Combination Vehicles knowledge test

CDL Coupling and Uncoupling Questions

Coupling and uncoupling is the most procedural topic on the Combination Vehicles test. It is a numbered sequence in the manual, the questions follow that sequence closely, and you will also have to demonstrate parts of it during the skills test. The reasoning behind each step is what makes it stick.

What the test asks about coupling and uncoupling

Almost every step exists to stop the trailer moving when it should not. You chock the trailer wheels if it has no spring brakes or you are not sure, because air can leak out of the trailer tank and release its emergency brakes. You set the parking brakes while the tractor is still pushing against the kingpin, which holds the rig with the pressure off the locking jaws. When you lower the landing gear on a loaded trailer you turn the crank a few extra turns in low gear to lift some weight off the tractor, but never so far that you lift the trailer off the fifth wheel. Read the steps as cause and effect and the order stops being arbitrary.

Where drivers lose marks

  • Lifting the trailer off the fifth wheel while cranking down the landing gear. The manual says to take some weight off the tractor, not to raise the trailer.
  • Skipping the wheel chocks because the trailer seems to be holding. If it has no spring brakes, or you are unsure, chock it.
  • Not checking clearance twice. There has to be clearance between the rear of the tractor frame and the landing gear so a sharp turn does not hit it, and between the top of the tractor tires and the nose of the trailer.
  • Forgetting the glad hands. Air lines come off the trailer and go onto the dummy couplers so they stay clean and dry.
  • Treating the kingpin and fifth wheel as a visual glance. You are checking that the kingpin is not bent or broken and that the fifth wheel is actually locked.

How to study itLearn it as a story with a reason attached to every step. If you can say why a step exists, you can reconstruct the order under exam pressure instead of hoping you remember the list.

50 Coupling and Uncoupling practice questions with answers

Every question below is followed by the correct answer and an explanation of why it is right. Read through them, then take the timed Combination Vehicles test to see how you do without the answers in front of you.

1. Why must you stow the landing gear crank handle securely before driving?

  • A. To prevent theft of the handle
  • B. To ensure the landing gear does not lower automatically
  • C. Because a loose handle could swing out and strike passing vehicles or pedestrians (correct answer)
  • D. To balance the weight of the trailer

Why: The crank handle must be securely fastened in its bracket so it does not swing out into traffic, causing damage or injury.

2. What is the proper angle of the fifth wheel before coupling a tractor to a semi-trailer?

  • A. Tilted down towards the rear of the tractor (correct answer)
  • B. Perfectly flat and horizontal
  • C. Tilted up towards the rear of the tractor
  • D. Tilted towards the left or right side of the frame

Why: Before coupling, the fifth wheel plate should be tilted down towards the rear of the tractor so the trailer apron can easily slide up onto it.

3. What color is the service air line glad hand usually painted?

  • A. Red
  • B. Blue (correct answer)
  • C. Green
  • D. Black

Why: The service air line and its glad hand are universally color-coded blue.

4. How can you visually verify that the fifth wheel locking lever is in the correct position after coupling?

  • A. It should be pulled all the way out
  • B. It should be pointing straight up
  • C. It should be pushed all the way in with the safety catch properly engaged (correct answer)
  • D. It should be removed and stored in the cab

Why: A proper coupling requires the release lever to be fully pushed in (retracted) and secured by a safety catch to prevent accidental unlocking.

5. After a successful coupling, how high should you crank the trailer landing gear?

  • A. Just high enough to clear the ground by a few inches
  • B. Halfway up
  • C. Fully raised with the crank handle secured in its bracket (correct answer)
  • D. Leave it touching the ground for stability

Why: The landing gear must be fully raised and stowed securely. If it is only partially raised, it could catch on raised ground, railroad tracks, or other obstacles.

