CDL
Air Brakes
A driver sees the air compressor stop pumping when tank pressure reaches about 125 psi and thinks the compressor has failed. What is actually happening?
- AThe safety valve has released the excess tank pressure.
- BThe compressor has run out of its own oil supply.
- CThe governor has stopped the compressor at cut-out pressure.
- DThe automatic drain valves have opened to release air.
Not A. The safety valve normally opens at about 150 psi, not at the governor's cut-out pressure of about 125 psi.
C is right. When tank pressure rises to the cut-out level, around 125 psi, the governor stops the compressor from pumping air, and it lets the compressor start again when pressure falls to the cut-in level, around 100 psi. This normal cycling is not a sign of a failed compressor; a low oil supply is a separate item you check before driving.