| Checking pressures in refrigerant circuit with air conditioner service station (with ignition switched off) |
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| All test requirements marked with an * are vehicle-specific and are described in the Workshop Manual for the relevant vehicle. |
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| Depending on the version of the air conditioner compressor, the high-pressure end of the air conditioner compressor may be fitted with a valve to prevent the backflow of liquid refrigerant into the compressor after switching off the air conditioning system. If an air conditioner compressor with this valve is fitted on a vehicle with a refrigerant circuit with an expansion valve, a relatively long period may elapse before the pressure on the high-pressure end decreases (the expansion valve is cold and the pressure on the low-pressure end increases rapidly after switch-off, the expansion valve closes and the refrigerant can only flow slowly to the low-pressure end). If the air conditioner compressor is switched on, the pressure on the low-pressure end decreases, the expansion valve opens and the refrigerant can flow to the low-pressure end. |
| Under certain operating conditions, residual moisture in the coolant circuit may lead to the formation of ice at the air conditioner compressor regulating valve. Such ice formation impedes the control of the air conditioner compressor. The evaporator is cooled down too much and ices up. An iced-up evaporator may cause the following problems: |
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| The air conditioner fails repeatedly or sporadically (no cooling/heating output) after long journeys; after the ignition is switched off and waiting a short while, the air conditioner functions properly again. |
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| Misting up of the windows on the inside after a long journey; the windows are initially not cleared even by pressing the Defrost button; the air conditioner functions properly again after a short delay following engine shut-off. |
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| In the case of vehicles as of Model Year 2001 and an air conditioner compressor with air conditioner compressor regulating valve -N280-, check the measured value of the evaporator output temperature sender -G263- (by way of the „Reading measured value block“ function). If the sender measured value is too low under the usage conditions outlined by the customer (at ambient temperature above 0 °C, colder than 0 °C for a lengthy period although -N280- is currently not being actuated) or too high (greater than approx. 10 °C although the air conditioner is functioning properly), the evaporator may ice up due to the incorrect measured value → Vehicle diagnostic tester („self-diagnosis“ or „Guided Fault Finding“ function for air conditioner) and → Heating, air conditioning; Rep. gr.87 or → Air conditioning; Rep. gr.87 (vehicle-specific workshop manual). |
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| On vehicles without a evaporator output temperature sender -G263- use for example footwell vent temperature sender -G192- to check vent temperature under operating conditions described by customer in the following settings: "Lo temperature" for driver's and passenger's side, 4 or 5 bars for fresh-air blower speed, air outlet to footwell and fresh-air mode. If the measured value of the sender is too low (at ambient temperature above 0 °C, colder than 0 °C for lengthy period). |
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| Check refrigerant pipe between evaporator and reservoir (thick pipe, low-pressure side) with engine running. If this pipe is severely iced up when the problem occurs (a thin ice layer is permissible), this is a further indication that the temperature in the evaporator is too low. |
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| Discharge refrigerant circuit, renew reservoir or receiver with dryer and then evacuate refrigerant circuit for at least 3 hours. |
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| Continuation of testing depending on design of refrigerant circuit → Chapter |
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