Compressed Air Supply: Compressor and Governor Operation
The compressor
The compressor is engine-driven (belt or gear) and supplies all the air the system uses. It draws in air, compresses it, and pushes it through the air dryer into the supply tank. The dryer removes water vapour and oil mist and purges collected moisture to atmosphere when the governor signals cut-out. Larger vehicles may use a priority valve so the compressor's output goes to the brake system before anything else.
The governor
The governor is the pressure switch that reads reservoir pressure and controls whether the compressor pumps:
- Cut-out - at roughly 105 psi (about 720 kPa) the governor signals the compressor's unloader to stop pumping.
- Cut-in - at roughly 85 psi (about 585 kPa) pumping resumes. The spread between the two is typically 15–20 psi, and it is normal to hear the change while idling.
A safety valve (pop-off) on the supply tank, set well above cut-out (commonly around 150 psi), protects the system if the governor fails.
Traps
- The safety valve is not the governor. If pressure climbs past cut-out, suspect the unloader/governor - never 'cure' it by adjusting the safety valve.
- A slipping or broken compressor belt means no replenishment; you are living on stored air.
- A compressor that passes oil contaminates the dryer desiccant and eventually the whole system.
- The dryer purge is a normal sound, not a leak.
Rule of thumb: watch the gauge over one full cycle. If the system will not reach cut-out, or the cut-in/cut-out spread is extreme, the compressor or governor needs attention before the vehicle moves.