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Combination vehicles

Selected knowledge practice, not a complete licensing course or practical training.

Combination vehicles

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Question 1 of 64
  1. 1. Which tractor-trailer condition particularly increases the chance of getting hung up on a raised railroad crossing?
  2. 2. You are driving a tractor without its semitrailer. What should you remember about braking?
  3. 3. The trailer wheels have locked during braking and the trailer begins to swing. What action addresses the cause of this trailer skid?
  4. 4. A curved backing maneuver with a semitrailer cannot be avoided. Which side does the handbook favor when a safe setup permits it?
  5. 5. During ordinary driving, which control should normally apply the tractor and trailer service brakes together?
  6. 6. Why is the trailer hand valve unsuitable as a parking brake?
  7. 7. A trailer air hose tears apart. What is the purpose of the tractor protection valve?
  8. 8. On a newer tractor with the standard red eight-sided trailer-air control, what does pushing it in do?
  9. 9. What does the air pressure in the trailer service line communicate?
  10. 10. Which pair of functions belongs to the trailer emergency or supply air line?
  11. 11. How do relay valves help the trailer service brakes respond promptly?
  12. 12. When standard red and blue air-line markings are used, which connection is correct?
  13. 13. Before joining the tractor and trailer glad hands, what should be checked?
  14. 14. After supplying trailer air, the trailer spring brakes will not release. What connection error should you investigate?
  15. 15. Why is being able to pull an older trailer without spring brakes not proof that its air lines are connected correctly?
  16. 16. After disconnecting a trailer, why should unused air hoses be attached to suitable dummy couplers?
  17. 17. Why might a large leak in a trailer service line become obvious when you first apply the brakes?
  18. 18. Before normal travel, how should unused service and supply shut-off valves at the back of the last trailer be set?
  19. 19. What problem can insufficient lubrication of a conventional fifth-wheel plate cause?
  20. 20. For the conventional fifth wheel described in the handbook, what position is needed before backing under the trailer?
  21. 21. Before coupling with a sliding fifth wheel, what should you confirm about the slide?
  22. 22. A pre-coupling inspection shows a bent or broken trailer kingpin. What should you do?
  23. 23. Why should the tractor be lined up directly with the semitrailer before backing underneath?
  24. 24. For the handbook’s conventional fifth-wheel coupling, how high should the trailer be?
  25. 25. How should connected air hoses be positioned before the tractor backs fully under a semitrailer?
  26. 26. While following the handbook’s coupling checks, you apply and release trailer brakes with the engine off. What should you listen and watch for?
  27. 27. How should the tractor be backed under the trailer to engage the kingpin?
  28. 28. The coupling resists a gentle pull against locked trailer brakes. What else is still needed?
  29. 29. Before entering the coupling inspection area, what does the handbook specify to prevent another person moving the truck?
  30. 30. A visible gap remains between the upper and lower fifth-wheel plates after attempted coupling. What does that mean?
  31. 31. Where should conventional fifth-wheel locking jaws close around the kingpin?
  32. 32. After the locking lever is in the locked position, what should be checked about the safety latch?
  33. 33. After inserting the trailer electrical plug, what should you do with its retaining safety catch?
  34. 34. After the coupling is verified, how far should the landing gear be raised before travel?
  35. 35. What should happen to the landing-gear crank after the supports are raised?
  36. 36. After the trailer’s full weight rests on the tractor, which clearance check matters?
  37. 37. Before uncoupling a loaded semitrailer, what must be true of the parking surface?
  38. 38. Why should the tractor and trailer be lined up straight before pulling apart?
  39. 39. During the properly secured uncoupling sequence, why gently back against the kingpin before releasing the fifth wheel?
  40. 40. When lowering landing gear on a loaded trailer before uncoupling, what does the handbook advise after firm ground contact?
  41. 41. How does the handbook advise stowing the electrical cable after uncoupling?
  42. 42. Why stop with the tractor frame still under the trailer after the fifth wheel clears it during uncoupling?
  43. 43. Before the tractor moves completely clear of an uncoupled trailer, what should be checked?
  44. 44. A pintle-hook mount has a missing fastener and visible movement. What is the proper response?
  45. 45. Before towing with a pintle coupling, what must be verified after the drawbar eye is seated?
  46. 46. Under the cited federal rule for air-braked towing units, in what pressure range must the tractor protection valve or similar device operate automatically?
  47. 47. For an ordinary two-axle trailer required to have brakes, outside driveaway-towaway operations, how long must its automatically applied breakaway brakes remain applied under the cited federal rule?
  48. 48. During the secured check for service air at the back of the last trailer, what must be applied before opening that service-line shut-off briefly?
  49. 49. Two otherwise similar semitrailers carry equal cargo weight. One load is stacked much higher. What is the important handling concern?
  50. 50. Before departure, a loaded trailer visibly leans because its cargo is concentrated on one side. Which action addresses the rollover risk?
  51. 51. A loaded tractor-trailer is approaching a tight freeway exit curve. When should you bring the rig down to a safe turning speed?
  52. 52. During an abrupt lane change, the tractor stays upright but the trailer begins to roll. Why is the tractor’s response not proof that the maneuver is safe?
  53. 53. After a hard brake application, which observation can give early warning that a trailer is beginning to skid sideways?
  54. 54. The tractor’s front wheels clear a curb on a tight low-speed turn. Why must you still watch the trailer’s rear-wheel path?
  55. 55. During a tight low-speed turn with more than one trailer, which rear wheels generally off-track the most?
  56. 56. Why should a driver avoid swinging left before a right turn and leave the rear of the combination close to the curb while maintaining safe clearance?
  57. 57. With a conventional tractor-semitrailer initially straight, you want the trailer’s rear to begin moving left while backing. Which initial movement of the top of the steering wheel starts that change?
  58. 58. You have started a trailer turning left while backing by initially steering right. What must you do as the trailer develops the intended angle?
  59. 59. While backing straight slowly, you notice the trailer’s rear beginning to drift right of the desired path. How does the handbook describe the initial steering correction?
  60. 60. During a backing maneuver you can no longer tell whether the trailer will clear an overhead obstacle. What is the safest next step?
  61. 61. The trailer is developing too much angle during a slow backing maneuver and a safe forward space is available. Which choice is appropriate?
  62. 62. A combination has ABS on the trailer but not the tractor. During braking the tractor begins to jackknife. Does the trailer ABS guarantee control of the tractor?
  63. 63. Your tractor and trailer have functioning ABS. How should that affect the following space you leave?
  64. 64. After unloading a tractor-trailer, a driver assumes it can always stop in less distance and cuts the following gap. What is wrong with this assumption?
Review all 64 questions with answers

