CDL combination vehicles practice test

1. If a vehicle has ABS only on the trailer, what effect does this have on vehicle control?

1. It prevents the tractor from jackknifing completely.
2. It makes the trailer less likely to swing out.
3. It eliminates the need to monitor the tractor and trailer while slowing down.
4. It allows the driver to follow other vehicles more closely.

2. When driving a combination vehicle with a trailer, how should a driver steer during a lane change to prevent the trailer from tipping over?

1. Steer gently and smoothly.
2. Make a sudden movement with the steering wheel to correct quickly.
3. Apply the trailer hand brake to stabilize the rear.
4. Increase speed before turning to maintain momentum.

3. According to the handbook, why is it important to keep the air supply clean?

1. To prevent water and dirt from getting into the coupler and air lines
2. To ensure the trailer wheels turn freely when parked
3. To allow the tractor protection valve to close automatically
4. To increase the pressure sent to the relay valves

4. How much following distance should a driver maintain behind other vehicles when driving a combination vehicle?

1. One second for every 10 feet of vehicle length, plus another second if going over 40 mph.
2. Two seconds regardless of speed or vehicle length.
3. One second for every 15 feet of vehicle length.
4. Three seconds if going over 40 mph, otherwise two seconds.

5. Why is it important to check that the sliding fifth wheel is not positioned too far forward?

1. To ensure the kingpin is not damaged during loading.
2. To prevent the tractor frame from hitting the landing gear or the cab hitting the trailer during turns.
3. To allow the electrical cord to be firmly plugged in and secured.
4. To ensure the glide plate is securely mounted to the trailer frame.

6. What is the correct procedure to verify that air flows to all trailers in a combination vehicle?

1. Open the emergency line shut-off valve at the rear of the last trailer and listen for escaping air.
2. Check the front glad hands only to ensure the electrical cord is firmly plugged in.
3. Apply the parking brake and observe the pressure gauge without opening any valves.
4. Disconnect the service line at the tractor and reconnect it to test the seal.

7. According to the handbook, what is the primary function of the Anti-lock Brake System (ABS) during hard braking?

1. It significantly shortens the stopping distance compared to non-ABS vehicles.
2. It helps the driver keep the vehicle under control by preventing wheel lockup.
3. It replaces the normal braking system entirely to provide more stopping power.
4. It automatically increases braking pressure to stop the vehicle faster than manual braking.

8. How should a driver test the trailer service brakes?

1. Pull gently on the trailer with the tractor after pulling out the trailer air supply control.
2. Apply trailer brakes with the hand control (trolley valve) while moving forward slowly.
3. Open the emergency line shut-off valve at the rear of the last trailer.
4. Check that the glide plate is securely mounted to the trailer frame.

9. What happens to the trailer emergency brakes when the emergency line loses air pressure?

1. The trailer emergency brakes release automatically.
2. The trailer emergency brakes come on.
3. The trailer brakes remain disengaged until the foot brake is pressed.
4. The tractor protection valve opens to supply more air.

10. Why is it important to keep cargo as close to the ground as possible in a combination vehicle?

1. To lower the center of gravity and reduce the risk of rollover
2. To increase the vehicle's overall height for better visibility
3. To allow for faster acceleration on steep grades
4. To prevent the cargo from shifting during lane changes

11. When testing the tractor protection valve, within what pressure range should the control close or move to the emergency position?

1. Between 10 and 30 psi
2. Between 20 and 45 psi
3. Between 30 and 60 psi
4. Between 45 and 70 psi

12. Why do large combination vehicles take longer to stop when they are empty compared to when they are fully loaded?

1. The empty vehicle has less weight to slow down.
2. Stiff suspension springs and strong brakes give poor traction and cause bouncing.
3. The driver must allow extra time to check mirrors for the trailer swing.
4. Empty trailers have larger tires that require more distance to rotate.

13. What is the correct procedure for checking if the trailer brakes are functioning properly after connecting the air lines?

1. Listen for the sound of brakes applying and releasing while checking the pressure gauge for major air loss.
2. Visually inspect the brake pads through the wheel spokes while the engine is running.
3. Apply the hand brake and ensure the trailer does not move forward.
4. Check the air pressure gauge only after driving the vehicle for one mile.

14. What is the correct procedure for connecting glad hands to ensure a proper seal?

1. Press the two seals together with the couplers parallel to each other.
2. Press the two seals together with the couplers at a 90 degree angle to each other.
3. Twist the couplers 180 degrees before pressing them together.
4. Align the couplers vertically before locking them.

15. What is the primary function of shut-off valves located at the back of trailers?

1. To close off air lines when another trailer is not being towed
2. To drain moisture from the trailer air tanks daily
3. To engage the emergency brakes when the air supply is lost
4. To lock the glad hands together when hoses are not connected

16. Why must a driver use wheel chocks when parking a trailer that does not have spring brakes?

1. Because the emergency brakes only hold as long as there is air pressure in the trailer air tank
2. Because the tractor protection valve prevents the trailer brakes from engaging
3. Because the service line pressure is too low to hold the trailer stationary
4. Because the relay valves require constant pedal pressure to maintain braking force

17. What is the correct procedure for stopping a trailer skid once it has started?

1. Apply the trailer hand brake to straighten out the rig.
2. Release the service brakes to get traction back.
3. Turn the steering wheel sharply in the opposite direction of the skid.
4. Accelerate gently to pull the trailer straight.

18. How are the emergency and service air lines typically color-coded to prevent mixing them up?

1. Blue is used for emergency lines and red for service lines.
2. Red is used for emergency lines and blue for service lines.
3. Yellow is used for emergency lines and green for service lines.
4. Black is used for emergency lines and white for service lines.

19. According to the handbook, what is the specific pressure range at which the tractor protection valve closes automatically?

1. Between 10 and 20 psi
2. Between 20 and 45 psi
3. Between 45 and 60 psi
4. Between 60 and 80 psi

20. When coupling a tractor to a semitrailer, how should the driver position the tractor relative to the trailer?

1. Back under the trailer at a sharp angle to align the kingpin quickly.
2. Position the tractor directly in front of the trailer.
3. Leave a gap of at least two feet to allow for easy alignment.
4. Back up only until the fifth wheel is halfway under the trailer.

21. What happens to the trailer emergency brakes when air pressure in the emergency line is lost?

1. The emergency brakes come on
2. The trailer wheels turn freely without braking
3. The tractor protection valve opens to restore pressure
4. The service line pressure increases to compensate