Air Supply and Compressor Systems
Core concepts
This domain tests the air supply chain: compressor, discharge line, air dryer, reservoirs, drains, and the plumbing that ties them together. Know that the compressor is engine-driven and lubricated/cooled from the engine; its unloader mechanism (controlled by the governor) lets it pump or idle. The discharge line carries hot, moisture-laden air to the air dryer, which removes water and oil vapor and holds a purge cycle.
Understand the air dryer's role: desiccant cartridge, purge valve, check valve, and heater (on many buses) to prevent freeze-up. Know the difference between the wet tank (first tank, collects condensate) and dry tanks (supply the brake circuits). Automatic drains and manual drains must be functional; a bus that builds pressure slowly or has excessive moisture is a diagnostic clue.
Traps in questions
- Blaming the compressor for slow build when the real cause is a leaking discharge line, stuck unloader, or saturated dryer.
- Forgetting the air dryer purge cycle and heater are part of freeze protection.
- Assuming all tanks are dry tanks; the wet tank is the condensate collector.
- Overlooking that a restricted intake or worn compressor rings cause oil carryover, not just low pressure.
Worked example
If a bus takes too long to build from 85 to 100 psi, first check for leaks (with engine off, listen and use soapy water), then verify governor and unloader operation, then inspect the dryer for restriction and the compressor for wear. Build-time specifications vary by manufacturer; always compare to the published spec, not a guessed number.