General Knowledge: Vehicle Control Systems

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

Sources: 49 CFR 393.51, 49 CFR 571.121

What you need to know

393.51(c)(1) requires a gauge visible from the normal driving position indicating the air pressure available for braking, in kPa or psi.

It must be visible from the normal driving position.

A pressure gauge, visible to a person seated in the normal driving position, which indicates the air pressure (in kilopascals (kPa) or pounds per square inch (psi)) available for braking.

49 CFR 393.51

49 CFR 571.121 S5.1.6.1(b) requires direct ABS control of at least one front axle and at least one rear axle on truck tractors built on or after 1 March 1997, with at least one axle independently controlled.

Other axles may be indirectly controlled. At least one axle must be independently controlled.

Each truck tractor manufactured on or after March 1, 1997, shall be equipped with an antilock brake system that directly controls the wheels of at least one front axle and the wheels of at least one rear axle of the vehicle, with the wheels of at least one axle being independently controlled

49 CFR 571.121

49 CFR 571.121 S5.1.6.2(a) requires the lamp to be mounted in front of and in clear view of the driver.

It must also illuminate briefly as a lamp-function check whenever the ignition is turned on.

shall be equipped with an indicator lamp, mounted in front of and in clear view of the driver, which is activated whenever there is a malfunction that affects the generation or transmission of response or control signals in the vehicle's antilock brake system

49 CFR 571.121

49 CFR 571.121 S5.2.3.3(c)(3) places it on a permanent structure on the left side of the trailer as viewed from the rear, near the red rear side marker lamp or on the front face of the left rear fender.

Knowing this matters because it is how a driver checks trailer ABS during a walk-around.

Each trailer that is not a trailer converter dolly and on which the malfunction indicator lamp cannot be placed within the location specified in S5.2.3.3(c)(1) shall be equipped with a lamp mounted on a permanent structure on the left side of the trailer as viewed from the rear

49 CFR 571.121

Check yourself

• What is your air pressure gauge required to show, and from where must it be visible?

• Under FMVSS No. 121, which axles must your truck tractor's antilock brake system directly control?

• Where must the ABS malfunction indicator lamp be mounted on your truck tractor?

• Where is your trailer's external ABS malfunction lamp located when it cannot be placed in the primary position?

Answers are in the practice test for this topic.