CDL General Knowledge TestAir Brakes (if applicable)Medium
During a pre-trip inspection, a driver observes that the air pressure gauge does not rise above 90 psi, even after several minutes of the engine running at operating RPM. The low air pressure warning light is illuminated. What is the most probable cause?
- AA stuck open safety relief valve.
- BA defective air compressor.
- CA faulty air pressure gauge.
- DA restricted air dryer inlet line.
Show answer & explanationAnswer & explanation
Correct answer: B. A defective air compressor.
If the air pressure does not build to the normal operating range (typically 100-125 psi) even after running the engine, and the low air warning is on, it points to a problem with the air compressor's ability to generate sufficient pressure.
Why the other options are wrong
- A. A stuck open safety relief valve would cause pressure to drop rapidly or prevent it from building, but it's less common than a compressor issue for a complete lack of build-up.
- C. A faulty gauge might show incorrect pressure, but the illuminated low air warning light suggests a genuine pressure problem, not just a display error.
- D. A restricted air dryer inlet line might slow down pressure build-up or cause other issues, but a complete inability to reach 90 psi suggests a more fundamental problem with the pressure source.
Air Compressor Function
The air compressor generates and stores compressed air in the air tanks, which is essential for the operation of air brakes and other air-powered systems.
- Engine-driven.
- Typically builds pressure to 100-125 psi.
- Governed by the air governor.
Memory trick: No Pressure = Compressor's Distress Call