Pressure Checking
TESTING WITH PRESSURE GAUGESDue to the constant temperature/pressure relationship of R-134a, approximate high side system temperature can be determined based on system pressure.
Pressure gauges may have one or more temperature scales in addition to the pressure scale. Since various refrigerants have different temperature/pressure relationships, each gauge is specific for a particular refrigerant.
Measuring pressure and temperature in a "switched-on" refrigerant system
Use the A/C refrigerant high pressure gauge to measure the high side system pressure between the compressor and restrictor (including condenser) with the A/C switched on.
The temperature display of the high pressure gauge applies only to a portion of the high pressure side. The high pressure gauge displays approximate temperature between the compressor outlet and restrictor.
Note
The temperature/pressure relationship only holds true when the refrigerant system contains liquid and vapor; but not when it contains only gas. In the gaseous state, the temperature is approx. 10-30° C (18-54° F) higher than indicated on the pressure gauge.
Measuring pressure and temperature in a closed container or "switched-off" refrigerant system
Due to the constant temperature/pressure relationship of R-134a, approximate temperature of refrigerant in a closed container or in a switched-off A/C system can be determined based on its pressure.
If temperature display on gauge is lower than the actual temperature of the refrigerant, the refrigerant container or system is empty (discharged).
Note
The measured temperature/pressure relationship only holds true when the refrigerant system contains liquid and vapor, but not when it contains only gas. In the gaseous state, the temperature is approx. 10-30° C (18-54° F) higher than indicated on the pressure gauge.
Before checking A/C system pressures, check A/C system cooling performance
Use A/C system high pressure service valve for checking A/C system pressure.
TEST CONDITIONS
^ condenser and radiator clean and free of obstructions (clean if necessary)
^ drive belts for compressor and Generator in good condition and properly tensioned
^ all air ducts, covers and seals OK and correctly installed
^ check On Board Diagnostic (OBD) Diagnostic Trouble Code (DTC) memory using VAG 1551 Scan Tool (ST) and eliminate/repair any malfunctions (DTC's) indicated (not affecting the A/C system)
^ all air distribution flaps reach end positions
^ engine at normal operating temperature
^ evaporator housing and heater box do not leak air (draw in secondary air) at high fresh air blower speed
^ with engine running, and system set to maximum cooling ("Auto operating mode with "LO" temperature setting selected or A/C and recirculation buttons pressed and temperature setting on full cold),
- fresh air blower runs
- coolant fan runs
- fresh air/recirculating air flap in "recirculating air" position
- temperature flap in "cooling position"
^ outside (ambient) temperature must be between 59°F - 113°F (15°C - 45°C)
CHECKING
- switch ignition OFF
- connect high pressure hose of refrigerant recovery/recycling/recharging unit Kent-Moore ACR4, or equivalent, to high pressure service valve
- leave low pressure hose of refrigerant recovery/recycling/recharging unit disconnected
Note
Only a high pressure service valve is used for A/C system servicing and checking pressures. A low pressure service valve is not installed in the A/C system. Perform all A/C system pressure checks through the high pressure service valve.
IF A/C REFRIGERANT SYSTEM BASE PRESSURE IS LESS THAN SPECIFIED ON CHART, SYSTEM IS LOW ON REFRIGERANT.
- check system for leaks using halogen leak detector Hitec HI400A-TEL or equivalent, repair system as necessary
- check pressure relief valve on A/C compressor.
IF THE PRESSURE RELIEF VALVE HAS BLOWN,
- check triggering of coolant fan according to wiring diagram
- check for pinched, kinked or damaged refrigerant lines/hoses
IF NO PROBLEMS ARE FOUND,
- discharge A/C system, then evacuate and recharge
IF A/C REFRIGERANT SYSTEM PRESSURE CORRESPONDS TO OR IS HIGHER THAN BASE PRESSURE SPECIFIED ON CHART,
- start engine
- adjust A/C to maximum cooling
^ A/C compressor clutch engages and compressor is driven
Note
If A/C clutch is not engaged, check voltage supply to A/C clutch according to wiring diagram. If voltage is OK, repair (replace) A/C clutch.
- raise Engine Speed (RPM) to 2000
- note pressure on A/C high pressure gauge Specified high pressure value
^ high side (discharge) pressure increases from base pressure (engine OFF) to maximum 20 bar (290 psi)
Note
For switching pressure specifications of A/C refrigerant low pressure switch and A/C refrigerant high pressure switch.