  1. Which tractor-trailer condition particularly increases the chance of getting hung up on a raised railroad crossing?

    Correct answer: Low clearance under the trailer or its landing gear.

    A raised crossing can catch low-slung equipment or landing gear. Check the route and clearance before entering; once stuck, get clear of the tracks and seek emergency help.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.4, Low-clearance rail crossing; printed 6-2; physical PDF page 98; reviewed September 21, 2026. ↗

  2. You are driving a tractor without its semitrailer. What should you remember about braking?

    Correct answer: Low weight on the rear drive axles can make smooth stopping more difficult.

    A bobtail tractor can have poor drive-axle traction and be difficult to stop smoothly. Allow adequate space; actual stopping performance depends on the vehicle, brakes, load and road.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.3, Bobtail braking; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

  3. The trailer wheels have locked during braking and the trailer begins to swing. What action addresses the cause of this trailer skid?

    Correct answer: Release the braking that caused the lockup so the trailer wheels can regain traction.

    A locked trailer wheel cannot provide normal directional grip. More trailer braking worsens this skid. This describes an actual wheel-lock trailer skid, not a general instruction to pump functioning ABS.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.5, Trailer-wheel skid; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  4. A curved backing maneuver with a semitrailer cannot be avoided. Which side does the handbook favor when a safe setup permits it?

    Correct answer: Back toward the driver’s side for better visibility.

    Driver-side backing generally gives a better view of the trailer and path. Straight backing is preferable when possible; inspect the area and stop to look again whenever uncertain.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Backing visibility; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  5. During ordinary driving, which control should normally apply the tractor and trailer service brakes together?

    Correct answer: The foot brake pedal.

    The foot pedal controls service braking throughout the combination. The trailer hand valve is for checking trailer brakes, not routine driving or straightening a skid.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.1, Trailer hand valve in traffic; printed 6-4; physical PDF page 100; reviewed September 21, 2026. ↗

  6. Why is the trailer hand valve unsuitable as a parking brake?

    Correct answer: Air can leak away and release the brakes it applied.

