Air Temperature Control
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 ATC module using LIN bus messages.
The ATC module 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 ATC module signals the stepper motor to adopt the new position.
When maximum cooling or maximum heating is selected, the ATC module overrides any previously selected options and sets the temperature, air source, blower speed and air distribution as detailed in the following table.
| Function | Maximum Cooling | Maximum Heating |
|---|---|---|
| Temperature control | Blend door fully closed | Blend door fully open |
| Air source | Recirculated air | Fresh air |
| Blower speed | Maximum | Maximum * |
| Air distribution | Face (with bleed to feet) | Feet and Screen |
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.