In a vehicle equipped with air brakes, what does the governor control?
- When air is released from the brake chambers
- The air pressure applied in braking
- The speed of the air compressor
- When air is pumped into the tanks ✓
Answer: D. When air is pumped into the tanks
The governor controls when air is pumped into the tanks: it cuts the compressor out at high pressure and cuts it back in when pressure drops.
Why this is the answer
The handbook explains that the compressor runs with the engine and can make more air than the brake system needs. The governor watches system pressure and stops the compressor from pumping when pressure is high enough, then starts it pumping again when pressure falls. The key-points passage puts it plainly: the governor controls the compressor cut-in and cut-out pressure. That means it governs the pumping cycle that puts air into the tanks, not the braking pressure you apply with the pedal.
Why the other answers are wrong
- A. When air is released from the brake chambers — The governor does not release air from the brake chambers; the passages describe it switching the compressor's pumping on and off as system pressure changes.
- B. The air pressure applied in braking — The governor reacts to system pressure, but it does not set the air pressure applied during braking; it controls the compressor cut-in and cut-out.
- C. The speed of the air compressor — The compressor runs whenever the engine runs, and the governor stops or starts the pumping, so it is not described as controlling compressor speed.
Remember it
High pressure, cut out; low pressure, cut in — the governor is the compressor's pump switch.