    A hand-valve application depends on air pressure and is not a secure parking method. Use the correct parking system and suitable chocks, especially for trailers without spring brakes.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.1, Hand valve is not parking; printed 6-4; physical PDF page 100; reviewed September 21, 2026. ↗

  7. A trailer air hose tears apart. What is the purpose of the tractor protection valve?

    Correct answer: Protect the tractor’s braking-air supply from escaping through the trailer system.

    The protection system limits the trailer failure’s effect on tractor air. It cannot repair the damaged connection or authorize continued operation of an unsafe combination.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.2, Tractor air protection; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  8. On a newer tractor with the standard red eight-sided trailer-air control, what does pushing it in do?

    Correct answer: Supplies air to the trailer system.

    The red control is pushed in to supply trailer air and pulled out to shut off supply and activate trailer emergency braking. Know any different controls on the actual vehicle.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.3, Red trailer-supply control; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  9. What does the air pressure in the trailer service line communicate?

    Correct answer: The amount of braking requested by the pedal or hand control.

    The service or control line carries the braking signal to relay valves. It is not the line whose principal job is charging the trailer reservoirs.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.4, Service-line signal; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  10. Which pair of functions belongs to the trailer emergency or supply air line?

    Correct answer: Charge the trailer tanks and control emergency braking when supply pressure is lost.

    The supply line fills trailer reservoirs. Loss of its pressure also activates the trailer emergency-brake system; it has a different job from the service signal line.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.4, Supply-line functions; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  11. How do relay valves help the trailer service brakes respond promptly?

    Correct answer: They deliver air from nearby trailer reservoirs in response to the service-line signal.

    The control signal tells relay valves how much reservoir air to send to the trailer brakes. Supplying that air locally speeds the response.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.6, Trailer relay valves; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  12. When standard red and blue air-line markings are used, which connection is correct?

    Correct answer: Red to emergency/supply and blue to service.

    The usual color code separates the red supply line from the blue service line. Verify markings and the vehicle’s connections rather than assuming every hose is correctly identified.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.5, Identifying air connections; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  13. Before joining the tractor and trailer glad hands, what should be checked?

    Correct answer: That the rubber seals and mating surfaces are clean and sound.

    Damaged or dirty seals can cause air leaks. Properly matched, clean couplers form the sealed connection needed for brake operation.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.5, Glad-hand seals; printed 6-5; physical PDF page 101; reviewed September 21, 2026. ↗

  14. After supplying trailer air, the trailer spring brakes will not release. What connection error should you investigate?

    Correct answer: The service and supply air lines may be crossed.

    Crossed lines can send supply air into the service circuit instead of filling the trailer reservoirs. Diagnose the fault before moving; do not force the vehicle against the brakes.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.5, Crossed lines and spring brakes; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  15. Why is being able to pull an older trailer without spring brakes not proof that its air lines are connected correctly?

    Correct answer: It may roll even though crossed lines have left it without working trailer brakes.

    An older trailer with depleted stored air may roll freely. Perform the required trailer-brake checks rather than using movement as evidence of a safe connection.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.5, Older trailer moves without braking; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  16. After disconnecting a trailer, why should unused air hoses be attached to suitable dummy couplers?

    Correct answer: To keep water and dirt out of the air connections.

    Dummy couplers protect the disconnected lines from contamination. Secure the hoses too, so they cannot drag or strike moving parts.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.5, Unused glad hands; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  17. Why might a large leak in a trailer service line become obvious when you first apply the brakes?

    Correct answer: That line is pressurized by a service-brake application.

    A service-line leak may not show as an obvious pressure loss until braking is requested. A sudden pressure drop during the check must be investigated.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.8, Service-line leak timing; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  18. Before normal travel, how should unused service and supply shut-off valves at the back of the last trailer be set?

    Correct answer: Closed.

    Close the lines where no following trailer is connected. Intermediate connections supplying another trailer must instead allow air through.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.2.7, Last trailer shut-offs; printed 6-6; physical PDF page 102; reviewed September 21, 2026. ↗

  19. What problem can insufficient lubrication of a conventional fifth-wheel plate cause?

    Correct answer: Excess friction that interferes with steering.

