General Information
All test conditions marked * are vehicle-specific and are described in the Service Information for the relevant vehicle.
Check cooling performance.
Connections with valve and service connections for measurement and testing. Refer to Refrigerant Circuit .
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 compressor once the A/C 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 is cold and the pressure in the low pressure side quickly increases after it is turned off, the expansion valve closes and the refrigerant flows slowly into the low pressure side). If the A/C compressor is switched on, the pressure on the low pressure side goes down, the expansion valve open and the refrigerant can flow of the low pressure side.
Under certain operating conditions, residual moisture in refrigerant circuit can lead to an ice build-up at A/C Compressor Regulator Valve. A/C compressor control is reduced by this ice build-up, 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 Action:
-- On vehicles as of model year 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"). Is the measured value from the sensor is below the operating condition described by the customer (at an temperature above 0 °C (32 °F), longer when it is lower than 0 °C (32 °F) although the A/C Compressor Regulator Valve -N280- is not currently activated) or too high (above 10 °C (50 °F) even though the A/C is working correctly). Can ice up caused by the incorrect measured value from the evaporator. Use the Vehicle Diagnostic Tester ("OBD" or "Guided Fault Finding for the A/C system") and Refrigerant Circuit .
-- In vehicles without the Evaporator Vent Temperature Sensor -G263-, for example via the Footwell Vent Temperature Sensor -G192-, check vent temperature at the adjustment: "Lo temperature" for driver and passenger side, 4 or 5 bar (58 or 73 psi) for fresh air blower RPM, air outlet in footwell and fresh air operation under operating conditions specified by customer. If measured value of sensor is too low (at ambient temperature above 0 °C (32 °F), colder than 0 °C (32 °F) for a long period of time).
-- Check refrigerant line from evaporator to accumulator (thick tube, low pressure side) with 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 refrigerant circuit, replace accumulator or receiver/dryer with dryer and evacuate refrigerant circuit for a minimum of three hours.