L322 Range Rover System Description and Operation
AIR CONDITIONING
82-40
DESCRIPTION AND OPERATION
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:
l
The temperature setting on the control panel
l
The ambient temperature
l
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 off the ATC ECU memorises 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:
l
Engine speed is more than 400 rev/min
l
Evaporator temperature is more than 3
°
C (37
°
F)
l
The refrigerant pressure is within limits
l
Battery voltage is less than 16 V
l
The blower is running
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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
°
C (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.
The compressor can also be disengaged by the ECM changing the grant message to a negative value or outputting
a hardwired signal direct to the ATC ECU. Changing the grant message involves a time delay of up to 4 seconds
before the ATC ECU de-energises the compressor clutch. The time delay allows the ECM to decrease throttle angle
and fuelling, in anticipation of the reduction in engine load when the compressor disengages, to prevent a sudden
increase in engine speed if the engine is at idle. The hardwired signal is used to obtain instant disengagement of the
compressor to ensure maximum acceleration when accelerator pedal demand suddenly goes from no load to full load.
When this occurs the ECM sends a 12 V signal on a hardwired connection direct to the ATC ECU. When it receives
the signal the ATC ECU immediately de-energises the compressor clutch.
