Pressure Checking, Vehicles with Expansion Valve and Receiver 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 and design of refrigerant circuit switches are vehicle-specific.
Pressures must be measured at service connections; component locations of these connections are vehicle-specific → 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 in bar

B - Low pressure in bar

C - 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 or expansion valve malfunctioning	Extract refrigerant from refrigerant circuit If quantity of refrigerant extracted roughly corresponds to specified capacity: Replace expansion valve Recharge refrigerant circuit Repeat test
High pressure normal Low pressure in line with value in graph Required cooling output is not attained		If quantity of refrigerant extracted is substantially less than specified capacity: Localize leak using leak detector and eliminate Recharge refrigerant circuit Repeat test

NOTE:

If no malfunction can be found and air conditioner operation is not OK when test is repeated, 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 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 Expansion valve malfunctioning	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 and replace expansion valve if necessary) If no fault is found: Clean refrigerant circuit (flush with refrigerant R134a or blow through with compressed air and nitrogen and replace expansion valve if necessary) Repeat test

NOTE:

If operation is not OK after cleaning refrigerant circuit (flushing with R134a → Refrigerant Circuit, Flushing with Refrigerant R134a or blowing through using compressed air and nitrogen → Refrigerant Circuit, Flushing with Compressed Air and Nitrogen ), expansion valve must be replaced.

Possible deviation from specification	Possible cause of trouble	Corrective action
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	Expansion valve malfunctioning Moisture in refrigerant circuit	Replace receiver (with dryer) and evacuate refrigerant circuit for at least 3 hours (see notes) Examine expansion valve for dirt or corrosion; replace if necessary 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:

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.
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.

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

NOTE:

If air conditioner operation is not OK when test is repeated, re-install old expansion valve, 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 ). Then replace A/C compressor and receiver.

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 and receiver

Possible deviation from specification	Possible cause of trouble	Corrective action
High pressure normal Low pressure too low (see graph) Required cooling output attained	Expansion valve or compressor malfunctioning	Replace expansion valve Recharge refrigerant circuit Repeat test

NOTE:

If air conditioner operation is not OK when test is repeated, re-install old expansion valve, 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 ). Then replace compressor and receiver.
With this fault, evaporator may ice up although quantity of refrigerant in circuit is OK.

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, the compressor must be drained and the receiver must be replaced. 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 .