The AC compressor has an electric clutch. the pulley rotates all the time, and when there is a need for cooling, a signal to a relay sends power to the clutch coil, which pulls the clutch part which is connected to the compressor. when activated, the pulley then provides movement for the compressor, which compresses the R134a, and pulls from the low side.
there are quite a few details, fluid dynamics, highly dependent on temperature and humidity. there are dryers, expansion valves, filters and screens. there is a high pressure sensor which can cut out, and the low pressure switch which can act up. the control is handled by the computers scattered around. the computers can decide not to operate the AC if other things are going on, like if the engine or transmission are overheating (or if it thinks it is) (Roadie may have more specific details)
to replace the compressor, you have to open up the refrigerant circuit. since the R134a is a potent greenhouse gas, and the older R12 was indirectly eating the ozone layer (the good one that blocks nasty parts of the solar radiation, not the bad lower layer kind that makes smog) you are expected to recover the refrigerant for reuse or recycling.
the R134a recovery is not a once in a while DIY.
the compressor comes with a clutch, and if you want to just get the clutch, you can do so. I searched on E-bay, and found a rebuilder willing to post a listing for a rebuilt clutch kit. coil, bearing, clutch, pulley, and shims (the clearance has to be just right. not too much, not too little.) you may be able to pull off the clutch plate, and remove one or two shims. the coil can have intermittent opens, or high resistance which makes it weak. the clutch can wear a bit over many years of on and off. which can misadjust. there also can be little bits blocking parts of the circuit, or the compressor can just wear out, jam up, and throw stuff through the pipes.
first step is to figure out what exactly is going on. for the AC Compressor clutch to be the problem, you want to verify that the signal is to the coil, but the coil is not engaging. (some folks have set up conditions for the ac and gave the clutch a gentle help with a broom handle or such, being careful not to get caught in the other moving parts)
my AC crapped out, but only when it is hot (over 95f. first happened last summer when it was over 100, and when stuck in traffic) I also had a slow leak. I put R134a with some stop leak, and have set up some software to watch the request for AC, AC relay signal, and the high pressure reading. also coolant and trans temperatures. my plan is to verify that it is the clutch crapping out, and when/if it does, try to change the clutch. I have heard you can get the clutch off without pulling the compressor off, and perhaps without pulling the fan clutch and fan blade (for clearance)
I ended at E-bay:
I searched for air parts inc.
And sent a message from inside E-Bay to airpartsinc. And told them what I wanted, (for car model, year and so on.)
They sent back a link to an e-bay post, which included shipping.
eBay My World - airpartsinc
and click on contact member ( you will need an e-bay account.)
if you do not have e-bay account, you could also call:
AIR PARTS INC. 1133 N. Magnolia Ave Ocala, Florida 34475 Toll Free # 800-223-7167
there are places on line which will sell you the various parts. with a compressor you may want to change the dryer and expansion valve. some compressors require a flush be performed for warranty. if you get it open and replace parts, you at least have to pull a vacuum to remove air and more important water. the right amount of oil has to be in, as well as the proper amount of R134a.
the AC is a great thing when it works, when it goes a little bad, it can drive you nuts.
my brother said he spent $800 a year on AC for his old Astro, every year a different thing like four years in a row. (his case included labor since he was not DIY'ing any of it)
Plenty of U tube to keep you busy figuring it out.