ELDT - Entry Level Driver Training Vehicle Systems and Malfunctions Questions and Answers 2 — Questions and Answers

Question 1: What should a CMV driver do if the low air pressure warning activates while driving?

Continue driving to the next truck stop
Immediately pull over safely and stop; do not drive until the air system is repaired (Correct answer)
Pump the brake pedal to build pressure
Turn off the warning buzzer and monitor the gauge

Correct answer: Immediately pull over safely and stop; do not drive until the air system is repaired

A low air pressure warning indicates the air brake system may fail. The driver must stop immediately because below a certain pressure, the spring brakes will apply automatically and the vehicle will have no service brakes.

The low air pressure warning (buzzer and light) activates at approximately 60 psi. This is a critical safety warning because: at around 20-45 psi, the spring brakes will apply automatically, which on a highway could cause a jackknife or collision; between the warning and spring brake application, braking effectiveness is severely reduced; and continued driving causes further air loss. The driver must safely pull off the road immediately and stop. The cause could be a ruptured air line, compressor failure, leaking brake chamber, or other air system component failure. The vehicle must not be moved until the air system is repaired and holds proper pressure.

Question 2: What is the function of the governor in an air brake system?

It controls the vehicle speed
It controls when the air compressor pumps air into the storage tanks (Correct answer)
It regulates fuel flow to the engine
It controls the trailer brake timing

Correct answer: It controls when the air compressor pumps air into the storage tanks

The governor controls the air compressor by cutting it in when tank pressure drops to about 100 psi and cutting it out when pressure reaches about 125 psi, maintaining proper system pressure.

The governor is an automatic pressure control device in the air brake system. It has two key functions: the 'cut-in' pressure (typically around 100 psi) at which it signals the compressor to start pumping air into the storage tanks, and the 'cut-out' pressure (typically around 125 psi) at which it stops the compressor. This maintains air pressure within the proper operating range. During the pre-trip inspection, the driver should build air pressure to the governor cut-out, note that the compressor stops, then check the air leak rate. If the governor is malfunctioning, the system may not maintain proper pressure or may over-pressurize, both of which are dangerous.

Question 3: What causes brake fade and how can it be prevented?

Using brakes too infrequently; prevented by braking harder
Overheating the brakes from continuous use; prevented by proper gear selection on downgrades (Correct answer)
Cold temperatures shrinking brake components; prevented by warming up brakes
Wet brakes losing friction; prevented by driving faster to dry them

Correct answer: Overheating the brakes from continuous use; prevented by proper gear selection on downgrades

Brake fade occurs when brakes overheat from continuous or excessive use, reducing their friction and stopping power. It is prevented by using proper gear selection and engine braking on long downgrades.

Brake fade is one of the most dangerous CMV malfunctions. When brakes are applied continuously (especially on long downgrades), friction generates extreme heat. As brake drums or rotors overheat, the friction material becomes less effective, requiring more pedal pressure for less braking effect. In severe cases, brakes can fail completely. Prevention methods include: selecting the right gear before a downgrade (same gear going down as needed going up); using engine braking/jake brake as primary retarder; applying wheel brakes only in brief snub applications; never riding the brakes; and ensuring brakes are properly adjusted. If brake fade occurs, the driver must find a way to stop immediately, including using a runaway truck ramp if available.

Question 4: What should you do if your engine overheats while driving?

Immediately shut off the engine
Pull over safely, keep the engine idling to circulate coolant, and investigate the cause (Correct answer)
Add cold water to the hot radiator immediately
Increase speed to get more air flowing over the radiator

Correct answer: Pull over safely, keep the engine idling to circulate coolant, and investigate the cause

If the engine overheats, pull over safely and let the engine idle (if safe to do so) to continue circulating coolant. Do not add cold water to a hot system as it can crack the engine block.

Engine overheating requires prompt but careful response: (1) Pull off the road to a safe location as soon as possible; (2) If the temperature is high but not critical, let the engine idle with the heater on full (this adds a second radiator to help dissipate heat); (3) If the gauge is in the red/critical zone or steam is visible, shut the engine off; (4) Never open the radiator cap on a hot engine - the pressurized coolant can cause severe burns; (5) Let the engine cool before investigating; (6) Check coolant level, fan belt, water pump, thermostat, and look for leaks. Adding cold water to an overheated engine can crack the block or head due to thermal shock. The vehicle should not be driven until the overheating cause is identified and corrected.

Question 5: What is the function of the slack adjuster in the air brake system?

It adjusts the air pressure in the tanks
It converts the push rod movement into the rotational force needed to apply the brake shoes (Correct answer)
It controls the timing of brake application between axles
It adjusts the ride height of the vehicle

Correct answer: It converts the push rod movement into the rotational force needed to apply the brake shoes

The slack adjuster is a lever that connects the push rod from the brake chamber to the camshaft. It converts the linear push of the rod into rotational force to turn the S-cam and apply the brake shoes against the drum.

The slack adjuster is a critical component in S-cam drum brakes (the most common brake type on CMVs). It is a lever arm connected between the brake chamber push rod and the brake camshaft (S-cam shaft). When air pressure pushes the brake chamber diaphragm, the push rod extends and pushes on the slack adjuster, which rotates the S-cam. The rotating S-cam pushes the brake shoes outward against the drum, creating friction to slow the wheel. As brake shoes wear, the slack adjuster compensates by adjusting the starting position. Automatic slack adjusters (ASAs) are required on all new CMVs and maintain proper adjustment. Manual adjusters require periodic driver or mechanic adjustment.

Question 6: What should a driver do if they experience a tire blowout on the highway?

Brake hard immediately
Grip the steering wheel firmly, stay off the brakes, and gradually slow down before pulling over (Correct answer)
Swerve to the shoulder immediately
Accelerate to maintain control

Correct answer: Grip the steering wheel firmly, stay off the brakes, and gradually slow down before pulling over

During a blowout, grip the steering wheel firmly to counteract the pulling force, do not brake (which can cause a skid), and let the vehicle slow naturally before safely steering to the shoulder.

A tire blowout creates a sudden violent pulling force toward the side of the blown tire, along with a loud bang and vehicle vibration. The correct response: (1) Grip the steering wheel firmly with both hands to maintain directional control; (2) Do NOT hit the brakes - the sudden loss of a tire already compromises traction, and braking can cause a skid or jackknife; (3) Let the vehicle slow down naturally through air resistance and rolling resistance; (4) If possible, gradually accelerate slightly to stabilize the vehicle before beginning to slow; (5) Once speed is reduced, gently steer to the shoulder; (6) Apply brakes gradually once at a lower speed. Steer tire blowouts are the most dangerous because they directly affect steering.

ELDT - Entry Level Driver Training Vehicle Systems and Malfunctions Questions and Answers 2

What should a CMV driver do if the low air pressure warning activates while driving?