    The plate needs the lubrication specified for that equipment. Excess friction between tractor and trailer can hinder steering; lubrication does not replace correct locking.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Fifth-wheel lubrication; printed 6-7; physical PDF page 103; reviewed September 21, 2026. ↗

  20. For the conventional fifth wheel described in the handbook, what position is needed before backing under the trailer?

    Correct answer: Plate tilted down toward the tractor’s rear, with the jaws open.

    The described preparation lets the trailer enter and the kingpin engage the fifth wheel. Follow the actual fifth-wheel manufacturer’s coupling procedure.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Fifth-wheel coupling position; printed 6-7–6-8; physical PDF pages 103–104; reviewed September 21, 2026. ↗

  21. Before coupling with a sliding fifth wheel, what should you confirm about the slide?

    Correct answer: Its locking mechanism is engaged.

    A sliding fifth wheel must be locked in its intended position. The trailer connection does not substitute for the slide’s own locks.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Sliding fifth-wheel security; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  22. A pre-coupling inspection shows a bent or broken trailer kingpin. What should you do?

    Correct answer: Do not couple and operate it until the defect is properly corrected.

    The kingpin is a critical coupling part. A low load or a forceful coupling attempt does not make a damaged pin safe.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Damaged kingpin; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  23. Why should the tractor be lined up directly with the semitrailer before backing underneath?

    Correct answer: An angled approach can push the trailer sideways and damage its landing gear.

    Approach straight and slowly. Side loading during coupling can damage supports or move the trailer unexpectedly.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Straight coupling approach; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  24. For the handbook’s conventional fifth-wheel coupling, how high should the trailer be?

    Correct answer: Low enough for the tractor to lift it slightly while backing underneath.

    A trailer set too high can miss proper engagement; too low can damage the nose. Adjust height and alignment before completing the coupling.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Trailer height for coupling; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  25. How should connected air hoses be positioned before the tractor backs fully under a semitrailer?

    Correct answer: Supported clear of places where they could be crushed or caught.

    Coupling movement can pinch or snag poorly routed hoses. Support them so the connection remains intact as the tractor moves under the trailer.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Air-hose support during coupling; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  26. While following the handbook’s coupling checks, you apply and release trailer brakes with the engine off. What should you listen and watch for?

    Correct answer: Brake movement and release air, without signs of major pressure loss.

    Listening helps confirm trailer application and release, while the pressure gauge can reveal major air loss. Perform the complete procedure with the vehicle secured.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Confirming trailer brake response; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  27. How should the tractor be backed under the trailer to engage the kingpin?

    Correct answer: Slowly, using the lowest reverse gear.

    A controlled low-speed approach avoids striking the kingpin too hard. Stop when the proper connection is made, then verify it.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Coupling speed; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  28. The coupling resists a gentle pull against locked trailer brakes. What else is still needed?

    Correct answer: A visual check of correct fifth-wheel engagement and locking.

    A gentle pull is one check, not a replacement for looking at the plates, jaws, kingpin and locking mechanism. Complete both parts of verification.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Tug test plus inspection; printed 6-8–6-9; physical PDF pages 104–105; reviewed September 21, 2026. ↗

  29. Before entering the coupling inspection area, what does the handbook specify to prevent another person moving the truck?

    Correct answer: Secure the vehicle, shut the engine off and take the key.

    The handbook specifies neutral, parking brakes, engine off and taking the key. Use the full safe inspection procedure and stay clear of pinch and wheel hazards.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Securing before coupling inspection; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  30. A visible gap remains between the upper and lower fifth-wheel plates after attempted coupling. What does that mean?

    Correct answer: The coupling may be incorrect and must be corrected before driving.

    A gap can indicate a high-hitched or otherwise incorrect connection. Do not drive until the actual cause is resolved and correct engagement is verified.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Gap between fifth-wheel plates; printed 6-8; physical PDF page 104; reviewed September 21, 2026. ↗

  31. Where should conventional fifth-wheel locking jaws close around the kingpin?

    Correct answer: Around the shank, not the head.

    The South Carolina walk-around checklist specifies jaws around the kingpin shank, not its head. Secure the rig and visually inspect the actual lock; a tug test alone does not establish correct engagement.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.5.1, Kingpin engagement; printed 6-13; physical PDF page 109; reviewed September 21, 2026. ↗

  32. After the locking lever is in the locked position, what should be checked about the safety latch?

    Correct answer: It is properly engaged as required for that fifth wheel.

