Test Requirements
Working on the refrigerant circuit with the A/C service station (check the pressure here) usually requires, although not necessary, the high-voltage system to be disabled.
Only de-energize the high-voltage system is procedures are necessary on the high-voltage system. Refer to SAFETY PRECAUTIONS WHEN WORKING ON VEHICLES WITH HIGH-VOLTAGE SYSTEM .
All Vehicles
The specific test requirements are vehicle-specific and described in the Service Information for the vehicle. Refer to COOLING OUTPUT, CHECKING .
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 reduces, the expansion valve opens and the refrigerant can flow to the low pressure side.
- Checking the cooling output. Refer to COOLING OUTPUT, CHECKING .
- Connections with valve and service connections for measurement and testing. Refer to Refrigerant Circuit .
- Connect the A/C service station. Refer to A/C SERVICE STATION ON REFRIGERANT CIRCUIT, CONNECTING .
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 vehicle and after a short time, A/C function is OK again.
- After a long drive, the vehicle windows fog up from the inside, vehicle windows are also not cleared by then pressing the Defrost button, after switching off the vehicle (or switching off the A/C system) and after a short time, A/C system function is OK again.
Corrective Measure:
-- On vehicles with an A/C 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 -N280- is not currently activated) or too high (above 10 °C (50 °F) even though the A/C system is working correctly). Can freeze-up due to incorrect activation of the A/C Compressor Regulator Valve -N280- of the incorrect measured value of the Evaporator Vent Temperature Sensor -G263-. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system. Refer to COOLING OUTPUT, CHECKING .
-- 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" or "cold" for the driver and passenger side, lowest setting (4 or 5 bar (58 to 72.5 psi)) for fresh air blower speed, air outlet in footwell and fresh air operation under operating conditions specified by customer. If the measured value of the sensor is too low (at an ambient temperature over 0 °C (32 °F), over a longer time colder than 0 °C (32 °F)) the evaporator can freeze-up due to the incorrect measured value. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function.
-- On vehicles with shut-off valves in the refrigerant circuit check the function and activation of these valves. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function checking the A/C system. Refer to Refrigerant Circuit and COOLING OUTPUT, CHECKING .
-- With the engine running, check the refrigerant line from the evaporator (or from the inner heat exchanger) to the A/C compressor (thick pipe, low pressure side). 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. Refer to Refrigerant Circuit .
-- Discharge the refrigerant circuit, replace each reservoir/receiver/dryer with dryer or the dryer cartridge, replace and evacuate the refrigerant circuit for at least three hours. Refer to Refrigerant Circuit and WORKING WITH A/C SERVICE STATION .
Test Requirements
- Pay attention to the general information for checking the refrigerant circuit pressure. Refer to GENERAL INFORMATION FOR CHECKING THE REFRIGERANT CIRCUIT PRESSURE .
All of the test requirements marked with a "*" are vehicle-specific and described in the Service Information for the vehicle. Refer to COOLING OUTPUT, CHECKING .
General test requirements (these only have an influence on the pressure gauge display for this measurement with the A/C system switched on)
- Radiator and condenser are clean (clean if necessary) *.
- The thermal insulation at expansion valve is OK and properly installed.*
- Ribbed belt is OK and properly tensioned. Ribbed belt for A/C compressor and generator are OK and correctly tensioned.*
- The drive unit for the A/C compressor is OK and installed correctly. *
- All air ducts, covers and seals are OK and properly installed *.
- The A/C system diagnostic does not detect any malfunctions (when the engine is running and the A/C system is switched on), and no compressor shut-off condition is displayed in the measured values (only for vehicles with "A/C system" diagnostic). Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function for the A/C system.
- Air flow through dust and pollen filter not obstructed by dirt *.
- All instrument panel vents are open *.
The A/C compressor is driven by different components depending on the engine (belt or input shaft). To protect these components and the engine an overload protection in installed in the belt pulley or the A/C compressor drive unit which disengages by a difficult to move A/C compressor. Refer to COOLING OUTPUT, CHECKING and A/C Compressor .
On specific versions the fan is switched on after the refrigerant circuit pressure specified value is exceeded. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function of the A/C system.
Additional test requirements (these have an influence on the display of the pressure gauge on this measurement)
- Vehicle is not exposed to direct sunlight. Refer to COOLING OUTPUT, CHECKING .
- The ambient temperature is greater than 15 °C (59 °F).
- The ignition is switched off.
- The A/C service station in connected to the refrigerant circuit. Refer to A/C SERVICE STATION ON REFRIGERANT CIRCUIT, CONNECTING .