The air conditioning system is switched on by pressing the air conditioning button. The function light signals that the air conditioning system is in standby mode.
The compressor only cuts in if the switch-on conditions are fulfillled. The evaporator then produces cold air which is raised to the required temperature in reheat mode with the aid of the heat exchanger.
To protect the evaporator from icing up, the outlet temperature is monitored by means of a sensor (evaporator sensor) and the compressor is controlled by means of an electromagnetic clutch.
If a mechanically controlled compressor is installed, the evaporator cutout temperature is dependent on the outside temperature. If the outside temperature increases above 20 degrees Celsius the cutout temperature drops to 1 degrees Celsius.
Note
The component coding can be checked via diagnosis "3 Service functions" , "2 Read coding data" . This data indicates what type of compressor (regulated/non-regulated) and interface (PWM signal/conventional) are installed and coded.
The refrigerant pressure in the air conditioning is registered by a pressure sensor and converted by a table into a torque and an auxiliary fan stage which is then sent to the DME/DDE via the K-bus/CAN bus.
The evaporator temperature controller is made up of a two-point controller on the basis of a variable positive switching threshold with 1 o K hysteresis. The refrigeration circuit is activated at an evaporator temperature > 3 o C and deactivated at an evaporator temperature < 2 o C. This function prevents the evaporator from icing up.
The instrument cluster electronics (IKE) features a coolant sensor on the radiator circuit. This analog sensor signal is processed in the IKE and read in by the IHKA via the K-bus.
To avoid the engine overheating, the cooling water is monitored by the air conditioning system. At a coolant temperature above 117 o C the compressor clutch cuts in and out at 20 s intervals. If the coolant temperature exceeds 120 o C, the compressor clutch is switched off until the temperature drops below 117 o C. The compressor clutch then cuts in once again at a rate of 100%.
In order to shorten the rev-up time of the engine from idle up to full load, the electromagnetic clutch is switched off for a limited period of time at full throttle. The cutout time is between 7 and 10 s depending on the type of engine.
Full load condition: Speed signal < 14 km/h and acceleration
To ensure smooth operation is not impaired by the compressor load, the control lines AC and KO are used for activation of the DME/DDE by the air conditioning control unit.
The required stage of the auxiliary fan requested via DME/DDE is determined via the A/C pressure sensor in the control unit.
A/C MAX button (from PU99)
The former residual heat button now bears the A/C MAX symbol . The air conditioning is switched on by this function. Only the ventilation flap is open at maximum blower output and in recirculating air mode . At terminal R the residual heat function is still allocated to this button.