6. When uncoupling an empty trailer, how should you adjust the landing gear?

  • A. Lower it until it makes firm contact with the ground (correct answer)
  • B. Lower it until it lifts the trailer high off the fifth wheel
  • C. Leave it halfway down
  • D. Keep it fully raised and use wood blocks instead

Why: For an empty trailer, simply lowering the landing gear until it rests firmly on the ground is sufficient before uncoupling.

7. When should you remove the wheel chocks from the trailer during the coupling procedure?

  • A. Before connecting the air lines
  • B. Before performing the tug test
  • C. After the coupling is complete, the landing gear is fully raised, and you are completely ready to drive away (correct answer)
  • D. As soon as the fifth wheel locks

Why: Wheel chocks should remain in place until the entire coupling procedure, including tests and raising the landing gear, is finished and you are ready to depart.

8. How do you properly perform a tug test to verify the coupling is secure?

  • A. Release all brakes and accelerate hard in reverse
  • B. Release the tractor brakes, leave the trailer brakes applied, and pull forward gently in low gear (correct answer)
  • C. Release the trailer brakes, leave the tractor brakes applied, and try to back up
  • D. Use the trailer hand valve to push the trailer backward

Why: A tug test involves applying the trailer brakes, releasing the tractor brakes, and gently pulling forward to ensure the fifth wheel jaws have firmly locked onto the kingpin.

9. When inspecting the coupling, you must use a flashlight to look under the trailer and verify that the locking jaws are closed around the:

  • A. Base of the kingpin
  • B. Head of the kingpin
  • C. Shank of the kingpin (correct answer)
  • D. Fifth wheel skid plate

Why: The locking jaws must close securely around the narrower middle part of the kingpin, known as the shank, to properly secure the trailer.

10. When checking clearance after coupling, you must ensure there is enough space between the tractor frame and the:

  • A. Tractor cab
  • B. Trailer landing gear to allow for sharp turns (correct answer)
  • C. Trailer axles
  • D. Exhaust stack

Why: You must check that the tractor frame and mudflaps have enough clearance to pass the landing gear during a sharp turn without catching.

11. In addition to the landing gear clearance, you must also ensure there is enough clearance between the top of the tractor frame and the:

  • A. Bottom of the trailer apron (correct answer)
  • B. Trailer roof
  • C. Top of the tractor cab
  • D. Trailer glad hands

Why: There must be adequate space between the tractor frame and the bottom of the trailer so they do not strike each other when going over bumps or dips.

12. Why is it dangerous to drive with a partially engaged or loose fifth wheel coupling?

  • A. It will cause the tires to wear out unevenly
  • B. It will increase the fuel consumption of the tractor
  • C. The trailer could detach from the tractor while driving, causing a catastrophic accident (correct answer)
  • D. The air compressor will have to work twice as hard

Why: A faulty or incomplete coupling means the trailer is not secured. It can drop off the tractor while in motion, causing severe property damage and loss of life.

13. When lowering the landing gear to uncouple a loaded trailer, you should turn the crank until the pads make firm contact with the ground and then:

  • A. Crank it until the trailer is lifted completely off the fifth wheel
  • B. Turn the crank in low gear a few extra turns to lift some weight off the tractor (correct answer)
  • C. Immediately fold up the crank handle without further turns
  • D. Release the air from the tractor suspension

Why: For a loaded trailer, after firm contact is made, turn the crank in low gear a few extra turns. This lifts some weight off the tractor making uncoupling easier, but you must not lift the trailer completely off the fifth wheel.

14. What is the specific purpose of the safety latch on the fifth wheel release lever?

  • A. To act as a secondary brake for the trailer
  • B. To prevent the release lever from accidentally opening while driving (correct answer)
  • C. To lock the glad hands in place
  • D. To keep the sliding fifth wheel from moving

Why: The safety latch ensures that the fifth wheel release arm cannot vibrate open or be accidentally pulled, keeping the trailer securely locked to the tractor.

15. What is the primary function of glad hand seals or grommets?

  • A. To conduct electricity between the tractor and trailer
  • B. To prevent air from escaping where the lines connect (correct answer)
  • C. To hold the glad hands to the trailer body
  • D. To lubricate the air lines

Why: Glad hand seals are rubber rings that create an airtight seal when the glad hands are locked together, preventing air pressure loss.

16. What controls the tractor protection valve, preventing total air loss in the tractor if the trailer breaks away?

  • A. The foot brake pedal
  • B. The trailer air supply valve in the cab (correct answer)
  • C. The steering wheel
  • D. The trailer hand valve

Why: The trailer air supply valve (the red octagon button) controls the tractor protection valve. If air pressure drops too low, the valve automatically pops out to seal off the tractor's air supply.

17. Why is easing pressure on the fifth wheel locking jaws necessary before uncoupling?

  • A. It engages the trailer spring brakes
  • B. It prevents the air lines from popping off
  • C. If the jaws are under tension from the kingpin, it will be very difficult or impossible to pull the release handle (correct answer)
  • D. It automatically lowers the landing gear

Why: When the tractor and trailer are parked, there is often pressure between the kingpin and the locking jaws. Easing this pressure makes pulling the release lever possible without strain.

18. Before pulling the fifth wheel release handle to uncouple a trailer, what should you do to make it easier to pull?

  • A. Back up gently to ease pressure on the fifth wheel locking jaws (correct answer)
  • B. Pull forward hard to stretch the locking mechanism
  • C. Lower the landing gear completely to lift the trailer entirely off the tractor
  • D. Disconnect the glad hands and electrical cord

Why: Easing the pressure by backing up gently and setting the parking brakes while pushing against the kingpin takes the tension off the locking jaws, allowing the handle to be pulled easily.

19. How can you test the trailer emergency brakes after coupling?

  • A. Push in the trailer air supply valve and step on the service brake
  • B. Pull out the trailer air supply valve and attempt to pull forward gently in low gear (correct answer)
  • C. Release the tractor parking brakes and coast in neutral
  • D. Use the trailer hand valve while driving at 10 mph

Why: Pulling out the red trailer air supply valve exhausts the air from the trailer, engaging the trailer emergency (spring) brakes. Gently pulling against them verifies they are holding.

20. If the tug test fails and the tractor moves forward freely without resistance from the trailer, what should you do?

  • A. Drive forward faster to test it again
  • B. Stop immediately, secure the tractor, and get out to inspect the coupling mechanism (correct answer)
  • C. Use the trailer hand valve to secure the connection
  • D. Turn the steering wheel sharply to lock the jaws

Why: If the tractor moves freely during a tug test, the coupling has failed. You must secure the vehicle immediately and physically inspect the fifth wheel and kingpin to correct the issue.

21. How do you supply air to the trailer to charge its air system?

  • A. Pull out the yellow parking brake valve
  • B. Push in the red trailer air supply valve (correct answer)
  • C. Step on the service brake pedal three times
  • D. Pull down the trailer hand valve

Why: Pushing in the red octagon-shaped trailer air supply valve allows air from the tractor to flow into the trailer air system, releasing the spring brakes and charging the tanks.

22. What color is the emergency air line glad hand usually painted?

  • A. Blue
  • B. Green
  • C. Yellow
  • D. Red (correct answer)

Why: The emergency air line and its glad hand are universally color-coded red to distinguish them from the service line.

23. During the uncoupling procedure, after properly lowering the landing gear, what is the next step?

  • A. Unlock the fifth wheel release handle
  • B. Disconnect the air lines and electrical cable (correct answer)
  • C. Pull the tractor forward
  • D. Remove the wheel chocks

Why: Once the landing gear is lowered and the trailer is supported, you must disconnect the air lines and electrical cable before unlocking the fifth wheel to ensure they are not stretched or torn when pulling away.

24. What is the purpose of lubricating the fifth wheel skid plate?

  • A. To prevent the kingpin from breaking
  • B. To keep the air lines from freezing
  • C. To reduce friction and wear between the tractor and the trailer apron during turns (correct answer)
  • D. To prevent rust on the landing gear

Why: The fifth wheel skid plate must be heavily lubricated with grease to allow the trailer apron to pivot smoothly against it when turning.

25. When positioning the tractor to couple with a trailer, how should the tractor be aligned?

  • A. At a 45-degree angle to the trailer
  • B. Directly in front of the trailer (correct answer)
  • C. Slightly to the left so you can see the trailer in your mirror
  • D. Slightly to the right to avoid striking the kingpin directly

Why: You must back the tractor directly in front of the trailer. If you back at an angle, you might push the trailer aside and damage the landing gear.

26. When preparing to couple, it is absolutely essential to chock the trailer wheels if:

  • A. The trailer does not have spring brakes or you are unsure if they are working (correct answer)
  • B. You are parking on a level surface
  • C. The trailer is fully loaded
  • D. You are driving a tractor with an automatic transmission

Why: Older trailers or trailers with defective air systems might not have functioning spring brakes. Chocking the wheels is the only way to ensure they do not roll backward during coupling.

27. Before coupling a tractor to a trailer, you should check your air pressure gauges to ensure the system is:

  • A. Completely empty of air
  • B. At normal operating pressure (usually 120-140 psi) to properly charge the trailer (correct answer)
  • C. Between 30 and 60 psi
  • D. Venting air rapidly

Why: You must have full air pressure before starting the coupling process so you have enough air to charge the trailer's air system and release its spring brakes.

28. How should the air lines and electrical cord be positioned after they are connected?

  • A. Tied tightly to the landing gear
  • B. Draped loosely over the fuel tanks
  • C. Suspended securely from the catwalk so they do not tangle, chafe, or drag on the frame (correct answer)
  • D. Wrapped around the drive shaft for safety

Why: Air lines and electrical cables must be properly supported so they do not rub against the tractor frame, drag on the ground, or catch on the catwalk, which could cause damage or failure.

29. When uncoupling, where is the safest place to park the combination vehicle?

  • A. On a soft shoulder
  • B. On a steep downhill grade
  • C. On a level, firm surface (correct answer)
  • D. On a gravel surface with a severe slope

Why: You should always attempt to park on a level, firm surface when uncoupling so the landing gear can support the weight of the trailer without sinking or tipping.

30. What is the correct way to connect glad hands together?

  • A. Push them together straight on until they click
  • B. Press the two seals together with the couplers at a 90-degree angle and twist them until they lock (correct answer)
  • C. Thread them together clockwise like a bolt
  • D. Snap them together using a locking pin

Why: Glad hands are connected by pressing their rubber seals against each other at a 90-degree angle and rotating them until they lock securely in place.

31. What is the most common cause of air leaking from the glad hands after they are connected?

  • A. The air pressure from the tractor is too high
  • B. Worn, damaged, or missing rubber seals (grommets) (correct answer)
  • C. The glad hands are made of different metals
  • D. The tractor protection valve is closed

Why: Glad hand rubber seals routinely dry out, tear, or fall out. Inspecting and replacing bad seals is the most common fix for air leaks at the connection.

32. What will happen if you forget to disconnect the air lines and electrical cord before pulling completely away during uncoupling?

  • A. The tractor protection valve will prevent damage
  • B. The lines will disconnect safely via quick-release mechanisms
  • C. The lines will be torn from the tractor or trailer, causing severe damage and brake failure (correct answer)
  • D. The trailer brakes will automatically release

Why: Forgetting to disconnect the air lines and electrical cable before pulling away will result in tearing them off, causing expensive damage and sudden air loss.

33. What is the most likely result if the trailer is too low before coupling?

  • A. The kingpin will lock perfectly
  • B. The tractor frame will strike the trailer nose and could cause damage (correct answer)
  • C. The glad hands will automatically connect
  • D. The fifth wheel will tilt forward automatically

Why: If the trailer is too low, the tractor frame will hit the nose of the trailer, potentially damaging the trailer or the tractor.

