The factory a/c compressor can be used, but with some drawbacks that turn most people away.
The factory compressors need to be pre-oiled. (definition- the air intake of the compressor needs to have an oiling element installed; they can be purchased from Kilby Enterprises at OnBoardAir.com) The reason for this relates to the coolant the compressor pumps.
Older a/c compressors used R12 refrigerant whereas the newer pumps use R134A.
The models using the older R12 fluid had separate oil and refrigerant chambers within the pump that used an oil sump design similar to a standard engine keeping the oil and coolant apart.
Newer R134A compressors use refrigerant that acts as the oil in the compressor itself as they lack a separate oil sump. The idea of pre-oiling the compressor is to simulate the coolant(R134A) the compressor would normally pump. With this in mind, the newer style pumps require more maintenance and attention than older pumps. It can be done with great success but at the cost of increased maintenance.
I have been running older R12 Ford rotary pumps from a very common 5.0L Ford engine (mustang, crown vic, thunderbird) from the early seventies to the early nineties. Another vehicle that used this pump was the Volvo 240DL from the eighties; the pump is located under the power steering pump on the driver's side. These pumps are harder to pluck from the vehicle but work equally well if no Fords exist. The rotary notation is based on the swash-plate style that differentiates it from the piston-style York pumps. The place to find said pumps is any standard junk yard; these cars are abundant and the pumps can be had for roughly $35 Southern California price. This is one pump that works great and has proven itself for over 300 psi on a daily driven truck that sees more than 100 miles daily.
Another pump that is commonly used is a dual piston-style model manufactured by York and carries the 210 nameplate. There are many other models available but the 210 is the highest displacement pump in the lineup. Accordingly it is also the largest case design; the 206 and 209 models have a smaller case and are usually found in the Air Lift Air Commander engine driven kits. These work well, but I highly recommend installing a used pump as the oil blow-by on the newer pumps is very high and takes quite some time to subside. I have used the York 210 Pump with great success on two more daily driven vehicles.
The choice between the York and rotary Ford pumps is one that can be made with the amount of room available in the target vehicle. In my Ford Ranger, I used the rotary pump because it has a lower deck height and easy to mount ears on the bottom of the pump. I used this same pump on a 2.2L Chevy s10 and it fit perfectly in the factory a/c pump location and looks very close to factory if left un-painted. On my 93 Toyota pickup with the 22re engine and turbo I was able to sqeak in the York 210 compressor which works awesome.
It is also important to note that both compressor styles require a coalescing filter (water/oil separator) and a steel brail lead line approximately 3 feet in length to dissipate the heat; trust me they get very hot.
-Written by Scott Thompson, MT username "REDOVAL"