A/C System Pressures, Checking
A/C Refrigerant System (R-134a) Pressures, Checking
Before checking system pressures, check A/C system cooling performance. A/C System Cooling Performance, Troubleshooting
Use A/C system high pressure service valve for checking A/C system pressures.
Test requirements
- 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 VAG1551 Scan Tool (ST) and eliminate/repair any malfunctions (DTCs) 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).
- Fresh air blower runs
- Coolant fan runs
- Fresh air/recirculating air flap in "recirculating air" position
- Temperature flap in "cooling position"
- outside (ambient) temperature above 15 °C (59 °F)
- 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 servicing pressure checking operations through the high pressure service valve.
Outside (ambient) temperature 1) Base pressure in refrigerant system (engine OFF)
+15 °C (59 °F) 3.9 bar (56.5 psi)
+20 °C (68 °F) 4.7 bar (68.2 psi)
+25 °C (77 °F) 5.6 bar (81.2 psi)
+30 °C (86 °F) 6.7 bar (97.2 psi)
+35 °C (95 °F) 7.8 bar (113.1 psi)
+40 °C (104 °F) 9.1 bar (132.0 psi)
+45 °C (113 °F) 10.5 bar (152.3 psi)
1) All components of A/C system must be at the specified temperature for an accurate pressure measurement.
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 H1400A-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 and flush with compressed air and nitrogen, then evacuate and recharge system.
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 compressor is not driven, check voltage supply to A/C compressor clutch -N25- according to wiring diagram. If voltage to compressor clutch is Ok, repair (replace) A/C compressor clutch.
- Raise engine speed (RPM) to 2000.
- Note pressure on A/C high pressure gauge.
Specified pressure values
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. The following low side pressure specifications are included as a guide when checking the A/C system high pressure.
- high side (discharge) pressure increases from base pressure (engine OFF) to maximum 20 bar (290 psi)
- low side (suction) pressure decreases from base pressure (engine OFF) to pressure value, see chart at left
A - High side (discharge) pressure (measured at service valve) in bar
B - Low side (suction) pressure (at compressor or accumulator)
C - Permissible tolerance range
D - Low side (suction) pressure (at evaporator)
E - Permissible tolerance range
NOTE: For switching pressure specifications of A/C refrigerant low pressure switch -F73- and A/C refrigerant high pressure switch -F118-.