CDL Combination Vehicles TestDriving Combination Vehicles SafelyHard
A commercial driver is performing a pre-trip inspection on a combination vehicle. When checking the air brake system, the driver notices that the low air pressure warning light and buzzer activate at 70 PSI. What does this indicate?
- AThis is a normal operational range for the low air warning system.
- BThe air pressure is dangerously low, and the vehicle should not be operated.
- CThe brake pedal should be pumped repeatedly to build more air pressure.
- DThe air compressor is functioning correctly and maintaining optimal pressure.
Show answer & explanationAnswer & explanation
Correct answer: B. The air pressure is dangerously low, and the vehicle should not be operated.
The low air pressure warning light and buzzer should activate before air pressure drops to 60 PSI (or 55 PSI in some older systems), typically between 60-100 PSI. If it activates at 70 PSI and continues to drop, it indicates a significant air leak or compressor problem that makes the vehicle unsafe to operate.
Why the other options are wrong
- A. The low air warning system is designed to alert the driver when pressure drops below a safe operating range, typically around 60 PSI. 70 PSI activating the warning suggests an issue.
- C. Pumping the brake pedal would use more air, exacerbating the problem if there's a leak or compressor failure, not building pressure.
- D. A correctly functioning compressor would maintain pressure well above 70 PSI during normal operation, and the low air warning would activate at a lower threshold.
Low Air Pressure Warning Threshold
The low air pressure warning light and buzzer should activate when air pressure drops below 60 PSI (or 55 PSI), indicating unsafe operating conditions.
- Warning indicates critical air loss.
- Below 60 PSI is unsafe for operation.
- Vehicle should not be driven until fixed.
Memory trick: 70 PSI = Serious Problem, Stop Immediately.