Most drivers aren’t aware of everything going on under the hood and behind the dash to keep them cool in the summer and the windshield clear in the winter. Thanks to the heat-transfer properties of refrigerant, driven by the car air conditioning compressor, comfort is just a button-press away.
Air Conditioning Parts
The air conditioning system itself is made up of several main parts. The refrigerant flows from the air conditioning (AC) compressor to the condenser, in front of the radiator, where it releases heat to the atmosphere. Then, heated refrigerant flows to the expansion valve or fixed-orifice tube. Just past this, in the evaporator, it absorbs heat from inside the cabin before heading back to the compressor. Along the way, the refrigerant (usually R-134a or R-1234yf) goes through tubes and hoses, a filter and a dryer.
Aside from the switch to engage the air conditioning, the electrical system uses pressure sensors to determine when to cycle, engage and disengage the electromagnetic (EM) clutch. On conventional vehicles, the clutch is driven by V-belts or the serpentine belt. On hybrid and electric vehicles, the air conditioning compressor is powered directly by a high-voltage electric motor, some of which go up to 400 volts. Less-common are adjustable-swash-plate compressors, which adjust their output without cycling.
Air Conditioning Compressor Basics
The typical car AC compressor is a 12-piston six-bore swash-plate unit. The swash plate is mounted off-axis to the drive shaft, “wobbling” as the shaft is driven by the EM clutch or electric motor. Six double-ended pistons are oscillated by the swash plate, each piston doing double-duty intake and compression. Reed valves control the flow of refrigerant. A fine oil mist circulates with the refrigerant, lubricating moving parts and sealing rubber hoses and seals.
Because it is a sealed system, the only maintenance that the typical AC compressor needs is regular use to keep lubricant dispersed throughout the system. When addressing air conditioning problems, refrigerant charge is important to consider, because excessive or insufficient pressure may disable the system. Drive belts need to be clean and tight for positive engagement. Finally, electrical checks confirm proper function of pressure sensors and switches; electromagnetic clutch fuse and relay; and the clutch coil.
Oil type and viscosity depend on the refrigerant and the compressor. For example, R-134a compressors are lubricated by polyalkylene glycol (PAG), which may not be compatible with proprietary R-1234yf compressor oils. Viscosity is also important, as lighter PAG 46 oils may not protect compressors requiring heavier PAG 100 or PAG 150 oils.
Replacing an Air Conditioning Compressor
Air conditioning compressors can fail due to age, wear or neglect. Many EM clutches are replaceable, but some EM-clutch and all motor-driven compressors are replaced as an assembly. When changing the AC compressor, such as with a re-manufactured air conditioning compressor, use a recovery machine to extract refrigerant and protect the atmosphere. On reassembly, ensure the proper charge of refrigerant and oil for performance and reliability. These specifications are in the repair manual.
Fortunately, car air conditioning isn’t overly complicated, but it is sensitive. If you’re suffering AC problems, the AC compressor might be at the root of them. Be sure to eliminate all other possibilities, however, before delving into this sealed system.