HOW DOES AIRCON WORK?
The engine drives a compressor (sometimes referred to just as a pump) that sucks the refrigerant gas in the pipe from the evaporator (that's the heat-exchanger next to the heater) and compresses it to quite a high pressure. This pressure can vary a lot but typically in summer would be around 250 psi. Gas compressed this much heats up considerably. This hot, high-pressure gas leaving the compressor is piped to the front of the car where it is connected to a radiator capable of containing these high pressures.
As cold air passes through the radiator it cools the gas sufficiently to turn it into a liquid in exactly the same way that if hot steam is cooled it condenses back into water, so by the time the refrigerant has passed right through this radiator it is in liquid form. The liquid (still at high pressure remember) comes out of the pipe from the bottom of this radiator (called a condenser) and is piped back towards the cabin of the car where it enters into the heat exchanger which provides the coldness.
The hot high pressure liquid is directed through a restriction in the pipe like a pin hole where it is squirted through in a fine spray similar to an aerosol into this heat exchanger (called the evaporator). This fine spray now finds itself in an area of very low pressure (remember this is where we came in - it is the area that the compressor is sucking). These tiny droplets of liquid now have the room to expand and turn back into a gas, and they long to do this, but to evaporate back into a gas they have to absorb some heat (stretch your mind back to school-days, 'Latent Heat of Vaporization and all that', is it coming back to you?).
It finds the heat necessary to return to a gas by stealing some of the heat from the car's interior - it takes what it needs and leaves you with just a little heat - perhaps five degrees Celsius. Aha - this is just what you want. You feel naturally that the AC is giving off cold, but what is really happening is that by evaporating it is taking your heat away and leaving you with a lack of heat - which is, of course, what we call coldness.
There are one or two niceties about the system that we don't need to go into here but that basically is the whole system, with the gas being compressed, condensed into liquid and returned to a gas in a continuous cycle. It is the change of state from liquid to gas which achieves the cooling, and if for any reason there is insufficient air passing through the front radiator (the condenser) then the gas will not change state but remain as a hot, high-pressure gas and will complete the circuit back to the compressor still as a gas and there will be no cooling.
It can be seen then that it is vitally important that the cooling fans are operating properly, both electro fans and any engine fans, whether they are directly coupled, viscous coupled or clutch operated. They may be perfectly adequate to keep the engine cool but the extra task of coping with the AC may sort the sheep from the goats. Viscous coupled fans in particular have a finite life and eventually need replacement, as they may no longer be capable of shifting the quantity of air they did in their younger days. An electro fan that may not have had to do any work for six months because the refrigerant has run down and has not been recharged for a time may easily seize up and refuse to work again (particularly common with early Ford Scorpios and some BMW’s).
When you press the AC button a sensor checks that there is sufficient refrigerant in the system to not damage the compressor and assuming that there is, it allows 12 volts to flow to an electro-magnetic clutch on the compressor. At this point you can usually hear a distinct click as the clutch is pulled in by the electro-magnet and the compressor starts to turn at the same speed as the belt pulley. Within 15 seconds some cooling can be felt but it may take a minute or so to achieve the lowest temperatures. It is normal for the system to have other sensors to monitor for excessively high pressures (over about 450 psi) and to turn off the compressor to avoid the possibility of damage, and also another to turn on or to increase speed of an electro fan when pressures rise to about 275 psi - this pressure will be rapidly reduced by this speeding fan.
The temperature of the evaporator (the heat-exchanger inside the cabin) is usually maintained at just over freezing point either by a thermostat or by a switch or a valve that controls the pressure and thence the temperature. This reservoir of coldness may now be blown in to the cabin very quickly or only gradually, depending on the settings you have chosen.
This description of a typical AC system is sufficiently accurate for illustration but some slight differences may be obvious in your own vehicle. For example many of the earlier systems used the GM Axial 6 cylinder compressor utilizing a strange looking 3 legged fuse, this system does not check for sufficient refrigerant pressure as stated in the paragraph above, but will deliberately blow the fuse if the compressor detects that it would be damaged by an insufficient refrigerant charge - Jaguars were the most frequent users in the UK.