Compressor HVAC: Description and Operation
WARNING:
^ The PAG oil dissolves fat and can irritate the skin. Always use protective gloves when you handle the oil.
^ Filling of refrigerant from a 725 gram refrigerant bottle may only be carried out' via the service connection on the low-pressure side. The reason for this is that it is necessary to start the engine to be able to fill the system with the correct amount of refrigerant. If the refrigerant bottle is connected to the high-pressure side when the engine is started, there is a risk that the bottle might explode.
CAUTION: If connections in the A/C system are open, they must be plugged in order to prevent moisture from entering the system. There are specially made plugs for this which can be ordered.
NOTE: During service on.the A/C system you must take the non-return valve into consideration and ensure that draining and filling of refrigerant are carried out on both the high-pressure and the low-pressure side simultaneously.
Technical Description
^ The new compressor is fitted with an over-heating cut-out, which disconnects the compressor when the temperature of the compressor body exceeds +140 °C. The compressor is reconnected when the temperature drops below +120 °C. If the compressor overheats because the amount of refrigerant in the A/C unit is too small, for instance, or if the expansion valve is clogged up, it will cycle (switch on and off a number of times) until the temperature falls.
^ The compressor is equipped with a non-return valve, which is fitted in the inlet on the low-pressure side. This non-return valve prevents the refrigerant from flowing back to the low-pressure side when the compressor stops operating. In this way we have eliminated the risk of noise which would otherwise occur when the compressor starts working again and liquid refrigerant rushes through the compressor.
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Mode Of Operation
The refrigerant arrives at the compressor from the evaporator at low pressure and as a gas. It is compressed in the compressor, which increases its pressure and temperature. The warm, gaseous refrigerant is then forced into the condenser.
The A/C compressor is always on in the Auto mode of operation. When the A/C is on, the ACC control module sends Batt+ to ICE.
If the coolant temperature does not exceed +126 °C, ICE sends Batt+ to the engine control module via the anti-freeze thermostat.
The engine control module compensates idling speed and after a short delay sends ground to the A/C relay via the low-pressure and high-pressure monitor.
When the A/C relay operates, the circuit to the magnetic coupling is closed.
The engine control module breaks the circuit to the A/C relay during heavy acceleration.
In the AUTO mode of operation during normal driving conditions and when the outside temperature is above about +1 °C, the anti-freeze thermostat decides whether the A/C compressor is to run or not. The reason why the A/C compressor runs even when cooling is not required is because the evaporator dries incoming moist air and so prevents the windshield from misting up in damp weather. The A/C compressor can be shut off with the ECON button.
Compressor Oil
PAG oil (poly alcylene glycol), a fully synthetic oil of glycol type, is used in the A/C system.
PAG oil is very hygroscopic, i.e. it absorbs moisture from the air very easily in the same way as brake fluid does. Water is bound chemically in the oil and cannot be removed by vacuum pumping. It is therefore important that the oil is stored in closed containers and that it is not exposed to the air for more than very short periods. The oil is supplied in tubes of 20 ml to minimize the risk of left-over oil being stored for the next time it is needed.