The function of the air compressor (Fig. 8) is to build up and maintain air pressure required to operate air brakes and air-powered accessories.

Air compressors are either gear driven directly from the engine or belt driven. Although most compressors use the truck’s lubrication and cooling systems, some are self-lubricated and some are air cooled. Self-lubricated compressors must have their oil checked and changed at regular intervals.

The compressor’s intake system draws air from either its own air filter or from the engine’s intake system.

Compressors that have their own filtration system must be serviced on a regular basis.

All compressors run continuously while the engine is running, but air compression is controlled and limited by a governor which loads or unloads the compressor. In the loaded stage, air is pumped into reservoirs. In the unloaded stage (with two cylinder compressors), the compressor pumps air back and forth between the two cylinders without supplying the reservoirs.

​​​​​​​The governor must take the compressor out of its pumping stage (unload/cut-out) when system air pressure reaches 120 to 145 psi (828 to 1,000 kPa), and also put it back into the pumping stage at a minimum of 100 psi (690 kPa).