Porsche P-80 2005, Manual

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911 Carrera (993) Model System
Climate Control Systems Diagnosis & Repair
Page 2.7
Sensors and Actuators
Sensors
Refrigerant pressure is monitored to prevent A/C system
operation when pressures are very low or high. High refrig-
erant pressure also signals increased condenser fan
speed for additional airflow and cooling.
A/C systems sometimes monitor the temperature of the
evaporator, ambient air and outlet air. Automatic climate
control systems use more temperature sensors than
manual A/C systems.
3-Way Refrigerant Pressure Switch (High/Low
Pressure Switch)
The 3-way refrigerant pressure switch (High/Low pressure
switch) has two switches that combine low pressure and
high pressure switch functions in one unit. The switch
states are an input to the Heating-A/C Control Unit.
The first switch is a three-position pressure switch in the
compressor clutch circuit. The switch is normally closed,
and the contacts will open when refrigerant pressure
exceeds very high or low thresholds. As long as
refrigerant pressure is between approx. 2.5-27 bar (36-
392 psi), the switch will be closed.
The second switch is a normally-open switch in the
condenser fan circuit. The contacts will close when refrig-
erant pressure exceeds 17.5 bar (254 psi). This will run
the condenser fan at high speed to move more air through
the condenser and to reduce high side refrigerant
pressure. The 3-way pressure switch is installed in the
system high side.
Evaporator Temperature Sensor
The evaporator temperature sensor sends a signal to the
Heating-A/C Control Unit for air temperature exiting the
evaporator. When the measured temperature is approx.
0-1° C (32-34° F), the circuit to the compressor clutch is
opened. This prevents condensation on the evaporator
from freezing and blocking airflow. With the compressor
OFF, evaporator temperature will rise. The compressor will
be allowed to turn ON again when the temperature has
increased to approx. 3° C (37° F).
Generally, systems with fixed displacement compressors
will have an evaporator temperature sensor. Systems with
variable displacement compressors generally do not use
them. These systems keep evaporator temperatures
above freezing by controlling low side pressure.
The temperature sensor is inserted between the
evaporator fins in the air distribution housing.
Ambient Temperature Sensor
The ambient temperature sensor supplies an analog signal
for outside temperature to the Heating-A/C Control Unit.
The sensor is identical to the sensor for the outside
temperature indicator. Both sensors are mounted together
in the front of the vehicle behind the left air intake grille.
Auxiliary Blower Temperature Sensor
The auxiliary blower temperature sensor supplies an
analog signal for cooling air temperature. It is used to
control the auxiliary blower.
Notes: