CDL Combination Vehicles Practice Test 3 — Questions and Answers
Question 1: After coupling, you should check that the fifth wheel is locked by:
- Pulling the tractor forward gently with the trailer brakes locked (Correct answer)
- Honking the horn twice
- Tapping the trailer tires
- Watching the air gauge rise
Correct answer: Pulling the tractor forward gently with the trailer brakes locked
With the trailer brakes locked, gently pulling forward against the kingpin (tug test) confirms the fifth wheel is firmly locked.
Question 2: What is the proper height for the fifth wheel before backing under the trailer?
- The trailer will be raised slightly when the tractor backs under it (Correct answer)
- Several inches above the trailer apron
- Resting flat on the ground
- Exactly level with the kingpin
Correct answer: The trailer will be raised slightly when the tractor backs under it
The fifth wheel should be low enough that the tractor raises the trailer slightly as it backs under, ensuring solid contact.
Question 3: Trailer brakes that come on automatically when the air supply is lost are called:
- Emergency (spring) brakes (Correct answer)
- Service brakes
- Parking pawls
- Trolley brakes
Correct answer: Emergency (spring) brakes
Spring brakes apply automatically when air pressure drops, providing an emergency braking function.
Question 4: When checking the coupling, the gap between the upper and lower fifth wheel should be:
- Wide enough to fit your fist
- There should be no space between them (Correct answer)
- About one inch
- Filled with grease only
Correct answer: There should be no space between them
There should be no space between the upper and lower fifth wheel; a gap means something is wrong and could cause separation.
Question 5: The 'crack-the-whip' effect in combination vehicles refers to:
- A sudden steering move amplifying trailer sway and risking rollover (Correct answer)
- Snapping the air lines during coupling
- The sound brakes make when cold
- Tire blowout vibration
Correct answer: A sudden steering move amplifying trailer sway and risking rollover
Quick steering movements can throw the trailer sideways, and this rearward amplification can cause the trailer to roll over.
Question 6: Glad hands are used to:
- Connect the service and emergency air lines between tractor and trailer (Correct answer)
- Lock the kingpin
- Raise the landing gear
- Adjust the fifth wheel angle
Correct answer: Connect the service and emergency air lines between tractor and trailer
Glad hands are the coupling devices used to connect the air brake lines between the tractor and trailer.
Question 7: If the service air line connection is crossed with the emergency line, what may happen?
- You may not have trailer service brakes when you press the brake pedal (Correct answer)
- The trailer lights will fail
- The engine will not start
- The fifth wheel will unlock
Correct answer: You may not have trailer service brakes when you press the brake pedal
Crossed air lines can leave you without trailer service braking, a dangerous condition you must check for before driving.
CDL Combination Vehicles Practice Test 3
After coupling, you should check that the fifth wheel is locked by: