ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 2 — Questions and Answers
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?
- Worn brake drums only
- Glazed or contaminated brake linings, or out-of-adjustment slack adjusters allowing excessive push rod travel (Correct answer)
- Failed ABS module
- Low engine oil pressure
Correct answer: Glazed or contaminated brake linings, or out-of-adjustment slack adjusters allowing excessive push rod travel
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?
- The power steering fluid is low
- 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) (Correct answer)
- The air conditioning system needs recharging
- The air suspension bags need inflation
Correct 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)
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?
- To connect the electrical lighting cables between tractor and trailer
- To connect the tractor's air supply and control (service) brake lines to the trailer's air brake system (Correct answer)
- To connect the fifth wheel greasing system
- To couple the tractor and trailer mechanically at the kingpin
Correct answer: To connect the tractor's air supply and control (service) brake lines to the trailer's air brake system
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?
- The alternator is overcharging
- The alternator voltage is below the normal charging range (13.8–14.7V); the charging system is underperforming and the batteries may slowly discharge (Correct answer)
- The voltage is normal for a heavy truck
- The battery voltage is too high — batteries must be replaced
Correct 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
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?
- To adjust brake lining clearance automatically
- To convert the linear push rod movement from the brake chamber into rotational force that spreads the brake shoes against the drum (Correct answer)
- To sense wheel rotation speed for ABS
- To hold the spring brakes in the released position
Correct answer: To convert the linear push rod movement from the brake chamber into rotational force that spreads the brake shoes against the drum
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.
Question 6: What is "clutch brake" on a heavy-duty manual transmission truck, and how is it correctly used?
- A brake that stops the engine from spinning during gear selection
- A small disc between the clutch disc and transmission input shaft that stops the transmission input shaft from spinning — it is engaged only at the last inch of clutch pedal travel when the truck is fully stopped, to enable first or reverse gear engagement without grinding (Correct answer)
- A device that automatically engages the parking brake when the clutch is depressed
- A brake system that only operates when the clutch is engaged
Correct answer: A small disc between the clutch disc and transmission input shaft that stops the transmission input shaft from spinning — it is engaged only at the last inch of clutch pedal travel when the truck is fully stopped, to enable first or reverse gear engagement without grinding
The clutch brake is a device found between the flywheel and the transmission input shaft on heavy-duty trucks. It stops the spinning transmission input shaft and gear cluster to enable first or reverse gear engagement when the vehicle is stopped. It is engaged only by pressing the clutch pedal all the way to the floor (last inch of travel). Using the clutch brake while the truck is moving damages it — it should only be used when fully stopped to shift into first or reverse.
ASE Practice Test (T1-T8: Medium-Heavy Truck Certifications) 2
A Class 8 truck driver reports the air brakes feel "spongy" and take longer to stop.
Which system component is MOST likely responsible?