A vehicle air conditioning system does not create cold. It moves heat out of the cabin and dumps it at the front of the vehicle, using a refrigerant that changes state as it goes round the loop. Everything you diagnose comes back to where in that loop the refrigerant is, and what pressure and temperature it is at.
The high side
The compressor draws low-pressure vapour from the evaporator and compresses it. The vapour leaves hot, typically 70 to 90 °C, and at 12 to 18 bar on a warm day. It travels through the discharge hose to the condenser, which sits in the airflow ahead of the radiator. As the airflow carries the heat away, the vapour condenses into a liquid.
That liquid passes through the receiver drier, where desiccant traps any moisture and a filter catches debris. On an accumulator system the drier sits on the low side instead, after the evaporator.
The metering point
The expansion valve or orifice tube is the boundary between the high and low sides. It restricts flow so that pressure drops sharply, and the liquid refrigerant begins to boil. A thermostatic expansion valve modulates that restriction using a sensing bulb on the evaporator outlet; an orifice tube is a fixed restriction and relies on the compressor cycling to control evaporator temperature.
The low side
In the evaporator, the boiling refrigerant absorbs heat from cabin air blown across the fins by the blower motor. Moisture condenses out on the cold surface and drains away, which is why a working system also demists the screen so quickly. The now low-pressure vapour returns to the compressor and the cycle repeats.
Why symptoms mislead
- Weak cooling only in traffic usually means airflow across the condenser has been lost, not that the compressor is failing.
- Cooling that starts well and fades after ten minutes points at moisture in the system freezing at the metering device.
- A compressor that will not engage is far more often a pressure switch, relay or low charge than a dead clutch coil.
- An oily film on a fitting is refrigerant oil escaping with the gas, and marks the leak point.