Pressure Checking, Vehicles with Restrictor and Reservoir with Internally Regulated Compressor

WARNING: This page does not describe the selected car, but rather 17 other vehicles, including the 2009 Audi S4, 2009 Audi A4, 2008 Audi S4, 2008 Audi RS 4, and 2008 Audi A4. However, it is still accessible from the selected car via links, so may be relevant.

NOTE:

Connecting A/C service station → A/C Service Station, Connecting
Observe test conditions → Refrigerant Circuit, Checking Pressures with A/C Service Station .
Set engine speed to 2000 rpm.
Observe pressure display (e.g. pressure gauge) of A/C service station.

NOTE:

Switching pressures for refrigerant circuit switches are vehicle-specific.
Connection with valve for low-pressure switch or at evaporator is only to be used for vehicles with no service connection on low-pressure side and inaccessible connection at compressor or reservoir (measurement accuracy). Only applies to certain vehicles → 87 - Air conditioning .

Specifications

High-pressure side:

Increasing from initial pressure (on connecting pressure gauges) to max. 20 bar

Low-pressure side:

Fig 1: Permissible Tolerance Range Graph

Courtesy of VOLKSWAGEN UNITED STATES, INC.

Decreasing from initial pressure (on connecting pressure gauges) to value in graph

A - High pressure (measured at service connection) in bar

B - Low pressure (measured at connection with valve at compressor or reservoir) in bar

C - Permissible tolerance range

D - Low pressure (measured at connection with valve for low-pressure switch or at service connection) in bar

E - Permissible tolerance range

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure remains constant or only increases slightly (above pressure with engine stopped) Low pressure quickly drops to value in graph or below Required cooling output is not attained	Not enough refrigerant in circuit	Localize leak using leak detector and eliminate Recharge refrigerant circuit
High pressure normal Low pressure in line with value in graph Required cooling output is not attained
High pressure normal Low pressure too low (see graph) Required cooling output is not attained

NOTE:

If no fault is found in relation to this complaint, clean refrigerant circuit (flush using refrigerant R134a → Refrigerant Circuit, Flushing with Refrigerant R134a or blow through using compressed air and nitrogen → Refrigerant Circuit, Flushing with Compressed Air and Nitrogen ).

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure only increases slightly above pressure with engine stopped Low pressure only drops slightly Required cooling output is not attained	Compressor malfunctioning	Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen ) Replace compressor

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure increases above specification Low pressure quickly drops to value in graph or below Required cooling output is not attained	Constriction or obstruction in refrigerant circuit	Run hand over refrigerant circuit to check for differences in temperature If difference in temperature is found at one component: Replace hose or pipe if kinked or constricted In the event of an obstruction, clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen ) If no fault is found: Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen )
High and low pressure normal at first After some time, high pressure increases above specification and Low pressure drops to value in graph or below Required cooling output is no longer attained	Moisture in refrigerant circuit	Check and if necessary replace reservoir (with dryer) and restrictor, then evacuate refrigerant circuit for min. 3 hours (see note) Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen )
High and low pressure normal at first After lengthy operating period, low pressure drops excessively (evaporator ices up)

NOTE:

If problem involving moisture in refrigerant circuit only occurs after a lengthy operating period or only infrequently (low pressure drops below specification and evaporator ices up), it is sufficient to replace the dryer (adjust quantity of refrigerant oil). Refrigerant circuit is then to be evacuated for at least 3 hours.
It is not initially necessary to clean the refrigerant circuit (flush using refrigerant R134a → Refrigerant Circuit, Flushing with Refrigerant R134a or blow through using compressed air and nitrogen → Refrigerant Circuit, Flushing with Compressed Air and Nitrogen ) when this problem occurs since normally, there is only a small quantity of moisture in the system which can be removed by lengthy evacuation.

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure normal Low pressure too low (see graph) Required cooling output attained	Compressor malfunctioning	Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen ) Replace compressor

NOTE:

Pay attention to the following as regards fault "High pressure normal, low pressure too low" : This fault may cause the evaporator to ice up or the A/C Refrigerant Low Pressure Switch F73 to shut off the compressor although the amount of refrigerant in the circuit is OK.
On the Audi 100, Audi A6 (up to and including Model Year 1997) and Audi V8, this fault may result in compressor being shut off by control head (if temperature at fresh-air blower drops below -3 C) → 87 - Air conditioning .

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure normal or too high Low pressure too high (see graph) Compressor noise (particularly after switch-on) Required cooling output is not attained	Too much refrigerant in circuit	Extract refrigerant from refrigerant circuit If quantity of refrigerant extracted roughly corresponds to specified capacity: Replace compressor If quantity of refrigerant extracted is substantially greater than specified capacity: Recharge refrigerant circuit Repeat test

Possible deviation from specification	Possible cause of trouble	Corrective action
High and low pressure normal Required cooling output is not attained	Too much refrigerant oil in circuit	Discharge refrigerant circuit Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen )
High and low pressure normal Compressor noise (particularly after switch-on) Required cooling output attained

NOTE:

Overfilling with refrigerant oil can occur if, for example, the compressor has been replaced without adjusting the quantity of refrigerant oil.
If there is too much refrigerant oil in the circuit, drain compressor and replace reservoir. After cleaning the refrigerant circuit (flushing with refrigerant R134a → Refrigerant Circuit, Flushing with Refrigerant R134a or blowing through using compressed air and nitrogen → Refrigerant Circuit, Flushing with Compressed Air and Nitrogen ), the correct quantity of refrigerant oil is filled into the circuit → Approved Refrigerant Oils .