When the system pressure falls below the low pressure limit, the duty cycle of the PWM signal is held at its lowest setting, so that the A/C compressor is maintained at the minimum stroke, to avoid depletion of lubricant from the A/C compressor. The protection algorithm is calculated at a high rate, to enable early detection of the rapid pressure changes possible if a system fault develops.

A/C Compressor Torque
The ATCM uses refrigerant pressure, evaporator temperature and engine speed to calculate the torque being used to drive the A/C compressor. The calculated value is broadcast on the medium speed CAN bus for the Engine Control Module (ECM), which uses the calculated value for idle speed control and fueling control. The ATCM also compares the calculated value with a maximum A/C compressor torque value received from the ECM over the medium speed CAN bus. If the calculated value exceeds the maximum value, the ATCM signals the refrigerant solenoid valve to reduce the refrigerant flow, to reduce the torque being used to drive the A/C compressor. By reducing the maximum A/C compressor torque value, the ECM is able to reduce the load on the engine when it needs to maintain vehicle performance or cooling system integrity.

Idle Speed Control
In order to maintain A/C cooling performance, the ATCM requests an increase in engine idle speed if the evaporator temperature starts to rise while the refrigerant solenoid valve is already set to the maximum flow rate. The increase in engine idle speed is requested in three stages, using a medium speed CAN bus message to the Engine Control Module (ECM).

The need for a change in idle speed is determined as follows:
^ If the evaporator temperature increases by 3 degree C (5.4 degree F), or to 6 degree C (10.8 degree F) above the target operating temperature, over a 10 seconds period, the first stage of idle speed increase is requested.
^ When the first stage of idle speed increase is set, if the evaporator temperature increases by 3 degree C (5.4 degree F), or increases to 12 degree C (21.6 degree F) above the target operating temperature, over a 9 seconds period, the second stage of idle speed increase is requested.
^ When the second stage of idle speed increase is set, if the evaporator temperature increases by 3 degree C (5.4 degree F), or increases to 15 degree C (27 degree F) above the target operating temperature, over a 10 seconds period, the third stage of idle speed increase is requested.
^ When an idle speed increase is set, if the evaporator temperature decreases by 3 degree C (5.4 degree F) over a 10 seconds period, the next stage down of idle speed increase is requested.

Electrical Load Management

The ATCM manages the vehicle electrical loads to:
^ Maintain the vehicle battery in a healthy state of charge.
^ Ensure adequate power is available for defrost demisting during engine warm-up.
^ Ensure adequate power is available for A/C during extended periods with the engine at idle speed.
^ To maintain system voltage within acceptable limits.
^ To provide adequate power to meet customer expectations.

Electrical load management is achieved by increasing the engine idle speed and controlling the electrical load of systems that do not affect the driveability or safety of the vehicle.

During the engine warm-up period, the ATCM manages the electrical load to make sure that the battery voltage is maintained above a pre-determined level. The battery voltage level that is maintained and the duration of the start period varies with ambient air temperature and engine coolant temperature. After the engine warm-up period, the ATCM manages the electrical load to make sure that the requested electrical load does not exceed the generator output.

The duration of the engine warm-up period depends on the ambient air temperature and the engine coolant temperature when the ignition is switched on, as detailed in the following table:

Engine Warm-up Times





The ATCM calculates the electrical load from the battery voltage and generator output voltage, and compares the result against the maximum load available from the generator. The calculation is averaged across the first 20 seconds after the engine starts, and subsequently averaged every 60 seconds. When the ignition is turned off, the ATCM stores the status of the electrical load management for 20 seconds. If the engine is re-started within the 20 seconds, the ATCM resumes electrical load management using the stored status. If the engine is re-started after the 20 seconds, the timers are reset and the ATCM re-calculates the status.

If the electrical load is more than the maximum load available, the ATCM requests an increase of engine idle speed using the medium speed CAN bus message to the ECM. If an electrical load imbalance remains after an increase in engine idle speed, or if the electrical load is more than the capacity of the charging system, the ATCM reduces the electrical load by reducing the power of some vehicle systems or inhibiting their operation. The number of systems controlled depends on the electrical load reduction required. The systems controlled, and the order in which their power is reduced or they are inhibited, are contained in three priority tables. The table used depends on the ambient air temperature, battery temperature and engine coolant temperature:
^ The cold start table is used when the ambient air temperature is less than 5 degree C (41 degree F) and the engine coolant temperature is less than 30 degree C (86 degree F).
^ The hot start table is used when the ambient air temperature is 5 degree C (41 degree F) or more and the engine coolant temperature is less than 30 degree C (86 degree F).
^ The continuous table is used when battery temperature is more than 5 degree C (41 degree F) and the engine coolant temperature is more than 50 degree C (122 degree F).
^ If none of above conditions are met, the ATCM adopts the last used table.

