A/C-Compressor Control (Functional Descriptions)

A/C Compressor Activation

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 fulfilled. The evaporator then produces cold air which is raised to the required temperature in reheat mode with the aid of the heat exchanger.

Activation of A/C compressor relay
The A/C compressor relay is activated by the engine control unit. The activation request is sent by the air conditioning control unit via CAN.

Full load cutout
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 4 and 10 seconds depending on the type of engine.

Full load condition: Speed signal < 14 km/h and acceleration

Idle increase (anti-stall)
The air conditioning control unit sends the AC and KO signals via the CAN bus to ensure engine operating refinement is not impaired by the compressor load.

When switching on the air conditioning system, the AC signal requests an increase in the idle speed from the engine control unit. When the signal is active, the engine control unit increases the idle speed irrespective of the magnetic clutch.

Compressor activation
With the KO signal, the A/C control unit informs the engine control unit of its intention to switch on the compressor. Consequently, the DME implements an interfering variable circuit, i.e. it moves the throttle or the idle speed control valve in the direction "more air" while at the same time increasing the injection volume.

On the E36 and E46 The A/C compressor relay is then activated by the engine control unit. Only relay activation is monitored by the diagnosis function of the engine control unit but not the compressor itself.

On the E38 and E39 the engine control unit sends a reply in the form of the DME_KOREL signal (compressor relay) to the A/C control unit to switch on the A/C compressor. If all conditions for switching on the A/C compressor are fulfilled, the A/C compressor is activated directly by the control unit.