Porsche P-80 2005, Manual

Page: 77 / 108

Cayenne Model System
Climate Control Systems Diagnosis & Repair
Page 4.19
Service and Diagnosis Information
Moisture condensing on the cold evaporator fins drains
out the bottom of the air distribution housing. Be sure that
this drain does not become obstructed or plugged. Water
coming from outlets at high blower fan speeds can
indicate drain blockage.
Evaporator fin temperatures cannot be allowed to drop
below the freezing point of water. If this happens, the
evaporator can become blocked with ice and airflow can
be reduced. In Cayenne models, evaporator temperature
is maintained by varying compressor displacement. This
controls the amount of refrigerant pumped through the
system and refrigerant pressure and temperature in the
evaporator. These systems are very tolerant of faults and
can be difficult to diagnose.
Cayenne On Board Diagnostics
The Climate Control Unit monitors its own function and
also various sensors and actuators. If a fault is detected, a
fault code is stored in non-volatile memory (EEPROM).
Sensor circuits are checked for signal plausibility and also
whether the signal is within range. If a sensor circuit fault
is detected, the system ignores the signal and substitutes
a stored default value. In this way, emergency operation of
the climate control system can be maintained.
CAN Bus Comfort High and Low
The Climate Control Unit receives information over the
CAN Bus from sensors which are not directly connected to
the control unit. The control unit also provides climate
control information to other components on the CAN Bus.
Information received by the Climate Control Unit over the
CAN Bus:
• Ignition ON (Terminal 15)
• Park Heater ON/OFF
• Country-specific information
• Temperature units
• Outside temperature
• Dimming signal
• Vehicle speed
• Terminal X
• Heated rear window actual value
• Windshield wiper
• Steering angle
• System information (e.g. MONO function)
• Engine speed
• Time
• Engine temperature
• Shutdown and reduction request
Information placed over the CAN Bus by the Climate
Control Unit:
• RPM increase (to stabilize the idle when the A/C
compressor is requested)
• Heated rear window
• Compressor status
• Heater OFF
• Refrigerant pressure
• Request condenser cooling
• ECON function
• Status of the engine residual heat function
• Booster heater (not used, value is always 0)
• Windshield heater
• Outside temperature
• Compressor load calculation
• Blower load
• KD fault