ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 2

Question 1

A Class 8 truck driver reports the air brakes feel "spongy" and take longer to stop. Which system component is MOST likely responsible?

Accepted Answer

Glazed or contaminated brake linings, or out-of-adjustment slack adjusters allowing excessive push rod travel

Answer

Spongy or slow air brakes on a heavy truck are most commonly caused by brake lining issues (glazing, contamination with grease, or excessive wear) or misadjusted slack adjusters. Slack adjusters control the push rod travel — if adjusted too loosely, air has to travel further before the brakes apply, resulting in a delayed, soft feeling. Federal regulations mandate brake adjustment within specific push rod travel limits, checked during CMV inspections.

Question 2

On a heavy-duty diesel truck, what does a low air pressure warning (typically at or below 60 PSI) indicate to the driver?

Accepted Answer

Air system pressure has dropped to a dangerous level — the driver must safely stop immediately, as spring brakes will automatically apply when pressure drops further (typically at 20-45 PSI)

Answer

Air brake systems on heavy trucks operate between approximately 90-120 PSI. Below 60 PSI, the low-pressure warning buzzer and light activate, alerting the driver to stop safely immediately. If pressure continues to drop below approximately 20-45 PSI (depending on spring brake activation pressure), the spring brakes automatically apply — locking the vehicle. This is a fail-safe design that prevents brake failure from driving without sufficient air pressure.

Question 3

What is the purpose of the "gladhand" (air hose coupling) on a semi-tractor-trailer combination?

Accepted Answer

To connect the tractor's air supply and control (service) brake lines to the trailer's air brake system

Answer

Gladhands are the quick-connect air hose couplings used to connect the tractor to the trailer. There are two: the "service" (blue) gladhand carries the brake signal air that applies the trailer brakes when the driver presses the brake pedal. The "supply" (red, or emergency) gladhand charges the trailer's air reservoir. If the supply line disconnects, the trailer emergency spring brakes automatically apply — a safety feature that prevents a runaway trailer.

Question 4

A truck technician is checking the alternator on a heavy-duty diesel truck and measures 13.2V at the batteries with the engine running. What does this indicate?

Accepted Answer

The alternator voltage is below the normal charging range (13.8–14.7V); the charging system is underperforming and the batteries may slowly discharge

Answer

A healthy charging system in a heavy truck should produce approximately 13.8–14.7V at the batteries with the engine running and a moderate electrical load. 13.2V is at or below battery resting voltage, indicating the alternator is not providing adequate charging voltage. Over time this will discharge the batteries. The technician should load-test the alternator (measure output current at full load) and check belt tension, alternator diodes, and connections to identify the cause of low output.

Question 5

On a heavy truck with an S-cam air drum brake system, what is the function of the S-cam?

Accepted Answer

To convert the linear push rod movement from the brake chamber into rotational force that spreads the brake shoes against the drum

Answer

The S-cam (named for its S-shaped profile) is the key mechanical link in an air drum brake. When the brake chamber push rod extends, it rotates the slack adjuster, which turns the S-cam shaft. The S-shaped cam rotates between the roller ends of the two brake shoes, pushing them apart and into contact with the drum. The S-cam converts the air brake actuator's linear force into the spreading action that applies the brakes. Worn or cracked S-cams cause brake imbalance.