Principals
IHKA Control System Principle of Operation
IHKA Controls
On the IHKA system all functions have automatic and some manual modes of operation. The automatic modes provide optimum control of the system and require no manual intervention. The manual modes allow individual functions of automatic operation to be overridden, to accommodate personal preferences.
The IHKA control system controls the operation of the refrigerant system and the control flaps in the heater assembly to control the temperature and distribution of air in the vehicle interior. It also outputs signals to the BC1 to control the fresh/recirculation air servomotor and to the blower to control the air volume.
Temperature Control
The IHKA receives a temperature input from the Temperature Control Switch. Inputs from the Ambient Air Temp Sensor, Evaporator Temp Sensor, Interior Temp Sensor and Heater Core Air Temp Sensor are evaluated and a Target Temperature is set. The Target Temperature is the required temperature of the outlet air to achieve the requested temperature.
The IHKA module then signals the servomotor controlling the blend flap in the heater assembly to move the flap to the appropriate position. Feedback information is received by the IHKA from the blend flap servomotor to indicate flap position and confirm flap movement.
The target temperature is constantly updated and, in the automatic mode, also used in further calculations to determine the blower speed and the air distribution.
Blower Control
IHKA equipped vehicles have a blower final output stage in place of the resistor pack. The output stage is in the same location as the resistor pack.
In AUTO mode, the blower is automatically regulated. In the manual mode, 8 blower speeds are available. These correspond to approximately 20, 30, 40, 50, 60, 75, 90 and 100% of battery voltage.
The blower relay is energized by the BC1 whenever the ignition switch is in position 1 or 2, and the IHKA module modulates the output stage to regulate the voltage across the blower motor to control blower speed.
In the automatic blower and fresh air mode, the blower speed is corrected for vehicle speed to compensate for the increase in ram effect on the inlet air as the vehicle speed increases.
IHKA IPO
In the automatic mode, if the temperature is set to LO or HI, the blower runs at maximum speed with correction only for vehicle speed. When the temperature is set to a specific temperature, blower speed corrections are added to compensate for the heater coolant temperature, external air temperature, and the solar load on the vehicle:
- During warm-up, to avoid blowing excessive amounts of cold air into the passenger compartment, the blower speed is limited if the heater core air temperature is below approximately 60° C. A similar limitation is applied when the exterior temperature falls below -10° C.
- During cool down, to purge the dueling of hot air, blower speed is limited for a period of approximately 5 seconds after the system is switched on. The blower speed is then progressively increased over a period of approximately 10 seconds.
- As the temperature in the passenger compartment approaches the selected temperature, blower speed is progressively reduced until, once the selected temperature has been established, blower speed stabilizes.
- Solar heating correction is employed and progressively increases the blower speed with increasing values of solar heating.
Air Distribution
In automatic mode, to control air distribution within the passenger compartment, the IHKA module signals the servomotor controlling the distribution flap in the heater assembly to move the flap to the appropriate position. When a specific temperature selection is set, air distribution is determined from the target air outlet temperatures. For higher target air outlet temperatures, air distribution is set to footwalls only. For lower target air outlet temperatures, air distribution is set to face level only. For intermediate target air outlet temperatures, air varies the bias between the footwalls and the face level outlets, in three stages, to provide a gradual transition of air distribution from footwalls only to face level only. Position and movement confirmation is received from a feedback potentiometer in the air distribution servomotor.
Fresh/Re-Circulation Air
The IHKA module outputs the required position of the fresh/re-circulated air flap to the BC1, which then operates the servo accordingly.
Air re-circulation will be controlled automatically if AUTO is selected. This may be overridden by pressing the air re-circulation control. Re-circulated air will then remain manually controlled until the air re-circulation control is pressed again.
Fresh or recirculated air is controlled by a flap, driven by a servomotor controlled by the BC1. The IHKA requests the position of the servomotor and flap by sending a K-Bus message indicating either fresh or recirculated air. The Body Control Module (BC1) responds with a K-Bus message that is used to switch the LED on/off. The fresh/recirculation servomotor is driven end to end, and then switched-off on a timer. No servo position feedback is available.
Programmed Defrost Function
The programmed defrost function is activated by a momentary push of the defrost switch. This function is only available when the engine is running, 0.5 secondsafter cranking is completed. The programmed defrost functions as follows:
- AUTO mode off.
- AC stays ON if ON before or OFF if OFF before.
- Fresh/recirculation function is set to fresh air mode.
- Desired temperature will be under manual control.
- Air distribution will be set to DEFROST vents.
- Blower speed 8 selected (depending on battery level).
- Heated rear window and (heated front screen if fitted) is requested on.
The display will show the fan speed at MAX (8 bars), and the desired temperature. The defrost LED will flash at 0.5 Hz to indicate programmed defrost function is active. The programmed defrost function is not automatically recalled after ignition off.
The programmed defrost function is de-activated by the following actions:
- Brief press of the screen distribution switch will cancel the programmed defrost function. The control panel will return to its state prior to selection of programmed defrost.
- Foot or face air distribution mode switches will provide manual air mode operation with all other settings returning to their state prior to selection of programmed defrost and cancel the programmed defrost mode.
- A press of the AUTO switch will restore auto operation of the panel and cancels programmed defrost mode.
- A press of the OFF button will switch the panel off. A subsequent press of the OFF button will switch on the panel and restore the programmed defrost function.
- A long push (>2 second) of the defrost switch will provide manual screen distribution and cancel programmed defrost mode.
Heated Front and Rear Windows (HFS/HRW)
Operation of the Heated Front and Rear Windows is the same in the IHKA system as in the IHKS system.