CDL Air Brakes TestAir Brake System PartsHard
A driver hears a constant hissing sound coming from the area of the air compressor governor and notices that the air pressure in the tanks is not building up. What is the most probable cause?
- AA ruptured air storage tank.
- BThe air compressor discharge valve is stuck open.
- CThe air compressor governor's unloader valve is stuck open.
- DThe air dryer purge valve is stuck open.
Show answer & explanationAnswer & explanation
Correct answer: C. The air compressor governor's unloader valve is stuck open.
The governor's unloader valve (or associated components) controls whether the compressor is pumping air into the tanks or idling. If it's stuck open, the compressor will run, but the air will be vented (creating a hissing sound) instead of being directed into the tanks, preventing pressure buildup.
Why the other options are wrong
- A. A ruptured air storage tank would cause a significant air leak and pressure drop, but the hissing would likely be from the tank itself, not the compressor governor area.
- B. A stuck-open compressor discharge valve would prevent air from reaching the tanks, but the sound would be from the compressor itself, not the governor, which controls the compressor's state.
- D. A stuck-open air dryer purge valve would cause a constant hiss and air loss, but the sound would typically originate from the air dryer, not the governor.
Air Compressor Governor (Unloader Valve)
The air compressor governor uses an unloader valve (or mechanism) to stop the compressor from pumping air into the tanks once the cut-out pressure is reached. If stuck open, it continuously vents compressed air.
- Controls compressor's pumping and idling states.
- Unloader valve opens to vent air when tanks are full.
- Stuck open causes continuous air venting and no pressure buildup.
Memory trick: Hissing governor, no pressure gain.