CDL General Knowledge TestAir Brakes (if applicable)Medium
A commercial vehicle driver is performing a pre-trip inspection. They notice that the air pressure gauge builds from 85 psi to 100 psi in 30 seconds with the engine at operating RPM. What does this indicate about the air compressor's performance?
- AThe air compressor needs immediate replacement due to a leak.
- BThe air compressor is over-pressurizing the system.
- CThe air compressor is performing within acceptable limits.
- DThe air compressor is performing below standard.
Show answer & explanationAnswer & explanation
Correct answer: C. The air compressor is performing within acceptable limits.
For a single vehicle, the air compressor should build air pressure from 85 psi to 100 psi within 45 seconds. Building it in 30 seconds is well within this standard, indicating good performance.
Why the other options are wrong
- A. A leak would cause slow build-up or a drop in pressure, not efficient build-up.
- B. The test is about build-up time, not maximum pressure. The governor controls over-pressurization.
- D. This is incorrect; 30 seconds is faster than the minimum requirement.
Air Compressor Build-Up Rate
The speed at which the air compressor can increase air pressure in the reservoirs from a specified low pressure to a specified high pressure.
- For single vehicles, 85 to 100 psi in 45 seconds or less.
- For combination vehicles, 85 to 100 psi in 45 seconds or less (engine at operating RPM, parking brake off, service brake released).
- This test verifies compressor efficiency.
Memory trick: Compressor build-up: Fast enough to keep you safe, slow enough to be a problem.