CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
Which of the following describes the correct procedure for testing the low air pressure warning system on a combination vehicle?
- AWith the engine running, continuously apply the service brakes until the warning activates.
- BStart the engine, build full air pressure, then turn off the engine and listen for the warning.
- CWith the engine off and brakes off, pump the brake pedal until the low air warning light and buzzer activate.
- DDrain all air from the tanks, then start the engine and wait for the warning to activate as pressure builds.
Show answer & explanationAnswer & explanation
Correct answer: C. With the engine off and brakes off, pump the brake pedal until the low air warning light and buzzer activate.
To test the low air pressure warning system, with the engine off and parking brakes released, pump the brake pedal repeatedly to gradually reduce air pressure. The warning light and buzzer should activate before the pressure drops below 60 psi.
Why the other options are wrong
- A. Continuously applying service brakes with the engine running will maintain pressure or deplete it slowly, but it's not the standard or most effective way to test the warning system's activation point.
- B. This method doesn't reduce air pressure in a controlled way to test the warning system's activation point.
- D. Draining all air and then building pressure tests the compressor, not the low air warning system's activation point.
Low Air Pressure Warning Test
A test performed with the engine off to verify that the low air pressure warning light and buzzer activate when air pressure drops to the specified threshold (typically 60 psi or above).
- Engine off, parking brakes off.
- Pump brake pedal to reduce pressure.
- Warning should activate at 60 psi or higher.
Memory trick: Engine off, brakes pump, watch for the warning, don't slump.