Checking the Pressure with the Ignition Switched On
The switching pressures for the activation of the A/C Compressor Regulator Valve -N280-, the A/C Clutch -N25- (if equipped) and the radiator fan (for example the Radiator Fan -V7- / Radiator Fan 2 -V177-) are vehicle specific. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function of the A/C system.
Pressures must be measured at the service connections; the component location for these connections are vehicle-specific. Refer to Refrigerant Circuit (vehicle-specific Service Information).
After completing the specified procedures disconnect the A/C Service Station from the refrigerant circuit and switch it off. Refer to A/C SERVICE STATION, SWITCHING OFF AND DISCONNECTING FROM REFRIGERANT CIRCUIT .
- The engine is warm (at an ambient temperature below 25 °C (77 °F)).
If the engine is not warm perform a road test.
For the cooling output test the engine must be warm but at an ambient temperature smaller than 25 °C (77 °F) a specific A/C system cooling output it is necessary that the engine and lower ambient temperatures are warm.
- Observe the test requirements. Refer to Test Requirements .
-- On vehicles with a high-voltage system, switch off (deactivate) the "auxiliary climate control" function. Refer to the Owner's Manual and Infotainment/MMI Operating Manual.
-- Switch off the ignition.
-- Connect the A/C Service Station. Refer to A/C SERVICE STATION ON REFRIGERANT CIRCUIT, CONNECTING .
-- Check the pressure in the refrigerant circuit which the engine not running.
-- Take the A/C Service Station pressure gauge readings (two possible results):
- Pressure in refrigerant circuit lower than indicated in following table. Refer to The Refrigerant Circuit Pressure is Lower than Indicated in Table .
- The refrigerant circuit pressure is in line with following table or is higher. Refer to The Refrigerant Circuit Pressure Is in Line with Table or Is Higher .
Refrigerant R1234yf vapor pressure table. Refer to REFRIGERANT VAPOR PRESSURE TABLE .
For vehicles with a high-voltage system and additional A/C system functions ("heat pump operation" or "cooling the high-voltage battery"), valves may be installed in the refrigerant circuit that cannot be opened without current. These valves are opened and closed for example via a step motor and after switching off the ignition are no longer activated. Depending on the last operating condition of the A/C system the specific area of the refrigerant circuit are shut off via these valves. Depending on the layout of the service connections in the refrigerant circuit they can be in the area which was shut off (for example the area with the high pressure side service connection). If necessary, these valves must be opened before checking the pressure. Refer to Refrigerant Circuit (vehicle-specific Service Information) and the Vehicle Diagnostic Tester in the "Guided Fault Finding" function.
The pressure specified in the following table is slightly lower than the specified vapor pressure table for the refrigerant R1234yf. The vapor pressure of the refrigerant in the refrigerant circuit adjusts itself to the coldest component and can for this reason be slightly lower than the vapor pressure table for the refrigerant R1234yf also with the refrigerant circuit correctly filled.
| Ambient Temperature | Pressure in Refrigerant Circuit Positive Pressure |
|---|---|
| +15 °C (59 °F) | 3.5 (50.76 psi) |
| +20 °C (68 °F) | 4.4 (63.81 psi) |
| +25 °C (77 °F) | 5.3 (76.87 psi) |
| +30 °C (86 °F) | 6.3 (91.37 psi) |
| +35 °C (95 °F) | 7.4 (107.32 psi) |
| +40 °C (104 °F) | 8.6 (124.73 psi) |
| +45 °C (113 °F) | 10.0 (145.03 psi) |
The temperature of the refrigerant circuit components should be equal to the ambient temperature. Pressure will deviate from the values in table if individual components of the refrigerant circuit are warmer or colder.
At absolute pressure, 0 bar (0 psi) corresponds to absolute vacuum. Normal ambient pressure (positive pressure) corresponds to 1 bar (14.5 psi) absolute pressure. 0 bar (0 psi) pressure corresponds to an absolute pressure of one bar on most pressure gauges (indicated for example by -1 bar (-14.5 psi) below 0).
On vehicles with a pressure sensor (High Pressure Sensor -G65-, Refrigerant Circuit Pressure Sensor -G805-, A/C Pressure/Temperature Sensor -G395- etc.), by which the pressure in the measured values of the diagnostic is displayed, if the measured value is aligned to the values in the table (or slightly higher). Use the Vehicle Diagnostic Tester in the "Guided Fault Finding" function of the A/C system and the appropriate service information (vehicle-specific Service Information).
Pressure is measured in different units: 1 MPa (mega Pascal) corresponds to 10 bar (145.04 psi) positive pressure or 145 psi. 1 bar (14.5 psi) absolute pressure corresponds to 0 bar (0 psi) positive pressure and thus to the ambient pressure (atmospheric pressure).
If the measured values are okay and there is no concern:
-- Remove (close) the hand wheel on the quick-release coupling connection. Refer to the A/C Service Station Owner's Manual.
-- Use the A/C Service Station to extract the refrigerant from both filling hoses according to the operating instructions. Refer to A/C Service Station Operating Instructions.
-- Switch off the A/C Service Station (and if necessary disconnect from the power supply). Refer to A/C SERVICE STATION, SWITCHING OFF AND DISCONNECTING FROM REFRIGERANT CIRCUIT .
If the measured values are okay and there is a concern:
-- Check the A/C system cooling output. Refer to the appropriate service information (vehicle-specific Service Information) and at the same time pay attention to the pressure in the refrigerant circuit.
- Refer to MECHANICALLY DRIVEN A/C COMPRESSOR, CHECKING PRESSURE WITH A/C SYSTEM SWITCHED ON .
- Refer to ELECTRICALLY-DRIVEN A/C COMPRESSOR, CHECKING PRESSURE WITH A/C SYSTEM SWITCHED ON
If the measured values are not OK:
-- Determine and correct the cause for the difference.
If the pressure is too low it can be due to little refrigerant in the circuit. Refer to Technical Data (vehicle-specific Service Information) and REFRIGERANT CIRCUIT, DISCHARGING .
If the pressure is to high it is possible that there is too much refrigerant or refrigerant oil in the circuit. Empty the refrigerant circuit with the A/C Service Station (refer to REFRIGERANT CIRCUIT, DISCHARGING ) if the extracted refrigerant quantity is greater than the specified capacity for the refrigerant R1234yf. Refer to Technical Data (vehicle-specific Service Information), evacuate the refrigerant circuit and fill again with the correct quantity or refrigerant R1234yf. Then repeat the test. If the extracted refrigerant quantity corresponds to approximately the specified capacity for the refrigerant R1234yf or is lower, clean the refrigerant circuit with refrigerant. Refer to REFRIGERANT CIRCUIT, CLEANING . Then fill the specified quantity of refrigerant oil, evacuate the refrigerant circuit, fill again with the correct quantity of refrigerant R1234yf and repeat the test again.
If the pressure is too low or high and contaminated refrigerant is determined by a gas analysis it is possible that this is the cause of the difference (contaminated refrigerant have different vapor pressures) empty the refrigerant circuit and then clean with refrigerant R1234yf. Refer to REFRIGERANT GAS ANALYSIS, PERFORMING , REFRIGERANT CIRCUIT, DISCHARGING and REFRIGERANT CIRCUIT, CLEANING .