    The latch or catch helps retain the locked mechanism. Some equipment requires placing it by hand; follow the actual manufacturer’s design and instructions.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Coupling safety latch; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  33. After inserting the trailer electrical plug, what should you do with its retaining safety catch?

    Correct answer: Fasten it and confirm the cord is secure.

    A loose plug can disconnect as the rig moves. Secure the connection and route the cord away from damaging contact.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Trailer electrical connection; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  34. After the coupling is verified, how far should the landing gear be raised before travel?

    Correct answer: All the way into its proper travel position.

    Partly raised supports can strike a crossing or other obstruction. Fully raise them and secure the operating handle before moving off.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Landing gear travel position; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  35. What should happen to the landing-gear crank after the supports are raised?

    Correct answer: Secure the handle in its proper stowed position.

    A properly stowed crank cannot swing into hazards or come loose during the trip. Check the manufacturer’s retention arrangement.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Landing-gear crank; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  36. After the trailer’s full weight rests on the tractor, which clearance check matters?

    Correct answer: Space between tractor components, landing gear and the trailer during turns.

    Check frame-to-landing-gear and tire-to-trailer clearance with the trailer’s weight on the tractor. Correct coupling must also leave adequate clearance for turning.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.1, Clearance after load transfer; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  37. Before uncoupling a loaded semitrailer, what must be true of the parking surface?

    Correct answer: It can support the trailer’s weight on its landing gear.

    Once uncoupled, the supports transfer weight to the ground. An unsuitable surface can let the landing gear sink or the trailer fall.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Uncoupling surface; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  38. Why should the tractor and trailer be lined up straight before pulling apart?

    Correct answer: Pulling out at an angle can damage the landing gear.

    The handbook calls for straight alignment to avoid side loads on the supports. Keep the rig secured while preparing to release the coupling.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Straight uncoupling alignment; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  39. During the properly secured uncoupling sequence, why gently back against the kingpin before releasing the fifth wheel?

    Correct answer: To ease pressure on the locking jaws.

    Relieving tension can make the locking lever easier to release. Follow the complete sequence, including trailer security and tractor parking brakes.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Relieving jaw pressure; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  40. When lowering landing gear on a loaded trailer before uncoupling, what does the handbook advise after firm ground contact?

    Correct answer: Ease tractor load without lifting the trailer off the fifth wheel.

    After firm ground contact, the handbook uses a few additional low-gear turns to take some weight off the tractor and ease release. Do not jack the trailer completely off the fifth wheel.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Loaded-trailer support setting; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  41. How does the handbook advise stowing the electrical cable after uncoupling?

    Correct answer: Hang it securely with the plug facing down to keep moisture out.

    The downward-facing plug helps prevent moisture entry. Support the cable so it cannot be damaged during bobtail travel.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Disconnected electrical cord; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  42. Why stop with the tractor frame still under the trailer after the fifth wheel clears it during uncoupling?

    Correct answer: The frame offers protection if the landing gear collapses or sinks.

    The intermediate stop provides a safeguard before fully leaving the trailer. Secure the tractor and inspect the supports before pulling completely clear.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Partial pull-out safeguard; printed 6-9; physical PDF page 105; reviewed September 21, 2026. ↗

  43. Before the tractor moves completely clear of an uncoupled trailer, what should be checked?

    Correct answer: That the ground supports the trailer and the landing gear is not damaged.

    Confirm the supports are sound and bearing the trailer safely. A failed support can let the trailer fall after the tractor moves away.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.2, Support verification before clearing; printed 6-10; physical PDF page 106; reviewed September 21, 2026. ↗

  44. A pintle-hook mount has a missing fastener and visible movement. What is the proper response?

    Correct answer: Have the unsafe mounting corrected before towing.

    The pintle hook and its mounting must be secure and undamaged. A locked latch cannot make an insecure mounting safe.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.3, Pintle-hook mounting; printed 6-10; physical PDF page 106; reviewed September 21, 2026. ↗

  45. Before towing with a pintle coupling, what must be verified after the drawbar eye is seated?

    Correct answer: The latch and any required locking pin are properly secured.

    Correct seating must be followed by the equipment’s locking procedure. An unsecured latch can permit separation; do not generalize one manufacturer’s handle geometry to every hook.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.4.3 Step 4, Lock Pintle Hook (continued on next page); printed 6-10–6-11; physical PDF pages 106–107; reviewed September 21, 2026. ↗

  46. Under the cited federal rule for air-braked towing units, in what pressure range must the tractor protection valve or similar device operate automatically?

