When checking the inside of the cab during a preventive maintenance inspection, the air compressor cut-in pressure is observed to be 75 psi. What would be the most likely cause of this problem?

• A. Worn compressor belt
• B. Defective air governor
• C. Faulty pressure switch
• D. Damaged air tank

Answer: (B)

Cut-in pressure is set by the air governor, which works with the pressure switch to start the compressor when system pressure falls to a preset level and stop it at the cut-out. If the observed cut-in is abnormally high, like 75 psi, the governor is the most likely culprit because it determines when the compressor should begin running. A governor that is out of calibration, leaking, or sticking will delay the start and push the cut-in to a higher value. A worn belt affects the mechanical drive and speed, not the starting pressure setting; a damaged air tank would cause leaks or an inability to hold pressure; and while a faulty pressure switch can cause improper cycling, the symptom described most directly points to the governor.