I would like to know how the a/c system works. My 90 IROC when I turn the system to max or norm, it doesn't get cold and I don't here the click from the compressor or the clutch,(not sure which one). Also when I turn it to deforst I don't here the clutch engaging. So can someone tell me how the system works and how I should start troubleshooting this?

Sounds to me like it's lost it's refrigerant, and is time for a recharge. Without any pressure in the system the pressure switch shuts it down. Although it could be something as simple as a blown fuse to.

A/C systems work off pressure variances and the heating and cooling of a refridgerant gas. You can search the subject on the net for details of just how an automobile system works.

The problem your car has is probably a lack of charge causing the low pressure switch to remain open as 1piece stated. You can get system gauges from Sears, SnapOn, MacTools, or even HarbourFreight/Northern Automotive.

You can also jump the low pressure switch on the Dryer canister to see if the system starts up while the car is running and the A/C is set to ON. Further you can check for power and ground at the connector at the compressor.
There is a diode that goes bad sometimes there.

You can get a charge kit from most automotive stores with a single can of R134A and the hose needed to put it in the system, or you can go to a reputable shop and have it checked out.

1990 is probably still R-12 instead of 134-A, unless it's been converted.

What wire or signal causes the compressor to turn on? And what wire to I jumper on the canister?