    Correct answer: Between 20 and 45 psi.

    49 CFR 393.43(a) specifies 20–45 psi. The exact set point and inspection procedure come from the equipment manufacturer; this is distinct from the low-air warning threshold.

    Source: 49 CFR 393.43(a); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  47. For an ordinary two-axle trailer required to have brakes, outside driveaway-towaway operations, how long must its automatically applied breakaway brakes remain applied under the cited federal rule?

    Correct answer: At least 15 minutes.

    49 CFR 393.43(d) requires immediate automatic breakaway application and at least 15 minutes of holding. The question excludes the driveaway-towaway exception and the separate three-or-more-axle application qualification.

    Source: 49 CFR 393.43(d), (f); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  48. During the secured check for service air at the back of the last trailer, what must be applied before opening that service-line shut-off briefly?

    Correct answer: The trailer hand brake or service brake pedal.

    The service line needs an application signal for this test. Close the valve after checking; all intermediate valves must pass air, and the complete procedure requires rollaway protection.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.5.2, Service air through the complete rig; printed 6-14; physical PDF page 110; reviewed September 21, 2026. ↗

  49. Two otherwise similar semitrailers carry equal cargo weight. One load is stacked much higher. What is the important handling concern?

    Correct answer: The higher load raises the center of gravity and increases rollover risk.

    Stacking cargo higher raises the center of gravity. Keep cargo as low as practical and reduce speed for turns; legal total weight alone does not establish that a loaded trailer is stable.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.1, High load and rollover; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

  50. Before departure, a loaded trailer visibly leans because its cargo is concentrated on one side. Which action addresses the rollover risk?

    Correct answer: Correct the loading so the cargo is centered and properly distributed.

    A load concentrated on one side makes a trailer more likely to overturn. Correct its distribution and secure it before travel; steering technique does not replace a properly balanced load.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.1, Side-to-side cargo balance; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

  51. A loaded tractor-trailer is approaching a tight freeway exit curve. When should you bring the rig down to a safe turning speed?

    Correct answer: Before entering the turn.

    Slow to a safe speed before the turn and steer smoothly. Waiting until the rig is already in the curve can force abrupt braking or steering when the trailer is least able to tolerate it.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.2, Speed before a curve; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

  52. During an abrupt lane change, the tractor stays upright but the trailer begins to roll. Why is the tractor’s response not proof that the maneuver is safe?

    Correct answer: A sudden steering input can be amplified toward the rear of the combination.

    The crack-the-whip effect can overturn a trailer even while the tractor remains upright. Look ahead, leave room and use gradual steering instead of judging the entire rig by the tractor alone.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.2, Rearward amplification; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

  53. After a hard brake application, which observation can give early warning that a trailer is beginning to skid sideways?

    Correct answer: The trailer starting to move out of its proper position in the mirrors.

    Check the mirrors during hard braking to see whether the trailer is staying in line. A swinging trailer can be difficult to recover once it leaves the lane; pressure and speed readings do not show its lateral position.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.5, Recognizing trailer skid early; printed 6-2; physical PDF page 98; reviewed September 21, 2026. ↗

  54. The tractor’s front wheels clear a curb on a tight low-speed turn. Why must you still watch the trailer’s rear-wheel path?

    Correct answer: The trailer’s rear wheels take a different, tighter path and may strike the curb.

    In a tight turn, the rear wheels do not follow the same path as the front wheels. Allow sufficient turning space for the trailer instead of using front-wheel clearance as proof that the whole rig fits.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.6, Offtracking clearance; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  55. During a tight low-speed turn with more than one trailer, which rear wheels generally off-track the most?

    Correct answer: The last trailer’s rear wheels.

    The handbook explains that offtracking increases toward the rear of a multiple-trailer combination. Plan clearance for the last trailer; checking the tractor or first trailer alone can miss an obstruction.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.6, Last-trailer wheel path; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  56. Why should a driver avoid swinging left before a right turn and leave the rear of the combination close to the curb while maintaining safe clearance?

    Correct answer: It reduces the opening that could tempt another driver to pass on the right.