Cold Start Electrical Load Management





Hot Start Electrical Load Management





Continuous Electrical Load Management





Engine idle speed changes, and electrical load changes of systems not under direct control of the ATCM (air suspension and entertainment), are initiated using the appropriate medium speed CAN bus message. When partial operation is requested:
^ The air suspension system still performs height changes but reduces air compressor operation by not replenishing the reservoir.
^ The entertainment system restricts the maximum volume level and reduces the output frequency bandwidth.

Cooling Fan Control
The ATCM determines the amount of condenser cooling required from the refrigerant pressure, since there is a direct relationship between the temperature and pressure of the refrigerant. The cooling requirement is transmitted to the ECM in a medium speed CAN bus message. The ECM controls the condenser cooling using the cooling fan.

Air Temperature Control
Air from the evaporator enters the heater assembly, where temperature blend doors direct a proportion of the air through the heater core to produce the required discharge air temperature. On the automatic control system two temperature blend doors operate independently to enable independent temperature selection for the left and right sides of the vehicle interior. The temperature blend doors are operated by a single stepper motor on manual systems and two stepper motors on automatic systems. The stepper motor(s) are controlled by the ATCM using LIN bus messages.

The ATCM calculates the stepper motor position required to achieve the selected temperature and compares it against the current position, which is stored in memory. If there is any difference, the ATCM signals the stepper motor to adopt the new position.

Air temperature is controlled automatically unless maximum heating or maximum cooling is selected. The required air temperature may be adjusted between 16 degree C (61 degree F) and 28 degree C (82 degree F) using the air temperature control switches. The control algorithms then attempt to maintain the desired set temperature.

Turning the temperature switches fully counterclockwise gives maximum available cooling. Turning the temperature switches fully clockwise gives maximum available heating. When maximum cooling or maximum heating is selected, the comfort algorithm adopts an appropriate strategy for the air distribution, blower speed, A/C and air source functions, except where a function is under manual control.

On the automatic system, the temperature control of one zone can be compromised by the other zone being set to maximum heating or maximum cooling. True maximum heating or maximum cooling can only be obtained with both controls set to the same maximum state.

When the economy mode is selected, the automatic temperature control function still operates, but with no cooling capability the minimum discharge temperature achievable will be ambient air temperature plus any heat pick up in the air intake path.

Air Distribution Control
When the A/C is in the automatic mode, the ATCM automatically controls air distribution according to a comfort strategy. Automatic control is overridden when one of the manual modes is selected. Air distribution remains manually controlled until the automatic mode is selected again. The distribution doors are operated by two stepper motors, which are controlled by the ATCM using LIN bus messages.

Blower Control
When A/C is selected or the blower speed is manually selected, the ATCM energizes the coil of the blower relay in the Battery Junction Box (BJB). The energized blower relay supplies battery power to the blower motor, which is grounded through the blower control module. The speed of the blower is controlled by a PWM signal from the ATCM to the blower control module. The blower control module regulates the blower motor voltage in relation to the PWM signal.

When the blower is in the automatic mode the ATCM determines the blower speed required from the comfort algorithms. When the blower is in the manual mode, the ATCM operates the blower at one of seven fixed speeds as selected on the control panel.

Programmed Defrost
The programmed defrost function automatically provides the maximum defrosting of the vehicle. When the programmed defrost function is selected, the ATCM configures the control system as follows:
^ Automatic mode off.
^ Air inlet to fresh air, manual control.
^ Selected temperature unchanged, automatic control.
^ Air distribution set to screen mode, manual control.
^ Blower speed set to speed 5, manual control.
^ Rear screen heater and windshield heater (if applicable) selected on.
^ A/C mode in automatic.

The programmed defrost function is cancelled by one of the following:
^ Selecting any distribution switch. The system response will be identical to the normal manual distribution control operation.
^ Selecting the automatic switch. This will restore the system to fully automatic operation.
^ Selecting the programmed defrost switch again. This returns the system to the state in use immediately before the programmed defrost function was first selected.
^ Turning the ignition off.

The blower speed can be adjusted manually without terminating the programmed defrost function.

Intake Air Control
The source of intake air is automatically controlled unless overridden by manual selection of recirculation. Under automatic control the ATCM determines the required position of the recirculation door from the comfort strategy and the input from the pollution sensor (if fitted). The recirculation door is operated by an electric motor, which is controlled by hardwired analog signals from the ATCM. A potentiometer in the motor supplies the ATCM with a position feedback signal for closed loop control.

Provided the intake air has not been manually selected to recirculation, the ATCM adjusts the recirculation door to reduce the ram effect produced by the forward motion of the vehicle.

When the ignition switch is turned off, the ATCM evaluates the ambient air temperature. If the ambient air temperature is less than a pre-determined value, the intake air source is set to recirculation, to prevent the ingress of damp air while the vehicle is parked.

When the vehicle is in the transportation mode, the ATCM sets the intake door to recirculation every time the ignition is turned off, regardless of the ambient air temperature.

