For an A/C system to be functioning correctly, you need:
In the cabin:
- something to exchange cold (an evaporator)
- something to move air over the evaporator (the fan)
- a working blend door that allows the user to select heat or cold
In the engine compartment:
- something to exchange heat (a radiator)
- something to move air over the radiator (radiator fan)
- a closed loop, containing refrigerant, through which refrigerant travels
- a compressor, which converts the gas into a liquid for 1/2 of the loop.
- a filter containing silica desiccant to remove moisture from the loop.
- a pressure switch which acts a safety in case the loop is not pressurized.
If the loop has pressure, and the switch is working correctly, the compressor activates. You will see the clutch on the compressor engage: this usually looks appears as an inner disc and outer disc. When the compressor is off, only the outer disc spins (the one attached to the serpentine). When the compressor is running, both discs spin as one.
Your lack of A/C could be due to:
- no compressor activity (due to insufficient refrigerant in the loop, a bad switch, or a faulty compressor)
- intermittent compressor activity, but not enough to cool (due to insufficient refrigerant in the loop or a faulty compressor
- a faulty fan in the cabin
- a broken blend door in the heater box.
Start troubleshooting this by observing the compressor. Does it engage when you turn the A/C system on? Ideally, have two people for this -- one watching the compressor, the other turning on the A/C. If it runs at all, does it only run for a few seconds, and then turn off? When the compressor kicks on, you will see the pressure drop substantially.
Additional checks: Does the heater fan work? When you switch between heat and cold, can you hear doors moving around? Can you turn off heat entirely on a cold day? (not being able to turn off heat indicates a blend door issue).
Last comment, and I'm reluctant to even bring this up: If the issue is your pressure switch, it is possible to "jump" this connection to force the compressor to come on. By closing the circuit that the switch is plugged into, it sends an "all clear" signal that the compressor can start. You do not want to do this for any significant amount of time. A working switch correctly senses low refrigerant levels; running your compressor without refrigerant will destroy it. If you bypass the switch and the compressor comes on, and the air starts to become cold, your switch probably needs to be replaced.