CDL Air Brakes TestAir Brake System PartsMedium
A driver is conducting a pre-trip inspection of a commercial vehicle's air brake system. They notice that the supply pressure gauge for the primary air tank is fluctuating rapidly while the engine is running. What is the most likely cause of this fluctuation?
- AA clogged air dryer.
- BA failing air compressor governor.
- CA stuck-open safety relief valve.
- DA leak in the air supply line leading to the gauge.
Show answer & explanationAnswer & explanation
Correct answer: D. A leak in the air supply line leading to the gauge.
Rapid fluctuation of a pressure gauge often indicates a restriction or leak in the small line leading directly to the gauge itself, rather than a problem with the main system pressure. The gauge struggles to get a stable reading.
Why the other options are wrong
- A. A clogged air dryer would affect air quality and moisture, not directly cause gauge fluctuation during normal operation.
- B. A failing governor would affect the cut-in/cut-out of the compressor, leading to pressure drops or over-pressurization, but not typically rapid, constant gauge fluctuation.
- C. A stuck-open safety valve would cause a continuous pressure drop, not necessarily rapid fluctuation of the gauge.
Supply Pressure Gauge Fluctuation
Erratic or rapid movement of the air pressure gauge needle, typically indicating a problem with the gauge itself or its supply line rather than the main system pressure.
- Often caused by a leak or restriction in the gauge's dedicated air line.
- Can also be caused by a faulty gauge internal mechanism.
- Does not necessarily mean the main air system pressure is fluctuating.
- Requires inspection of the gauge and its connections.
Memory trick: Gauge needle dances, not a chance, check the line, give it a glance.