Evaporator control (compressor control)

Evaporator control is 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.

Actuating the A/C button initiates the following steps:

The DME is requested to increase the idle speed to ensure the A/C compressor already generates sufficient refrigerating capacity with the engine idling.
Following the ”switch on A/C compressor” request from the control panel/control unit, the DME responds with an enable signal (Pin 15).
Consequently, the control panel/control unit activates the magnetic clutch for the A/C compressor.

The following information is transmitted in the form of a telegram on the K-bus to the instrument cluster and from here via the PT-CAN bus to the digital motor electronics DME:

A/C compressor information to increase the engine idling speed
Information on the load torque of the A/C compressor
Information on the set stage of the electric fan

Switching conditions for the magnetic clutch of the A/C compressor

A/C compressor ON	A/C compressor OFF
Actuating of A/C button and	Actuating of A/C button or
Evaporator temperature > 2°C (depending on the outside temperature) and	Evaporator temperature < -0.5°C (depending on the outside temperature) or
Outside temperature > -10°C and	Outside temperature < -10°C or
Vehicle voltage ≥ 9.7 V and	Vehicle voltage < 9 V or
Within permissible pressure range	Outside permissible pressure range or
	Blower zero position or
	Terminal 15 OFF¹

¹ With terminal 15 OFF, the current function ”A/C system” is stored in the EEPROM. The function light is switched off. Derived from the key memory (personalizing), the function last selected is activated at terminal 15 ON.

Pressure monitoring

The pressure of the refrigerant circuit is monitored by means of a pressure sensor on the dryer. Derived from the signal of the pressure sensor on the dryer: The load torque to be expected when switching on the A/C compressor and during its operation is derived from the signal of the pressure sensor on the dryer: The load torque in transferred via the K-bus and PT-CAN bus to the DME. The DME responds with corresponding engine control (adapted air and fuel volume).