Discovery 2. Manual - part 293

AIR CONDITIONING

DESCRIPTION AND OPERATION

82-11

ATC ECU
The ATC ECU is installed in the centre of the fascia, below the radio. An integral control panel on the ATC ECU 
contains switches for system control inputs and a LCD to provide system status information.

Inputs from sensors and the control panel switches are processed by the ATC ECU, which then outputs the 
appropriate control signals.

ATC ECU connectors

ATC ECU connector pin details

Connector/Pin 

No.

Description

Input/Output

C0791

1

Battery power supply

Input

2

Ignition power supply

Input

3

Sensor power supply

Output

4

Earth

-

5

Display illumination

Input

6

Not used

-

7

Not used

-

8

Sensor earth

-

C0792

1

Night lighting/dimming

Input

2

Vehicle speed

Input

3

Hand of drive

Input

4

Distribution flaps position

Input

5

Heater coolant temperature

Input

6

External air temperature

Input

7

In-car air temperature

Input

8

Blower power transistor collector voltage

Input

9

Not used

-

10

Not used

-

11

Windscreen heater status

Input

12

Rear screen heater status

Input

13

Rear air conditioning ON

Input

AIR CONDITIONING

82-12

DESCRIPTION AND OPERATION

Control panel

1 LH temperature switch
2 Economy mode (ECON) switch
3 Blower switch
4 Display
5 Distribution switch
6 External air temperature (EXT) switch

7 RH temperature switch
8 Fresh/Recirculated air switch
9 Defrost mode switch

10 Automatic mode (AUTO) switch
11 A/C on/off switch

14

Driver's blend flaps position

Input

15

Passenger's blend flaps position

Input

16

LH solar heating load

Input

17

RH solar heating load

Input

18

Evaporator

Input

19

Not used

-

20

Not used

-

C0793

1

Blower power transistor base current

Output

2

Blower relay

Output

3

Windscreen heater request

Output

4

Rear screen heater request

Output

5

Passenger's blend flaps servo motor, drive to hot

Output

6

Driver's blend flaps servo motor, drive to hot

Output

7

Distribution flaps servo motor, drive to windscreen and side 
windows demist

Output

8

Fresh/Recirculated air servo motor, drive to recirculated air

Output

9

Cooling fan request (diesel models)

Output

10

Power relay

Output

11

Compressor clutch request

Output

12

Cooling fan request (V8 models)

Output

13

Passenger's blend flaps servo motor, drive to cold

Output

14

Driver's blend flaps servo motor, drive to cold

Output

15

Distribution flaps servo motor, drive to footwells

Output

16

Fresh/Recirculated air servo motor, drive to fresh air

Output

Connector/Pin 

No.

Description

Input/Output

AIR CONDITIONING

DESCRIPTION AND OPERATION

82-13

The control panel switches are all non-latching pushswitches except for the LH and RH temperature switches, which 
are centre-off rocker switches. The switches have the following functions:

LH and RH temperature switches. Enabled only while the system is on:

l

Each press increases or decreases the related temperature setting, in steps of 1 

°

C (2 

°

F), between 16 and 28 

°

C (60 and 84 

°

F).

l

If the decrease side of the switch is pressed when a temperature of 16 

°

C (60 

°

F) is set, the display changes to 

LO (maximum cold).

l

If the increase side of the switch is pressed when a temperature of 28 

°

C (84 

°

F) is set, the display changes to 

HI (maximum hot).

l

If a switch is kept depressed, step changes occur every 0.4 seconds.

A/C on/off switch. Switches the system on and off. When used to switch the system on, the system resumes the 
configuration in use prior to the previous off selection.

Blower switch. Enabled only while the system is on. Provides manual control of blower speed:

l

Each press changes the blower speed, in sequence, through off (only available if economy mode is selected on) 
and five incremental speeds.

l

If the switch is kept depressed, after 1 second subsequent speed increments occur every 0.4 second until blower 
reaches high speed. Releasing and then pressing the switch again changes blower back to off or low speed.

