Evaporator control (compressor control)
Evaporator controlis activated by pressing the A/C button in the operating unit. A/C standby mode is indicated by the LED in the A/C button. If controlled compressors are installed, the cutout threshold is dependent on the outside temperature and may be between 1 °C and 3 °C. If a standard compressor is installed, the threshold is reduced by 1 °C at high outside temperatures (> 30°C). Disengagement of the compressor clutch is scarcely achieved by the mechanical control of the compressor output on the controlled compressor . The A/C compressor is controlled directly by the heating/air conditioning control unit and the auxiliary fan by the digital motor or diesel electronics (DME/DDE) . Requests for A/C compressor and auxiliary fan are sent via the CAN/K-bus to the DME/DDE. The pressure sensor also monitors the refrigerant pressure and switches the A/C compressor off if the pressure is too high or too low. A pressostat that checks the refrigerant pressure and, if necessary, interrupts the supply line to the compressor clutch is only installed on the M51 diesel engine.
Switching conditions for the magnetic clutch
ON:A/C button ON and evaporator temperature > 4°C (depending on the outside temperature) and coolant temperature < 117°C and terminal 30 at control unit > 9.7 Volt.
OFF: A/C button OFF or evaporator temperature < 3°C (depending on outside temperature) or blower stage 0 or coolant temperature > 120°C or terminal 15 OFF or terminal 30 < 9 Volt.
DME_AC idle speed increase
When the A/C button is pressed, the heating/air conditioning system requests an idle speed boost via the K-bus in order to provide adequate cooling capacity at idle speed.
DME_KO compressor activation
The DME receives the instruction to switch on the compressor via the K-bus . If no full load cutout is set a feedback signal enables the DMD/DDE to activate the compressor clutch. The load moment of the A/C compressor is derived from the pressure sensor signal and passed on via the K-bus to the DME.
Infinitely variable auxiliary fan control
The refrigerant pressure in the air conditioning is registered by means of a pressure sensor and converted by means of a table into an auxiliary fan stage which is then sent to the DME/DDE via the K-bus/CAN bus . From this, the electronic engine control system then generates the control voltage for the infinitely variable auxiliary fan motor .