This air conditioning compressor circulates refrigerant through the sealed loop of the cab climate-control system. It is driven by a belt from the engine and compresses low-pressure vapor from the evaporator into high-pressure gas for the condenser. A failing compressor typically produces warm airflow from the vents, unusual noise from the belt-driven pulley, or visible oil seepage around the clutch hub. The manufacturer machines the swash-plate and piston bores to the same tolerances as the original assembly, which maintains the correct displacement and avoids excessive clutch cycling. When replacing, transfer the original clutch and pulley if they are serviceable, and evacuate the system before opening the refrigerant lines.