Pollution Sensing
With a pollution sensor fitted to the vehicle, the ATCM controls the intake air source to reduce contamination of the intake air by external pollutants. This function is fully automatic, but can be overridden by manual selection of the intake air source.

Humidity Sensing
With a humidity sensor fitted, the ATCM controls the moisture content of the air in the vehicle. This is achieved by raising the evaporator temperature to increase the humidity of the air entering the vehicle, and reducing the evaporator temperature to reduce the humidity of the air entering the vehicle.

Front Seat Heaters
The front seat heaters are enabled when the ignition switch is position II, and operate at one of two temperature settings. With the first press of a front seat heater switch the ATCM adopts the higher temperature setting, supplies a power feed to the related front seat heater elements and illuminates two amber LED's in the switch. At the second press of the switch the ATCM adopts the lower temperature setting and extinguishes one of the LED's. At the third press of the switch the ATCM de-energizes the heater elements and extinguishes the second LED. The seat heaters remain on until selected off or the ignition is turned off.

The ATCM receives an input from a temperature sensor in each front seat, and regulates the power feed of the heater elements to control the seat temperature at the appropriate temperature setting between 35 and 45 degree C (95 and 113 degree F). The actual temperature settings vary with the type of seat covering, to allow for the different heat conduction properties of the different materials.

When the front seat heaters are activated at the higher temperature setting, the ATCM automatically resets them to the lower temperature after a time delay. The length of the time delay depends on the in-vehicle temperature.

Temperature Reset Time Delay





To protect the heater elements, the ATCM disables front seat heating if battery voltage exceeds 16.5 ± 0.3 volts for more than 5 seconds. Front seat heating is re-enabled when battery voltage decreases to 16.2 ± 0.3 volts.

The ATCM monitors the power feeds to the heater elements and disables the applicable front seat heating if a short or open circuit is detected. The ATCM also disables seat heating if the seat temperature rises significantly above the target temperature setting.

The plausibility of the temperature sensor inputs is also monitored by the ATCM. When seat heating is selected, if one of the temperature sensor inputs is within 5 degree C (9 degree F) below the target temperature, the ATCM monitors the sensor input for a temperature increase and checks that it is between the minimum and maximum working temperatures. If a temperature sensor input is at the high end of the working range, while the ambient air temperature and the engine temperature are within 10 degree C (18 degree F) of each other, the ATCM disables front seat heating until the input decreases below the target temperature setting. The ATCM interprets a temperature sensor input value of -45 degree C (-49 degree F) or below as an open circuit, and temperature sensor input value of 100 degree C (212 degree F) or more as a short circuit.

Rear Window Heater
The ATCM controls operation of the rear window heater using medium speed CAN messages to operate the rear window heater relay in the Central Junction Box (CJB). The control module in the CJB interprets the CAN messages and switches the ground connection of the relay coil to operate the rear window heater. While the rear window heater relay is energized, a battery power feed is connected to the rear window heater elements. Rear window heater operation is only enabled when the engine is running.

The ATCM operates the rear window heater in heating cycles of varying power and time. The heating cycle used depends on the ambient air temperature and whether it is the initial or subsequent operation during the current ignition cycle.

When the rear window heater switch is pressed, the ATCM illuminates an LED in the switch and initiates the appropriate heating cycle. The LED remains illuminated until the rear window heater is selected off, the heating cycle is completed or the engine stops. If the engine stalls or the ignition is turned off, rear window heating resumes if the engine is re-started within 20 seconds.

On the initial selection of rear window heating, the ATCM uses a short or long defrost phase at full power, followed by a low power phase. The defrost phase used depends on the ambient temperature. During the low power phase, the rear window heater relay is cycled off for 80 seconds and on for 40 seconds.

On subsequent operations, during the same ignition cycle, the ATCM operates the rear window heater at full power for a fixed time period.

Rear Window Heating Phases





Windshield Heater
The ATCM controls operation of the windshield heater using the windshield heater relay in the BJB. The ATCM switches the ground connection of the relay coil to operate the windshield heater. While the windshield heater relay is energized, a battery power feed is connected to each of the two windshield heater elements. Windshield heater operation is only enabled when the engine is running.

The ATCM operates the windshield heater in heating cycles of varying power and time. The heating cycle used depends on the ambient air temperature and whether it is the initial or subsequent operation during the current ignition cycle.

When the windshield heater switch is pressed, the ATCM illuminates an LED in the switch and initiates the appropriate heating cycle. The LED remains illuminated until the windshield heater is selected off, the heating cycle is completed or the engine stops. If the engine stalls or the ignition is turned off, windshield heating resumes if the engine is re-started within 20 seconds.

On the initial selection of the windshield heater, the ATCM uses a short or long defrost phase at full power, followed by a low power phase. The defrost phase used depends on the ambient temperature. During the low power phase, the windshield heater relay is cycled off for 80 seconds and on for 40 seconds.

On subsequent operations, during the same ignition cycle, the ATCM operates the windshield heater at full power for a fixed time period.

Windshield Heating Phases