That York is an AC compressor but from a different vehicle. It works just like a shop compressor, a motor turning a compressor (which intakes air, compresses it then discharges it). In this case the motor is the 4Runner motor and the compressor is the York. Yorks are popular because they are plentiful, can be had for cheap, can be mounted on their side, have a sealed oil crank case and they have high output.
It's hooked up to a pressure switch, just like an AC compressor. When the pressure is below a set pressure the clutch engages and the compressor starts sucking in air and compressing it, sending it through my system. When the high side set pressure is achieved the clutch disengages and the compressor stops sucking in air.
It's all pretty easy as the compressor does the work. I bought a kit but they aren't made any more. It would be easy to make. A bracket, combo pulley to add an additional belt, switch, pressure switch and fittings/air lines. For as simple as it is it can be pretty expensive. I'd say it could be done for less than $100 if you had some scrap metal, know how and good imagination. Mine was more :bigok:
