Porsche P-80 2005, Manual

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A/C Refrigerant System Service & Diagnosis
Climate Control Systems Diagnosis & Repair
Page 5.9
Diagnosing with Pressure Gauges
The A/C compressor is a pump that creates high side and
low side system pressures. The compressor pumps
against a restriction created by the expansion valve in
order to raise high side pressures. Any mechanical fault in
the system that prevents the compressor from pumping
efficiently will result in pressures higher or lower than
desired. The quantity of refrigerant also affects pressures.
Checking high side and low side pressure can help to
diagnose poor A/C performance. Remember that the
compressor’s job is to increase pressure on the high side
and reduce pressure on the low side. Variable
displacement compressors can compensate for many
conditions and can make this diagnosis more difficult.
Normal Pressures and Conditions
Normal A/C pressures will vary with changes in tempera-
ture and humidity. In general, as the temperature and
humidity increase, heat load and pressures also go up.
Variable displacement compressors tend to keep low side
pressures relatively constant. High side pressures go up
and down with changes in heat load.
The charts below show normal pressures for both fixed-
displacement and variable displacement compressors.