The a/c is controlled via the engine ecu. When you hit the dash switch, it
calls to the ecu to engage the compressor, which it will do it conditions
allow- engine temp, throttle position etc. The twin condensor fans are also
controlled by the ecu, and when a/c is called for, these will start about 3
seconds after pressing the button, followed by the compressor clutch 2-3
seconds after that. You'll probably hear a click as it does.

Looking at the a/c compressor with the engine running, you'll see the pulley
spinning with the belt, and if the clutch is engaged the hub of the pulley
will also be spinning. The low pressure (larger) line coming from the
evaporator at the bulkhead will be covered in moisture and be cold.

There are two safety switches in the system, which disable it if there is
insufficent gas or if the pressure rises too high.

If your a/c is dead altogether first check the fuse in the engine bay fuse
box, and then suspect its low on gas.

Recharging is normal practice on a healthy system about every 3 years,
through natural loss. If it needs more than this, you have a leak. Properly
and regularly exercising the a/c all year round is the key to keeping the
gas in it- 5-10 mins per week, every week. Without use, the seals dry up.
This is potentially costly to fix due to finding the source of the leak.

The evap anti-freeze thermostat cuts the system out at around 4 deg c, which
is why the compressor cycles. Exercising it when the ambient temp is around
4 deg requires heating the cabin and using the recirc button.

If you have no experience of a/c then i strongly recommend you entrust
regassing to a specialist, - shop around and go more on good reports from
other people than price.

On a poorly working system when re-gassing, its far better to vacuum the
system entirely and re charge with dye and the sticker recommended weight.

If its not working at all- vacuum, charge with nitrogen and check for leaks,
which they'll surely be, before vacuuming again, adding oil and gas.

Hope this helps abit.

Tim..