34. When backing the tractor under a trailer, at what speed should you move?

  • A. Fast enough to ensure a strong coupling impact
  • B. At least 5 mph
  • C. Use the lowest reverse gear and back very slowly (correct answer)
  • D. Coast in neutral to save fuel

Why: Coupling should always be done very slowly using the lowest reverse gear to avoid damaging the trailer, the landing gear, or the fifth wheel mechanism.

35. According to the official CDL coupling procedure, why should you connect the air lines and supply air to the trailer before backing fully under it?

  • A. To cool down the trailer brakes
  • B. So you can engage the trailer brakes and ensure the trailer will not roll back when the tractor pushes against it (correct answer)
  • C. To turn on the trailer brake lights
  • D. So you can test the electrical system first

Why: Connecting the air lines allows you to supply air to the trailer and then pull the red valve out to lock the trailer brakes. This prevents the trailer from rolling backward when the fifth wheel strikes the kingpin.

36. What will happen if you cross the air lines on a newer trailer equipped with spring brakes?

  • A. The trailer brakes will work normally
  • B. The trailer spring brakes will not release because they are not getting constant air pressure (correct answer)
  • C. The tractor air compressor will overheat
  • D. The trailer will steer to the right

Why: If air lines are crossed, the emergency line will not provide the constant air supply needed to compress the springs, meaning the spring brakes will remain locked.

37. What should you do with the glad hands when driving a bobtail tractor (without a trailer)?

  • A. Leave them dangling behind the cab
  • B. Attach them to dummy couplers or lock them together (correct answer)
  • C. Tie them to the exhaust stack
  • D. Store them inside the cab

Why: When not in use, glad hands should be connected to dummy couplers (dead-end receptacles) on the back of the cab, or locked to each other, to keep dirt and moisture out of the air system.

38. How do you use the trailer hand valve to check the trailer service brakes after coupling?

  • A. Push it up and accelerate hard
  • B. Pull down the hand valve, release the tractor brakes, and try to pull forward gently (correct answer)
  • C. Use it to bleed air from the air tanks
  • D. Tap it repeatedly while driving at highway speeds

Why: You can test the trailer service brakes by applying the trailer hand valve, releasing the tractor parking brakes, and gently pulling against the trailer to ensure the brakes are holding.

39. Why is it critical to lock the trailer brakes before backing the tractor fully under the trailer?

  • A. To prevent the trailer from moving backward when the tractor pushes against it (correct answer)
  • B. To test the air compressor governor
  • C. To allow the landing gear to lower automatically
  • D. To activate the tractor protection valve

Why: When the fifth wheel strikes the kingpin, it hits with significant force. If the trailer brakes are not locked, the trailer will simply roll backwards away from the tractor.

40. Before backing under the trailer, you should visually inspect the trailer kingpin for:

  • A. Proper color coding
  • B. Air leaks
  • C. Bends, cracks, or excessive wear (correct answer)
  • D. Proper tire pressure

Why: The kingpin is the crucial link between the tractor and trailer. It must be perfectly straight and free of cracks or excessive wear.

41. If the fifth wheel skid plate is completely lacking lubrication, what driving issue are you most likely to experience?

  • A. Brake failure
  • B. Steering problems due to the trailer resisting turning (correct answer)
  • C. Engine overheating
  • D. Loss of air pressure

Why: Without lubrication, the friction between the trailer and the fifth wheel will be so high that the trailer will push the tractor straight instead of pivoting, causing severe steering issues.

42. You are checking the back of the tractor before backing under a trailer. What are you primarily looking for?

  • A. Ensure the license plate is clean
  • B. Ensure there is nothing on the back of the tractor that will catch on the trailer during coupling (correct answer)
  • C. Check the tread depth of the drive tires
  • D. Ensure the exhaust stack is perfectly vertical

Why: Before backing, you must ensure the path is clear and nothing on the tractor frame will snag or crash into the trailer nose or undercarriage during the coupling maneuver.

