BMW 1999-2005 M3 Service Manual page 1125

Heating and Air Conditioning
7 of 22
http://ebahn.bentleypublishers.com/BMW/3/E46/Repair%20Manuals/6...
The operation of the A/C compressor is
modulated, eliminating noticeable on/off
cycling. In order to reduce fuel
consumption and improve vehicle
performance, the system default is with
the compressor OFF.
The A/C system uses a variable
displacement compressor. The swash
plate of the compressor is hinged so that
is can vary piston travel based on output
requirements of the system.
A/C compressor function is controlled by
the Engine Control Module (ECM) based
on inputs from the IHKA control panel.
Pressing the "snowflake" button is a
request for A/C activation. As long as the
evaporator temperature is above 2°C
(36°F), the IHKA signals the ECM to
activate the compressor.
The IHKA control module sends the
following signals to the ECM over the
K-bus and CAN-bus via the instrument
cluster:
Request for A/C activation
Load torque for switching the
compressor
Requested cooling fan speed
The IHKA determines the load torque for
compressor activation and required
engine cooling fan speed from the
pressure sensor mounted on the
receiver/dryer. The pressure sensor
provides a linear voltage input signal (0 -
5 volts) to the IHKA control module. The
module processes this signal and
determines the load torque of the system
(0 to 30 Nm with a variable displacement
compressor). The higher the pressure in
the system, the higher the voltage input
signal to the IHKA module. The output
2/25/2009 11:16 PM