Porsche P-80 2005, Manual

Page: 54 / 108

Page 3.20
Climate Control Systems Diagnosis & Repair
Sports Cars System
Service and Diagnosis Information
(Except Boxster/S
(987) w/Manual A/C)
Climate Control Unit Diagnosis
When the A/C system is in the automatic mode, the A/C
Control Unit monitors heat load requirements by means of
a calculation called the Energy Requirement Level (ERL).
All control functions depend upon the ERL value. The ERL
variable is derived from:
• Passenger compartment actual temperature
• Passenger compartment desired temperature
• Ambient temperature
• Solar energy level
• A/C ON-OFF status
The ERL value is between 0-255, with 0 = max. cooling
and 255 = max. heating.
The A/C function is ON with ERL values < 85, and the
heating function is ON with ERL values > 150.
The chart below shows temperature mixing flap position vs
ERL.
Climate Control Unit Communications
Through M.Y. 2000, the A/C Control Unit communicates
with the PIWIS Tester through the K data line. From M.Y.
2001, the control unit communicates via low speed
comfort CAN to the cluster, which then communicates via
high speed CAN to the DME.
The A/C Control Unit outputs a pulse width modulated
(PWM) signal to the DME control unit. The signal indicates
the load that the climate control compressor is placing on
the engine so that the DME can compensate.
Blower Control
The A/C Control Unit regulates voltage to the blower when
in the automatic mode. The following conditions affect
blower voltage:
Cold Start – Blower voltage is reduced by 50% during a
cold start. This prevents the unpleasant discharge of cold
air.
Low Ambient Correction – At ambient temperatures
below -10° C (14° F), blower voltage and speed is
increased to increase heating.
Solar Radiation Correction – A radiation-dependent
correction in blower voltage compensates for increased
heat load from solar radiation.
Recirculation Mode Correction – Blower speed is
reduced during manual recirculation to reduce noise.
Blower OFF Mode – When the blower is turned OFF
manually, air distribution is set to defrost and recirculate.
See the chart below for blower voltage values.