Discovery 2. Manual — part 294

AIR CONDITIONING

DESCRIPTION AND OPERATION

82-15

Dual pressure switch
The dual pressure switch protects the refrigerant system from extremes of pressure. The normally closed switch is
installed in the top of the receiver drier. If minimum or maximum pressure limits are exceeded the switch contacts
open, causing the compressor clutch to be disengaged. The minimum pressure limit protects the compressor, by
preventing operation of the system unless there is a minimum refrigerant pressure (and thus refrigerant and
lubricating oil) in the system. The maximum pressure limit keeps the refrigerant system within a safe operating
pressure.

Dual pressure switch nominal operating pressures

Evaporator temperature sensor
The evaporator temperature sensor is an encapsulated thermistor that provides the ATC ECU with an input of the
evaporator air outlet temperature. The evaporator temperature sensor is installed in a clip which locates in the
evaporator matrix in the heater assembly. The ATC ECU uses the input to prevent the formation of ice on the
evaporator.

In-car temperature sensor
The in-car temperature sensor is an encapsulated thermistor that provides the ATC ECU with an input of cabin air
temperature. The sensor is integrated into the inlet of an electric fan, which is installed behind a grille in the fascia
outboard of the steering column. The fan runs continuously, while the ignition is on, to draw air through the grille and
across the sensor.

Sunlight sensor
The sunlight sensor consists of two diodes that provide the ATC ECU with inputs of light intensity, one as sensed
coming from the left of the vehicle and one as sensed coming from the right. The inputs are used as a measure of the
solar heating effect on vehicle occupants. The sensor is installed in the centre of the fascia upper surface.

Heater coolant temperature sensor
The heater coolant temperature sensor is an encapsulated thermistor that provides the ATC ECU with an input related
to heater matrix coolant temperature. The sensor is installed in the casing of the heater assembly and presses against
the end tank of the heater matrix.

Ambient temperature sensor
The ambient temperature sensor is an encapsulated thermistor that provides the ATC ECU with an input of external
air temperature. The sensor is attached to the cooling fan mounting bracket in front of the condenser.

Limit

Opening pressure, bar (lbf.in

2

)

Closing pressure, bar (lbf.in

2

)

Minimum

2.0 (29.0), pressure decreasing

2.3 (33.4), pressure increasing

Maximum

32 (464), pressure increasing

26 (377), pressure decreasing

AIR CONDITIONING

82-16

DESCRIPTION AND OPERATION

Operation

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:

l

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.

l

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.

l

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 .

AIR CONDITIONING

DESCRIPTION AND OPERATION

82-17

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:

l

In the automatic mode, by pressing the fresh/recirculated air switch for 1.5 seconds or more; the audible warning
sounds twice.

l

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:

l

In the automatic mode, by pressing the fresh/recirculated air switch for less than 1.5 seconds; the audible
warning sounds once.

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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:

l

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).

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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.

l

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.

l

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 CONDITIONING

82-18

DESCRIPTION AND OPERATION

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:

l

If one is set to LO and one is set to a specific temperature, to face level only.

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If one is set to HI and one is set to a specific temperature, to footwells only.

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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:

l

Temperature indications in

°

C (in some markets a conversion connector is fitted to the ATC ECU to change the

default temperature scale to

°

F).

l

LH and RH outlet temperatures of 22

°

C (72

°

F).

l

audible warning switched on.

l

Warm-up air distribution (to face level and footwells) function switched on.

l

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.