Operation - 1
Operation
General
Both the low and high line systems operate on the reheat principle. The air entering the heater assembly is cooled to a constant value by the evaporator and then reheated as necessary by the heater matrix to produce the temperature(s) selected on the control panel.
To determine the various system settings the ATC ECU derives a reference value (called the Y factor) from:
- The temperature setting on the control panel
- The ambient temperature
- The in-car temperature
The reference value is measured in % where -27.5% means maximum cooling is required and 100% means maximum heating is required. On the high line system, separate reference values are produced for the LH and RH sides of the heater assembly.
On both the low and high line systems the reference value is used for temperature control. On the high line system the driver's side reference value is also used for flap positioning and blower speed calculations.
When the ignition is turned oft the ATC ECU memorizes the system settings and resumes the same settings the next time the ignition is switched on.
Compressor Control
The compressor is engaged by pressing either the automatic mode switch, defrost switch, A/C switch or maximum A/C switch. To prevent a dip in engine speed when the engine is at idle a time delay of approximately 0.5 second is built into the compressor engagement process. The time delay allows the ECM to increase throttle angle and fuelling in anticipation of the additional load on the engine when the compressor engages. When it receives an input to engage the compressor, the ATC ECU sends a message to the ECM, via the K bus, instrument pack and CAN bus, to advise that it wants to engage the compressor. Provided there are no engine management problems the ECM responds by increasing throttle angle and fuelling and sending a message granting the request to the ATC ECU over the CAN bus, instrument pack and K bus. When it receives the grant message, the ATC ECU energises the compressor clutch provided the following conditions exist:
- Engine speed is more than 400 rev/mm
- Evaporator temperature is more than 3 °C (37 °F)
- The refrigerant pressure is within limits
- Battery voltage is less than 16 V
- The blower is running
- There are no faults detected by the ATC ECU
The compressor remains engaged until selected off or the required conditions no longer exist. If the evaporator temperature decreases to approximately 2 °C (36 °F) the compressor is disengaged, then re-engaged when the evaporator temperature increases to more than 3 °(37 °F) again. If battery voltage exceeds 16 V for more than 5 seconds the compressor is disengaged, then re-engaged when voltage decreases to less than 15.8 V.
Engine Idle Speed Control
The ATC ECU signals the ECM to increase engine idle speed settings if a high current application is switched on or if low battery voltage is detected. The signal consists of a message transmitted via the K bus, instrument pack and CAN bus, and increases idle speed between 100 and 200 rev/mm. The ATC ECU requests the engine idle speed increase when the following high current applications are switched on:
- Rear window heater
- Blower at speed 6 or 7
- Headlamp man beam
The engine idle speed increase is also requested if battery voltage decreases to less than 11.4 V. The speed increase request is discontinued when battery voltage increases to more than 12.2 V again.
Engine Cooling Fan Control
The engine's electric cooling fan is controlled by the ECM. When the compressor is engaged the ATC ECU sends a fan speed message between 0 (off) and 15 (maximum speed) to the ECM, via the K bus, instrument pack and CAN bus. The ATC ECU requests the cooling fan on when:
- The vehicle speed is less than 44 mph (70 km/h)
- There is no fault with the compressor clutch circuit
- Battery voltage 5 less than 16 V
The fan speed requested depends on the pressure in the refrigerant system. The higher the pressure the faster the fan speed that is requested. When vehicle speed increases to more than 50 mph (80 km/h) the ATC ECU changes the fan speed request to 0 until vehicle speed decreases to less than 44 mph (70 km/h) again.
If battery voltage increases to more than 16 V for 5 seconds the ATC ECU changes the fan speed to 0 until the voltage decreases to less than 15.8 V.
Temperature Control
To determine the amount of heat required in the form of coolant flow through the heater, the ATC ECU compares the reference value with the heater matrix temperature and incorporates a correction factor to compensate for changes of coolant flow rate with engine speed. From the resultant value the ATC ECU determines the open duration of the coolant valve and outputs the appropriate control signal. On the high line system, separate values and output signals are generated for each coolant valve to produce the individual temperature control for the LH and RH sides of the passenger compartment.
When the temperature setting on the control panel is set to maximum, the ATC ECU signals the coolant valve to remain open. When the temperature setting on the control panel is set to minimum, the ATC ECU signals the coolant valve to remain closed.
On the high line system, the minimum setting is over-ridden when the defrost programme maximum A/C, rest heating or FBH functions are selected.
Heater Coolant Circuit Without FBH
Flap Control - Low Line System
While the ignition is on, the position of the recirculation flaps is manually controlled by selection of the recirculation switch on the control panel and the steering wheel switch.
The positions of the distribution flaps are manually controlled by selection of the distribution rotary switch on the control panel. The flaps are also automatically positioned by the ATC ECU to open the:
- Face and footwell outlets, when parked ventilation is initiated
- Windscreen outlets when the ignition is switched off and the ambient temperature is less than 0 ° or windscreen and footwell outlets when the ignition is switched off and the ambient temperature is 0 ° and above
Flap Control - High Line System
The position of the ram air flaps is automatically controlled by the ATC ECU. The positions of the recirculation flaps and the distribution flaps are either automatically controlled by the ATC ECU or manually controlled by the related switches.
