CDL Air Brakes TestDual Air Brake SystemsMedium
During a pre-trip inspection, a driver observes that the low air pressure warning light and buzzer activate when the primary air system pressure drops to 70 psi, but the secondary system's warning activates at 85 psi. What is the most appropriate action for the driver to take?
- AContinue the trip, as the primary system's warning is within regulations.
- BPlace the vehicle out of service until both warning systems activate at or above 60 psi.
- CAdjust the secondary system's warning activator to match the primary system.
- DDrive to the nearest service center, carefully monitoring the primary system's pressure.
Show answer & explanationAnswer & explanation
Correct answer: B. Place the vehicle out of service until both warning systems activate at or above 60 psi.
Federal regulations (49 CFR 393.51) require the low air pressure warning to activate at or above 60 psi for both systems. While the primary system is within limits, the secondary system's warning activating at 85 psi means it is not giving a timely warning, indicating a potential malfunction that requires repair.
Why the other options are wrong
- A. While the primary system is within regulation (above 60 psi), the secondary system's warning is not functioning correctly, making the vehicle unsafe.
- C. Adjusting components without proper diagnosis and tools is unsafe and may violate regulations. The issue is likely a faulty sensor, not an adjustment.
- D. Operating a vehicle with a malfunctioning safety feature, such as an incorrect low air warning, is unsafe and against regulations. The vehicle should be repaired before use.
Low Air Warning Activation Threshold
The low air pressure warning for a dual air brake system must activate when air pressure drops to or below 60 psi for EACH system.
- Mandated by federal regulations (e.g., 49 CFR 393.51).
- Applies to both primary and secondary systems.
- Ensures timely warning of critical pressure loss.
Memory trick: Sixty is the safety line for sound and sight.