Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Checking Pressure With Service Station
All test conditions marked * are vehicle-specific and are described in the Repair Information for the relevant vehicle.
Check cooling performance.
Connections with valve and service connections for measurement and testing, refer to Diagnosis and Testing (vehicle-specific repair 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 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 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 vehicle and after a short time, A/C function is OK again.
- After a long drive, windows fog up from inside, windows are also not cleared by 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 below the operating condition described by the customer (at a temperature above 0° C (32 °F), longer when it is lower than 0° C (32 °F) although the A/C compressor regulator valve is not currently activated) or too high (above 10° C (50 °F) even though the A/C is working correctly). An incorrect measured value can cause the evaporator to ice up. Refer to the Vehicle Diagnostic Tester("OBD" or "Guided Fault Finding for the A/C system") and Diagnosis and Testing (vehicle-specific repair information).
-- If the vehicle does have an Evaporator vent temperature sensor -G263-, check the "Lo temperature" for driver and passenger side, 4 or 5 bars for fresh air blower RPM, air outlet in footwell and fresh air operation under operating conditions specified by customer, for example, using the Footwell vent temperature sensor -G192-. 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 fluid reservoir with dryer and evacuate refrigerant circuit for a minimum of 3 hours.
- Test requirements, refer to TEST PREREQUISITES .
- Pressures, checking, refer to PRESSURES, CHECKING .
- Continuation of testing depending on design of refrigerant circuit, refer to CONTINUATION OF TESTING DEPENDING ON DESIGN OF REFRIGERANT CIRCUIT .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.