Distribution switch. Enabled only while the system is on. Provides manual control of air distribution:

l

Each press changes the air distribution, in sequence, through footwells only, footwells and windscreen/side 
windows demist, windscreen/side windows demist only, face level only, face level and footwells.

l

If the switch is kept depressed, after 1 second subsequent distribution changes occur every 0.4 seconds until 
distribution reaches face level and footwells. Releasing and then pressing the switch again changes distribution 
back to footwells only.

External air temperature (EXT) switch. Enabled while the system is on or off. Switches the external temperature output 
on and off:

l

If the system is already on, the temperature output overrides the system outputs for approximately  7 seconds, 
then the display reverts to system outputs.

l

If the system is switched on while the external temperature output is on, the system outputs override the external 
temperature output.

Fresh/Recirculated air switch. Enabled only while the system is on. Provides manual control of inlet air selection.

Defrost mode switch. Starts the system in, or switches the system to and from, defrost mode.

Automatic mode (AUTO) switch. Starts the system in, or switches the system to and from, the automatic mode.

Economy mode (ECON) switch. Enabled only while the system is on. Provides manual on/off control of the refrigerant 
system compressor, to reduce fuel consumption when there is no requirement for cool or dehumidified air, e.g. when 
the ambient temperature is lower than the LH and RH temperature settings.

Temperature settings: The LH and RH temperature settings are reference inputs used by the control system and give 
an approximation of the temperatures that will be established in the cabin. They are not necessarily actual distribution 
outlet temperatures, or the temperatures at specific points in the cabin.

Audible warning: A 'beep' is emitted from the ATC ECU each time it receives a control switch input. This audible 
warning can be switched off and on by pressing and holding the AUTO switch, then pressing and holding the A/C on/
off switch until the audible warning sounds (approximately 3 seconds). While switched off, the audible warning still 
sounds when:

l

Switching between 

°

F and 

°

C on the display.

l

Switching the audible warning from off to on.

l

Switching the timed feet function on and off.

l

Switching the timed recirculated inlet air on and off.

l

Switching the latched recirculated inlet air on and off.

l

When there is a fault warning.

l

Running the self diagnostic routine.

AIR CONDITIONING

82-14

DESCRIPTION AND OPERATION

Display outputs

1 LH temperature
2 Economy mode
3 Recirculated air
4 Automatic mode

5 Manual distribution
6 RH temperature
7 Blower speed
8 External air temperature

Outputs on the display are shown at full brightness when the exterior lights are off, and dimmed when the exterior 
lights are on.

LH temperature. Illuminates to show the LH temperature selection, the temperature scale or that the ATC ECU is in 
diagnostic mode.

Economy mode. Illuminates when the compressor is manually selected off with the ECON switch.

Recirculated air. Illuminates when the inlet air is manually selected to recirculated mode.

Automatic mode. Illuminates the AUTO legend and related symbol(s) when the blower speed, air distribution or 
temperature control are in the automatic mode.

Manual distribution. Illuminates the appropriate symbol(s) to show the manually selected air distribution. Also 
illuminates in the automatic modes when one of the temperatures is set to LO or HI.

RH temperature. Illuminates to show the RH temperature selection, external air temperature or diagnostic fault code.

Blower speed. Illuminates when the blower speed is manually selected. Also illuminates in the automatic modes when 
one of the temperatures is set to LO or HI.

External air temperature. EXT illuminates to show that external air temperature is selected on.

All temperature indications on the display are in either 

°

C or 

°

F. For 1 second after the system is first switched on, the 

display shows only 

°

C or 

°

F, in the LH temperature window, to indicate which temperature scale is in use. After 1 

second, the 

°

C or 

°

F indication goes off and the display shows all relevant outputs.

Temperature conversion: While the system is on, the temperature indications on the display can be switched between 
the two scales by pressing and holding the fresh/recirculated air switch, then pressing and holding the A/C on/off 
switch until the audible warning sounds (approximately 3 seconds).