LR3/Disco 3
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. For additional
information, refer to
Communications Network
(418-00 Module Communications Network)
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 °C (61 °F) and 28 °C (82 °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
