MONTE CARLO Factory 1986 AC; where do I start?

Here's a tip...easy way to check to see if the compressor is locked up before you put a belt on it...

Turn ignition on. Turn on A/C to Max or Norm, whatever, as long as the compressor clutch should be "on". If you hear the clutch "click" in and engage the compressor, then you likely have enough system pressure to keep the low pressure cutout switch from keeping the clutch disengaged. You'll hear the clutch kick in because it's kind of a distinct click noise. If in doubt, move it back to off, then back to where you had it and listen for it. If the clutch does engage, then try to turn the pulley by hand, preferably in the same direction it would as if it were running. This will tell you if it is free or locked. This will require a little effort to turn, but if it's locked, you likely won't budge it at all. The compressor is a pump so even if it's free, it will require varying degrees of effort as you spin it around. You can feel it trying to work. Likely a good sign if you get that far.

If the clutch does not engage when you turn on the A/C system, turn it back off and get under the hood on the passenger side. If you have a small wire to jump the low pressure cut out switch plug terminals (switch is located on back of the dryer- your last picture shows the top of your dryer and the switch is at the top) you can check the clutch by bypassing that low pressure switch. I don't recommend doing that except for this test if it goes that far. Just squeeze the opposite areas of the clips on the plug to release the clips so you can pull it straight off. While it's disconnected, use the jumper wire in the plug from prong to prong to complete the circuit. Then check the A/C clutch action again by turning the A/C back on. If you hear the clutch engaging, then your system is very low or empty, and likely suffered a leak somewhere.

Hope this helps you start diagnosing where you stand at least. As always, any advice given here comes with no liability or assurances on my part that it will work as described. In other words, consider it and use it at your own risk. I'm sure others with a lot of experience and a better set of tools can guide you better. But I've checked clutches this way before and it worked for me. Good luck.

Brian is right R-12 is not outlawed I should have used a less dramatic term. But it may as well be as the last time I saw an original 30lb can of it for sale it was going for over $1000!?! Is there an R-12 cartel supplying underground R-12? I agree that if everything is going to be replaced anyway you may as well change over to R134a and call it a day. But again, my Bonnewagon is running the original 35 year old components except for the compressor and I bought that as a used remanufactured 4 Seasons unit from a guy on here for like $50. I probably changed the accumulator too as that is the right thing to do on an opened system. I used ES-12 which is an R-12 equivalent freon and in my Mom's Cutlass Cierra I used something called Freeze 12, another R-12 equivalent freon. Both worked great and nothing else had to be converted.

Bonnewagon is right about the accumulator, it should be changed as well....I have been fixing too many late model imports with driers on the condensers, i guess.

Save yourself some headaches and call Vintage Air and order a new under dash unit for your car , they work great,and you don't have to use 30 year old technology.

No problem , they are located in San Antonio, Tx I use them all the time .

Are you sure the compressor itself turns freely, or just the clutch? You would need to turn the shaft of the compressor with a socket and bar to see if it actually turns, unless the system's pressure switch was jumpered to "make" it run.

I have been involved with aftermarket air in a couple streetrods, work great, but prefer the OE system for originality/appearance. The OE system works well, when it is in optimum condition.

Engine swap, AC upgrade all sound good. But in the meantime why not just get a belt and see what you have. Looking back over your pics the system looks to be all original and in very good condition. That is rare. I also see you have a "V" belt for the AC and a serpentine belt for everything else. That may be why it is missing. I would invest in a belt and if the compressor is locked up you will know immediately. But if it runs and there is any pressure at all still in the system, then a couple of cans of R-12 equivalent may get you up and running. Some cans even come with a small go/no-go gauge on the charge hose.