    An early swing left can invite a vehicle into the space beside the trailer. Use a safe turning setup, monitor the right side and allow the front enough room as the turn develops without running over the curb or pedestrians.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.6, Right-turn squeeze hazard; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  57. With a conventional tractor-semitrailer initially straight, you want the trailer’s rear to begin moving left while backing. Which initial movement of the top of the steering wheel starts that change?

    Correct answer: Turn it to the right.

    A trailer initially responds opposite to the top of the steering-wheel movement during backing. Start the turn slowly, then adjust to follow the trailer; this is not an instruction to hold the initial steering angle throughout the maneuver.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Starting a trailer backing turn; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  58. You have started a trailer turning left while backing by initially steering right. What must you do as the trailer develops the intended angle?

    Correct answer: Turn the wheel the other way as needed so the tractor follows the trailer.

    After initiating a backing turn, change the steering direction to follow the trailer and manage its angle. Continue slowly and monitor both sides; leaving the initial input unchanged can let the angle grow too far.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Following a backing trailer; printed 6-3; physical PDF page 99; reviewed September 21, 2026. ↗

  59. While backing straight slowly, you notice the trailer’s rear beginning to drift right of the desired path. How does the handbook describe the initial steering correction?

    Correct answer: Turn the top of the wheel toward the right-hand drift, then adjust as needed.

    Correct drift promptly by turning the top of the steering wheel toward the direction of the drift. Use small controlled corrections and follow the trailer as alignment improves; brakes are not a substitute for steering the backing path.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Correcting trailer drift; printed 6-4; physical PDF page 100; reviewed September 21, 2026. ↗

  60. During a backing maneuver you can no longer tell whether the trailer will clear an overhead obstacle. What is the safest next step?

    Correct answer: Stop, secure the vehicle and get out to recheck the path and clearance.

    When unsure of the backing path, stop and look again. The handbook specifically calls for checking side and overhead clearance; a clear path at tire height does not establish that the trailer roof will fit.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Uncertain backing clearance; printed 6-4; physical PDF page 100; reviewed September 21, 2026. ↗

  61. The trailer is developing too much angle during a slow backing maneuver and a safe forward space is available. Which choice is appropriate?

    Correct answer: Stop and use a controlled pull-up to reposition before continuing.

    Use pull-ups when needed to improve the vehicle’s position. Repositioning safely is preferable to continuing a poorly aligned backing movement; check that the forward path is clear before moving.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.7, Pull-up to reposition; printed 6-4; physical PDF page 100; reviewed September 21, 2026. ↗

  62. A combination has ABS on the trailer but not the tractor. During braking the tractor begins to jackknife. Does the trailer ABS guarantee control of the tractor?

    Correct answer: No; ease braking if safe to do so to regain control of the tractor.

    Trailer ABS can reduce trailer wheel lockup, but it does not control unprotected tractor wheels. The handbook advises easing braking when safe if steering is lost or a tractor jackknife begins so control can be recovered.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.3.2, ABS on trailer only; printed 6-7; physical PDF page 103; reviewed September 21, 2026. ↗

  63. Your tractor and trailer have functioning ABS. How should that affect the following space you leave?

    Correct answer: Keep a safe gap; ABS is not permission to follow more closely.

    ABS helps control wheel lockup but does not remove the need for space, suitable speed and careful driving. A functioning system is not a guarantee that the combination can stop in a shorter distance.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.3.2, ABS and following distance; printed 6-7; physical PDF page 103; reviewed September 21, 2026. ↗

  64. After unloading a tractor-trailer, a driver assumes it can always stop in less distance and cuts the following gap. What is wrong with this assumption?

    Correct answer: A lightly loaded combination can have poorer traction and may take longer to stop.

    Strong brakes and stiff suspension on a lightly loaded combination can contribute to poor traction and wheel lockup. Keep ample following distance when empty as well as loaded; actual stopping performance also depends on the vehicle and road.

    Source: South Carolina Commercial Driver’s License Manual, March 2025; Section 6.1.3, Empty combination braking; printed 6-1; physical PDF page 97; reviewed September 21, 2026. ↗

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Combination vehicles

64 questions from our CDL question set. Read each explanation to review the material.

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Study questions checked against SCDMV licensing information, its posted CDL manual and cited federal rules. Review date: September 21, 2026. This is selected study coverage, not a complete course. Follow the cited source and the actual vehicle manufacturer’s procedure; equipment and brake-system differences matter. These quizzes do not replace supervised inspection or hands-on training.