CDL Air Brakes TestAir Brake System PartsHard
A driver is conducting a pre-trip inspection of a commercial vehicle's air brake system. They notice that the supply pressure gauges for both the primary and secondary air tanks are fluctuating erratically, even when the engine RPM is steady and no brakes are being applied. What is the most probable cause?
- AThe air dryer is constantly purging, causing pressure drops.
- BThe air compressor governor is stuck in the cut-out position.
- CA faulty air pressure gauge or its associated sending unit.
- DA major air leak in one of the service brake lines.
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty air pressure gauge or its associated sending unit.
If both supply pressure gauges are fluctuating erratically while the engine is steady and no brakes are applied, it points to an issue with the gauges themselves or their sending units, rather than a system-wide pressure problem. A real pressure fluctuation of this nature would typically trigger other symptoms or warnings.
Why the other options are wrong
- A. The air dryer purges intermittently, causing a momentary dip, not continuous erratic fluctuation, and is usually a single event.
- B. If the governor were stuck in cut-out, the compressor wouldn't build pressure, leading to a steady drop in pressure, not erratic fluctuation.
- D. A major air leak would cause a steady, continuous drop in pressure, not erratic fluctuation, and would likely be audible.
Supply Pressure Gauge Fluctuation
Erratic fluctuation of air pressure gauges, especially without corresponding system events (like braking or leaks), often indicates a fault with the gauge itself or its sending unit.
- Gauges provide critical pressure information.
- Faulty gauges can give misleading readings.
- Must be accurate for safe operation.
Memory trick: Gauges dancing, no real flow? The sensor's faulty, time to know!