Ram Air Flaps
The ATC ECU progressively closes the ram air flaps in four steps as vehicle speed increases and opens them again as vehicle speed decreases. At the maximum closed position, 90% of the fresh air inlet area is closed off. A hysteresis of 9 mph (15 km/h) prevents the ram air flaps from hunting at a constant vehicle speed.
Ram Air Flap Positions
Recirculation Flaps
Unless the recirculation flaps are manually selected closed, using the recirculation switch on the control panel or the steering wheel switch, they are normally open but automatically closed by the ATC ECU under certain conditions.
Rapid Cooling:
The ATC ECU opens the recirculation flaps after 12 minutes (temperate climates) or 20 minutes (hot climates),if the driver's side reference value increases to more than -5% or if the ambient temperature decreases to 6 °C(43°F).
Pollution: When the ATC ECU detects a high level of pollutants, it sets the recirculation flaps to the recirculation position for
- 3 minutes if the NO system is in the heating mode
- 10 minutes if the A/C system is in the cooling mode
After the recirculation time period, the ATC ECU returns the recirculation flaps to the fresh air position. After 1 minute, if the pollutants are still present, the ATC ECU repeats the recirculation cycle.
Ignition Off; If the recirculation flaps are in the recirculation position when the ignition is switched off, the ATC ECU automatically sets them to the fresh air position, to ensure that fresh air is available if a fault occurs with the recirculation flap drive circuit at the next ignition on. When the ignition is switched on, the ATC ECU sets the recirculation flaps to the position they were in at ignition off.
Distribution Flaps
When a manual distribution selection is made on the control panel, the ATC ECU moves the distribution flaps to preset positions. When the system is in the automatic mode the ATC ECU uses maps of the driver's side reference value to determine the position of the distribution flaps. Each distribution flap has a separate map and there are separate maps for temperate and hot climates. The mapped positions of the face level distribution flaps are given a correction factor from the sunlight sensor inputs. The mapped positions of the distribution flaps are overridden by special programmes, as follows:
Condensation Prevention: Jo prevent condensation on the windscreen, the ATC ECU closes the windscreen flaps for 12 seconds each time the engine starts. If a manual distribution selection is made during the 12 seconds the windscreen flaps revert to normal control.
The ATC ECU also closes the windscreen flaps when the reference value is less than 90% with the recirculation flaps closed and the compressor is disengaged.
Cold Start: When the engine starts, if the system is in the automatic mode, the driver's side reference value is 100% and the heater matrix temperatures is less than 30 °C (86 °F), the ATC ECU closes the face level and footwell flaps and opens the windscreen flaps.
Duct Purging: After a cold start, if the system is in the automatic mode and the reference value is 100%, the ATC ECU keeps the face level flaps closed until one of the heater matrix temperatures exceeds 60 °C (140 °F). The ATC ECU then fully opens the face level flaps for 25 seconds to purge the ducts of cold air. After the purge the face level flaps revert to normal automatic control. If, before or during the purge process the reference value decreases below 100%, the ignition has been on for 15 minutes, or a manual distribution selection is made, then the purge process is cancelled.
Blower Control
In the automatic mode, blower speed is determined from reference temperature maps. In general terms, the more heating or cooling required to achieve the temperature selected on the control panel with the driver's side temperature switch, the faster the blower speed.
When maximum cooling is first required in the automatic mode the ATC ECU runs the blower at full speed for a fixed period regardless of any drop in passenger compartment temperature. Different time periods are incorporated for the following markets:
- NAS and Gulf States
- Europe and ROW
- Japan
NAS and Gulf States have the longest time period with the blower at full speed and Japan the shortest. TestBook/T4 can be used to change the market setting so, in some cases, if a customer complains of the duration the blower runs at full speed, changing the market setting could resore the complaint.
When heating is required, blower speed is reduced if the engine is cold and then progressively increased to the nominal setting as the engine coolant temperature increases from 20 to 50 °C (68 to 122 °F).
On the high line system, blower speed is also influenced by the sunlight sensor. At high light levels blower speed will increase to increase the cooling effect.
When the blower is selected off temperature regulation is no longer possible so the ATC ECU disengages the compressor, de-energises the auxiliary water pump and closes the coolant valves. Pressing any of the A/C system switches restores blower operation and activates the required function. If the blower is selected off when the ignition is switched off, the blower remains off if the ignition is switched on again within 15 minutes. If more than 15 minutes elapses between the ignition being switched off and back on again the ATC ECU switches the blower on at speed 1.
If battery voltage is low the ATC ECU reduces blower speed to conserve power. If battery voltage decreases to less than 12 V the ATC ECU decreases the blower signal voltage and thus the voltage to the blower motor in direct proportion to the reduction in battery voltage.
Defrost Programme (High Line System Only)
When the defrost programme is selected the ATC ECU:
- Opens the windscreen distribution flaps and closes the face and footwell flaps
- Sets the recirculation flaps to the fresh air position
- Runs the blower at speed 7
- Activates the A/C compressor
- Activates the rear window heater
- Activates the windscreen heater (where fitted)
When the ambient temperature is 10° C (50 °F) and below, the ATC ECU signals the coolant valves to remain open. When the ambient temperature is more than 10 °C (50 °F), the ATC ECU checks the existing heater matrix temperature and, if necessary, increases the open time of the coolant valves to produce a minimum heater matrix temperature of 30 °C (86 °F).
The defrost programme is cancelled by pressing the defrost switch again or pressing the automatic, A/C or maximum A/C switches.
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