General Knowledge: Safety Control Systems

5 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 392.33, 49 CFR 393.51, 49 CFR 571.121

What you need to know

393.51(c)(2) specifies a continuous warning whenever service reservoir pressure is at 379 kPa (55 psi) and below, or one-half of the compressor governor cutout pressure, whichever is less.

The cutout-based figure matters on systems that run at lower pressures than usual.

A warning signal that is audible or visible to a person in the normal driving position and provides a continuous warning to the driver whenever the air pressure in the service reservoir system is at 379 kPa (55 psi) and below, or one-half of the compressor governor cutout pressure, whichever is less.

49 CFR 393.51

393.51(c) requires both a pressure gauge and a warning signal.

The gauge must be visible from the normal driving position and indicate pressure available for braking.

equipped with service brakes activated by compressed air (air brakes) or a commercial motor vehicle towing a vehicle with service brakes activated by compressed air (air brakes) must be equipped with a pressure gauge and a warning signal.

49 CFR 393.51

393.51(a) sets a general rule covering every bus, truck and truck tractor, subject to the exception in paragraph (f).

Hydraulic, air and vacuum systems each then have their own specific requirements. Each must have a signal warning the driver of a service brake failure.

Every bus, truck and truck tractor, except as provided in paragraph (f), must be equipped with a signal that provides a warning to the driver when a failure occurs in the vehicle's service brake system.

49 CFR 393.51

392.33(a) prohibits driving when required lamps or reflective devices are obscured by the tailboard, the load or its covering, dirt, or other equipment.

The only exception in 392.33(b) concerns conspicuity treatments on trailer front end protection devices. Reflective devices count the same as lamps.

No commercial motor vehicle shall be driven when any of the lamps or reflective devices/material required by subpart B of part 393 of this title are obscured by the tailboard, or by any part of the load or its covering, by dirt, or other added vehicle or work equipment or otherwise.

49 CFR 392.33

49 CFR 571.121 S5.5.1 requires that an ABS malfunction must not increase the actuation and release times of the service brakes.

You keep normal braking and lose only the anti-lock function, so the correct response is to brake normally and get the fault repaired, not to stop using the brakes. The malfunction must not increase the actuation or release times of the service brakes.

a malfunction that affects the generation or transmission of response or control signals of any part of the antilock system shall not increase the actuation and release times of the service brakes

49 CFR 571.121

Check yourself

• At what service reservoir pressure must your low air pressure warning signal activate, for power units to which the standard did not apply?

• What two devices does the rule require on an air brake equipped CMV?

• Which vehicles must have a brake system failure warning signal?

• May you drive with a tarpaulin partly covering a required rear lamp?

• What does the federal brake standard say about your service brakes when the ABS lamp lights?

Answers are in the practice test for this topic.