Air Brakes · Air Brake Warning and Control Devices
Federal regulation requires every air-braked commercial motor vehicle to warn the driver when air pressure drops too low to brake safely. At what point does that warning device activate?
Explanation
A low air pressure warning signal, visible or audible in the driver's normal field of view, must activate whenever air pressure in the service reservoir system drops below 60 psi. This threshold comes directly from FMVSS No. 121, codified at 49 CFR 571.121 S5.1.5, and matches the WA manual's own 60 psi figure exactly; an older 55 psi figure found elsewhere in 49 CFR 393.51(c)(2) applies only to vehicles manufactured before FMVSS 121 took effect on March 1, 1975, essentially no vehicle on the road today. 100 psi is wrong because that is roughly the compressor's normal cut-in pressure during ordinary operation, not a warning threshold. Requiring the driver to manually pull the parking brake control is wrong because the warning is automatic and tied to pressure alone, not a driver action. 20 psi is wrong because that is near the bottom of the range where spring brakes begin to apply automatically, a separate and later failsafe, not the pressure at which the driver is first warned.
Source: 49 CFR 571.121 (FMVSS No. 121) S5.1.5
Related Air Brake Warning and Control Devices questions
- If the low air pressure warning activates while a vehicle is moving, what should the driver do?
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