43. When chocking the trailer wheels before coupling, where should the chocks be placed?

  • A. Only in front of the front axle
  • B. In front of and behind the trailer wheels (correct answer)
  • C. Only behind the rear axle
  • D. Between the dual tires

Why: Wheel chocks should be placed firmly in front of and behind the trailer wheels to prevent the trailer from rolling forward or backward during the coupling process.

44. What is the very first step when preparing to couple a tractor to a semi-trailer?

  • A. Connect the air lines
  • B. Back under the trailer
  • C. Inspect the fifth wheel and the trailer (correct answer)
  • D. Supply air to the trailer

Why: The official procedure states that the first step is always to inspect the fifth wheel mechanism and the trailer components before beginning any coupling maneuvers.

45. During uncoupling, why must you stop the tractor and pull only partially clear of the trailer before inspecting the landing gear?

  • A. To keep the air lines from overstretching
  • B. To prevent the trailer from dropping to the ground if the landing gear collapses (correct answer)
  • C. To make it easier to reconnect the electrical cable
  • D. To test the tractor protection valve

Why: Pulling the tractor forward just enough to disengage the fifth wheel but keeping the frame under the trailer prevents the trailer from falling to the ground if the landing gear is faulty or sinks into soft ground.

46. If you see a gap between the trailer apron and the fifth wheel skid plate after coupling, what does this indicate?

  • A. The coupling is not secure and the kingpin may be resting on top of the jaws (correct answer)
  • B. The trailer is fully loaded and ready to drive
  • C. The fifth wheel needs additional lubrication
  • D. The landing gear is fully raised

Why: There should be no space between the upper and lower fifth wheel. A gap means the kingpin may be resting on top of the closed locking jaws, meaning the trailer is not safely secured.

47. Before backing under the trailer, you should check its height. The trailer is at the proper height when:

  • A. The kingpin clears the fifth wheel by at least two inches
  • B. The trailer apron rests firmly on the tractor frame
  • C. The trailer lifts slightly when the tractor backs under it (correct answer)
  • D. The landing gear is completely suspended off the ground

Why: The trailer should be just low enough that it is raised slightly by the tractor when backing under it. This ensures the kingpin will engage properly without jumping over the fifth wheel.

48. What is the best way to verify that your air lines are not crossed after coupling?

  • A. Drive forward at 15 mph and slam on the brakes
  • B. Push in the trailer air supply valve, turn off the engine, and listen for air escaping when you apply and release the trailer brakes (correct answer)
  • C. Look at the color of the glad hands from the driver's seat
  • D. Check the air pressure gauge while accelerating

Why: By charging the trailer, turning off the engine, and using the hand valve or service brake, you can listen for the sound of air operating the trailer brakes. If you hear it, the lines are correct.

49. What is likely to happen if the trailer is too high before coupling?

  • A. The trailer nose will crush the tractor cab
  • B. The landing gear will collapse
  • C. The tractor frame will strike the trailer nose
  • D. The kingpin may pass over the top of the fifth wheel and lock behind it (correct answer)

Why: If the trailer is too high, it will not guide into the fifth wheel opening and the kingpin may jump over the fifth wheel, getting trapped behind it.

50. After unlocking the fifth wheel during uncoupling, what should you do next?

  • A. Pull the tractor completely out from under the trailer immediately
  • B. Pull the tractor forward slightly until the fifth wheel comes out from under the trailer, but leave the tractor frame under the trailer (correct answer)
  • C. Raise the landing gear back up
  • D. Release the trailer brakes

Why: You pull forward just enough to clear the kingpin from the fifth wheel but stop with the frame still under the trailer. This catches the trailer if the landing gear fails.

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