412-04 Control Components: Description And Operation (page 15 of 21)

operation.  
Selecting the automatic switch. This will restore the system to fully automatic operation.  
Selecting the programmed defrost switch again. This returns the system to the state in use immediately before the 
programmed defrost function was first selected.  
Turning the ignition off.  

The blower speed can be adjusted manually without terminating the programmed defrost function. 
 

Intake Air Control 

The source of intake air is automatically controlled unless overridden by manual selection of recirculation. Under 
automatic control the ATCM determines the required position of the recirculation door from the comfort strategy and the 
input from the pollution sensor (if fitted). The recirculation door is operated by an electric motor, which is controlled by 
hardwired analogue signals from the ATCM. A potentiometer in the motor supplies the ATCM with a position feedback 
signal for closed loop control. 
 
Provided the intake air has not been manually selected to recirculation, the ATCM adjusts the recirculation door to reduce 
the ram effect produced by the forward motion of the vehicle. 
 
When the ignition switch is turned off, the ATCM evaluates the ambient air temperature. If the ambient air temperature is 
less than a pre-determined value, the intake air source is set to recirculation, to prevent the ingress of damp air while the 
vehicle is parked. 
 
When the vehicle is in the transportation mode, the ATCM sets the intake door to recirculation every time the ignition is 
turned off, regardless of the ambient air temperature.  
 

Pollution Sensing 

With a pollution sensor fitted to the vehicle, the ATCM controls the intake air source to reduce contamination of the intake 
air by external pollutants. This function is fully automatic, but can be overridden by manual selection of the intake air 
source. 
 

Humidity Sensing 

With a humidity sensor fitted, the ATCM controls the moisture content of the air in the vehicle. This is achieved by raising 
the evaporator temperature to increase the humidity of the air entering the vehicle, and reducing the evaporator 
temperature to reduce the humidity of the air entering the vehicle. 
 

Front Seat Heaters 

The front seat heaters are enabled when the ignition switch is position II, and operate at one of two temperature settings. 
With the first press of a front seat heater switch the ATCM adopts the higher temperature setting, supplies a power feed to
the related front seat heater elements and illuminates two amber LED's in the switch. At the second press of the switch 
the ATCM adopts the lower temperature setting and extinguishes one of the LED's. At the third press of the switch the 
ATCM de-energizes the heater elements and extinguishes the second LED. The seat heaters remain on until selected off 
or the ignition is turned off. 
 
The ATCM receives an input from a temperature sensor in each front seat, and regulates the power feed of the heater 
elements to control the seat temperature at the appropriate temperature setting between 35 and 45 °C (95 and 113 °F). 
The actual temperature settings vary with the type of seat covering, to allow for the different heat conduction properties of 
the different materials. 
 
When the front seat heaters are activated at the higher temperature setting, the ATCM automatically resets them to the 
lower temperature after a time delay. The length of the time delay depends on the in-vehicle temperature. 
 

Temperature Reset Time Delay 

In-vehicle 
Temperature, °C (°F) 

<-15 (5) 

-15 to -10 
(5 to 14) 

-10 to 0 
(14 to 32) 

0 to 15 (32 
to 59) 

15 to 25 
(59 to 77) 

>25 
(77) 

Time Delay, minutes 

Remains at higher temperature until  20 

15 

10 

5 

3