Pressures, Checking
- Observe the test requirements. Refer to TEST REQUIREMENTS .
-- Turn off the ignition.
-- Connect the A/C service station. Refer to A/C SERVICE STATION, CONNECTING .
-- Take the pressure gauge readings (two possible results):
- Pressure in refrigerant circuit lower than indicated in table
- Pressure in refrigerant circuit in line with table or higher
| Ambient temperature in °C (°F) | Pressure in refrigerant circuit in bar (psi) positive pressure |
|---|---|
| 15 (59) | 3.9 (57) |
| 20 (68) | 4.7 (68) |
| 25 (77) | 5.6 (81) |
| 30 (86) | 6.7 (97) |
| 35 (95) | 7.8 (113) |
| 40 (104) | 9.1 (132) |
| 45 (113) | 10.5 (152) |
Temperature of refrigerant circuit components should be equal to ambient temperature. Pressure will deviate from values in table if individual components of refrigerant circuit are warmer or colder.
At absolute pressure, 0 bar/psi corresponds to absolute vacuum. Normal ambient pressure (positive pressure) corresponds to 1 bar (14.5 psi) absolute pressure. 0 pressure corresponds to an absolute pressure of 1 bar (14.5 psi) on most pressure gauges (indicated by -1 bar (-14.5 psi) below 0).
On vehicles with High Pressure Sensor -G65-, Refrigerant Circuit Pressure Sensor -G805- or A/C Pressure/Temperature Sensor -G395- for which measured pressure is displayed in measured value block, pressure measured should coincide. Refer to Vehicle Diagnostic Tester ("OBD " or "Guided Fault Finding " for the A/C system) and OVERVIEW - REFRIGERANT CIRCUIT (vehicle-specific repair information).
Pressure is measured in different units: 1 MPa (145 psi) corresponds to 10 bar (145 psi) positive pressure. 1 bar (14.5 psi) absolute pressure corresponds to 0 bar/psi positive pressure and thus to the ambient pressure (atmospheric pressure).
Pressure in refrigerant circuit lower than indicated in table
Not enough refrigerant in refrigerant circuit
-- Determine refrigerant circuit leaks. Refer to REFRIGERANT CIRCUIT, DETERMINING LEAKS .
-- Check the pressure relief valve.
If pressure relief valve has responded:
-- Check the coolant fan activation.
-- Check the refrigerant lines and hoses for cross-section constrictions caused by inadequate bending radii.
-- Check the refrigerant lines and hoses for external damage.
-- If no fault is found, clean the refrigerant circuit (flush using refrigerant R134a. Refer to REFRIGERANT CIRCUIT, CLEANING (FLUSHING), WITH REFRIGERANT R134A ; or blow through using compressed air and nitrogen. Refer to REFRIGERANT CIRCUIT, FLUSHING WITH COMPRESSED AIR AND NITROGEN ).
Pressure in refrigerant circuit in line with table or higher
-- Start the engine.
-- Set the A/C system to maximum cooling output.
On vehicles with A/C Compressor Regulator Valve -N280-, the control current can be read in the measured value block. Refer to Vehicle Diagnostic Tester (Function "OBD " or "Guided Fault Finding " of the Air Conditioning) and ADDITIONAL INFORMATION FOR VEHICLES WITH A/C COMPRESSOR WITHOUT A/C CLUTCH, (ONLY WITH A/C Compressor Regulator Valve -N280-) (vehicle-specific repair information).
If A/C compressor is not driven with the engine running or regulating valve is not actuated:
-- Determine and eliminate cause, for example by checking A/C system DTC memory.
-- Observe the test conditions.
-- Check the A/C Clutch -N25- voltage supply. If it is OK, repair the A/C clutch.
-- Check the A/C Compressor Regulator Valve -87- activation. Refer to Vehicle Diagnostic Tester (Function "OBD " or "Guided Fault Finding " of the Air Conditioning) and ADDITIONAL INFORMATION FOR VEHICLES WITH A/C COMPRESSOR WITHOUT A/C CLUTCH, (ONLY WITH A/C Compressor Regulator Valve -N280-) (vehicle-specific repair information).
If the low pressure switch was removed to connect the service station, bridge the electrical connections in the corresponding connector for the pressure measurement.
A/C compressor is driven by the engine via A/C Clutch -N25-.
The A/C Compressor Regulator Valve -N280- is activated by the Front A/C Display Control Head -E87- or Climatronic Control Module -J255-. Refer to Vehicle Diagnostic Tester (Function "OBD " or "Guided Fault Finding " of the Air Conditioning) and ADDITIONAL INFORMATION FOR VEHICLES WITH A/C COMPRESSOR WITHOUT A/C CLUTCH, (ONLY WITH A/C Compressor Regulator Valve -N280-) (vehicle-specific repair information).
- Continuation of testing depending on design of refrigerant circuit. Refer to CONTINUATION OF TESTING DEPENDING ON DESIGN OF REFRIGERANT CIRCUIT .