Refrigerant Circuit, Checking Pressure With Service Station: Notes
All test conditions marked * are vehicle-specific and are described in the service information for the relevant vehicle.
Check the cooling output *.
Connections with valve and service connections for measurement and testing. Refer to Refrigerant Circuit (vehicle-specific service information).
Depending on the A/C compressor version, there may be a valve installed on the high pressure side of the A/C compressor, which prevents the liquid refrigerant from flowing back into the A/C compressor once the A/C system is turned off. If an A/C compressor with this valve is installed in a vehicle with a refrigerant circuit having an expansion valve, then it may take some time until the pressure in the high pressure side decreases (the expansion valve is cold and the pressure in the low pressure side quickly increases after it is turned off, the expansion valve closes and the refrigerant can flow slowly to the low pressure side). If the A/C compressor is switched on, the pressure on the low pressure side decreases, the expansion valve opens and the refrigerant can flow to the low pressure side.
Under certain operating conditions, residual moisture in refrigerant circuit can lead to an ice build-up at compressor regulator valve. This ice build-up interferes with A/C compressor regulation. The evaporator is cooled too intensely and freezes. The freeze-up of the evaporator can be the cause for the following customer complaints:
- After a long drive, A/C system repeatedly or sporadically fails (no cooling or heating performance). After switching off the vehicle and after a short time, A/C function is OK again.
- After a long drive, windows (front, side and rear windows) fog up from inside, windows are also not cleared by then pressing the Defrost button, after switching off vehicle and after a short time, A/C function is OK again.
Corrective Measure:
-- On vehicles from MY 2001 equipped with a compressor with A/C Compressor Regulator Valve -N280-, check measured value of evaporator outflow temperature Evaporator Vent Temperature Sensor -G263- (via function "Read measuring value block"). If the sensor measured value is too low under the operating conditions described by the customer (at an ambient temperature above 0 °C (32 °F), longer when it is lower than 0 °C (32 °F) although the A/C Compressor Regulator Valve A/C Compressor Regulator Valve -N280- is not currently activated) or too high (above 10 °C (50 °F) even though the A/C system is working correctly). Can ice up due to the incorrect evaporator measured value. Use the Vehicle Diagnostic Tester ("OBD" or "Guided Fault Finding for the A/C system" function) and refer to Refrigerant Circuit (vehicle-specific service information).
-- If the vehicle does not have an Evaporator Vent Temperature Sensor -G263-, check the vent temperature via, for example, the Footwell Vent Temperature Sensor -G192- at the "Lo temperature" for the driver and passenger side, 4 or 5 bars for fresh air blower speed, air outlet in footwell and fresh air operation under operating conditions specified by customer. If the sensor measured value is too low (at ambient temperature above 0 °C (32 °F), colder than 0 °C for a long period of time).
-- Check the refrigerant line from the evaporator to the reservoir (thick tube, low pressure side) with the engine running. If this line is thickly iced-up when complaint occurs (a thin layer of ice is permitted), this also indicates that the temperature in the evaporator is too low.
-- Discharge the refrigerant circuit, replace the reservoir or the receiver/dryer with dryer and evacuate the refrigerant circuit for a minimum of three hours.