Which component controls when the air compressor starts and stops and maintains system pressure in an air-braked bus?

• A. The brakes pedal
• B. The air dryer
• C. The air governor
• D. The compressor

Answer: (C)

In air-braked buses, the air governor is the device that decides when the compressor should run and when to stop, keeping the system pressure within the proper range. It constantly monitors the air stored in the tanks and uses two thresholds: when pressure drops to the lower limit, the governor allows the compressor to start building pressure; when it climbs to the upper limit, the governor stops the compressor to prevent over-pressurization. This cycling maintains a ready supply of compressed air for brakes and other systems, while reducing unnecessary engine load by unloading the compressor when not needed. The air dryer’s job is to remove moisture from the air, the brakes pedal is used to apply the brakes, and the compressor is the air source itself, but its on/off operation is controlled by the governor.