Porsche P-80 2005, Manual

Page: 73 / 108

Cayenne Model System
Climate Control Systems Diagnosis & Repair
Page 4.15
Sensors
A/C Pressure Sensor
Cayenne models use a refrigerant pressure sensor instead
of pressure switches. The sensor outputs a digital pulse
width modulated (PWM) signal that varies with changes in
refrigerant high side pressure.
The signal is an input to the Climate Control Unit. The
sensor provides more information to the control unit than
would be provided by a switch because it senses pressure
throughout the entire operating range, not just at high or
low pressure extremes. An integrated circuit and a silicon
crystal in the sensor receive a reference voltage. As refrig-
erant pressure is applied to the crystal, it deforms and its
resistance changes.
The integrated circuit converts this voltage to a PWM
signal at a frequency of 50 hz. The Climate Control Unit
uses this input to manage compressor displacement
during normal operation and prevents A/C operation at
pressure extremes. The pressure sensor is installed in the
system high side near the expansion valve.
Evaporator Temperature Sensor
The evaporator temperature sensor measures air tempera-
ture after it passes through the evaporator fins. The evap-
orator temperature sensor is located right behind the
evaporator.
The control unit monitors this sensor to prevent condensa-
tion on the evaporator fins from freezing and blocking
airflow. When the control unit detects evaporator tempera-
ture lower than desired, it commands the compressor to
reduce displacement and pump less refrigerant.
The evaporator temperature sensor “standard value” is
between 3-12° C (37-54° F). This is a calculated value
which changes based on the desired interior temperature
setting and ambient temperature.
Ambient Temperature Sensor
The ambient temperature sensor supplies an analog signal
for outside air temperature to the Climate Control Unit.
The system uses the signal from this sensor and the
intake air temperature sensor to calculate outside air
temperature. The sensor is mounted in the front of the
vehicle near the front bumper.
Intake Air Temperature Sensor
The intake air temperature sensor supplies a signal for
ambient temperature to the Climate Control Unit. The
sensor is mounted in the air distribution housing fresh air
intake. The Climate Control Unit compares the values from
both the ambient temperature sensor and the intake air
temperature sensor to calculate outside air temperature.
Compressor operation is not permitted at ambient temper-
atures < 2° C (36° F).
Notes: