Combination Vehicles CDL Practice Test

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Combination Vehicles is one of the three knowledge tests required for a Class A CDL. Our $15 Class A ELDT course covers coupling, stability, and combination-vehicle inspection in full.

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  1. How many times more likely is a fully loaded combination rig to roll over in a crash compared to an empty rig?
  2. What is one of the two key driving behaviors the AAMVA CDL Manual says will help prevent a rollover in a combination vehicle?
  3. Why does an off-center cargo load that makes a trailer lean increase rollover risk in a combination vehicle?
  4. What does a rearward amplification value of 3.5 for a triple-trailer combination mean?
  5. What causes the dangerous 'crack-the-whip' effect in combination vehicles?
  6. A tractor-trailer is traveling at 45 mph. Using the AAMVA following-distance guidance for combination vehicles, what should the driver add beyond the base rule of 1 second for each 10 feet of vehicle length?
  7. Which statement correctly describes stopping distance for large combination vehicles?
  8. Why does an empty or lightly loaded combination vehicle have poor traction when braking hard?
  9. Compared to a tractor-semitrailer loaded to maximum gross weight, how does a bobtail tractor (pulling no semitrailer) typically perform when stopping?
  10. Which trailer configuration is specifically identified by the AAMVA CDL Manual as prone to getting stuck on raised railroad-highway crossings?
  11. A single-axle tractor pulling a long trailer faces an elevated risk of getting stuck on a railroad crossing when which condition exists?
  12. If a combination vehicle becomes stuck on railroad tracks, what does the AAMVA CDL Manual say to do first?
  13. A trailer jackknife, where the trailer's wheels lock up and the trailer swings around, is MOST likely to happen when the trailer is:
  14. What is the earliest and best way to recognize that a trailer has started to skid?
  15. A driver feels the trailer begin to skid after braking hard. According to the AAMVA manual, what is the correct way to stop the skid?
  16. What is "off-tracking" (also called "cheating") in a combination vehicle?
  17. In a rig pulling two trailers (doubles), which wheels off-track the most during a turn?
  18. You are making a right turn and cannot complete it without entering another traffic lane. What does the AAMVA manual recommend?
  19. According to the AAMVA manual, how does a vehicle's length affect off-tracking?
  20. When backing a trailer, unlike backing a car, straight truck, or bus, which way do you first turn the steering wheel to make the trailer go left?
  21. If you must back a trailer along a curved path rather than a straight line, which side should you back toward, and why?
  22. Before backing a trailer into a tight space, what does the AAMVA manual recommend as the first step?
  23. While backing a trailer, you notice it beginning to drift off the intended path. What should you do?
  24. What is the trailer hand valve, also called the trolley valve or Johnson bar, intended to be used for?
  25. Why does the AAMVA manual say the foot brake is safer than the trailer hand valve for normal driving?
  26. Why does the AAMVA manual say you should never use the trailer hand valve to hold a trailer in place for parking?
  27. A trailer has no spring brakes and must be left parked. According to the AAMVA manual, what should the driver do in addition to setting the parking brakes?
  28. What causes the tractor protection valve to close automatically?
  29. On a newer tractor, what happens when the driver pulls out the red eight-sided trailer air supply knob?
  30. A driver inspecting an older tractor finds a lever instead of a push-pull knob for the trailer air supply control. Which statement is correct?
  31. What is the primary function of the service air-line in a combination vehicle?
  32. What happens if a driver crosses the service and emergency air-lines when coupling a trailer that has spring brakes?
  33. A driver crosses the air-lines while coupling an older trailer that has no spring brakes. What danger does the AAMVA manual describe?
  34. What is the correct way to connect a pair of glad hands?
  35. On a tractor-trailer's glad hands, what does the color blue identify?
  36. What is the purpose of "dead-end" or dummy couplers on a tractor when no trailer is attached?
  37. What fills the air tanks on a trailer or converter dolly?
  38. How does the relay valve on a trailer know how much air pressure to send to the trailer brakes?
  39. Why does the AAMVA manual instruct drivers to open each trailer air tank's drain valve daily, even if the tank has an automatic drain?
  40. On a set of doubles or triples, which shut-off valves (cut-out cocks) must be in the CLOSED position?
  41. What is the purpose of shut-off valves (also called cut-out cocks) on a combination vehicle?
  42. Where are shut-off valves (cut-out cocks) found on a combination vehicle?
  43. Which combination-vehicle components are NOT required to have spring brakes?
  44. On a trailer that has no spring brakes, what powers the emergency brakes, and when do they engage?
  45. Why is it important to use wheel chocks when parking a trailer that does not have spring brakes?
  46. A major leak develops in a trailer's service line. When will the driver typically notice a problem?
  47. As of what date are trailers and converter dollies required to be equipped with ABS?
  48. On a trailer equipped with ABS, what color is the ABS malfunction lamp, and where is it located?
  49. Converter dollies manufactured on or after March 1, 1998 are required to have an ABS malfunction lamp located where?
  50. On an older trailer where it isn't obvious whether ABS is installed, how can a driver check?
  51. How does ABS affect a combination vehicle's normal braking capability?
  52. A combination vehicle has ABS on the trailer only. What effect does this have, and what should the driver do if steering control is lost or a tractor jackknife begins?
  53. How should a driver brake a combination vehicle equipped with ABS, compared to one without it?
  54. If the ABS malfunction lamp comes on during a trip, what should the driver do, and what does having ABS NOT allow a driver to do?
  55. When coupling a tractor to a semitrailer, how should the tractor be positioned before backing up to the trailer?
  56. Before backing the tractor under a semitrailer, why does the AAMVA coupling procedure require the driver to check and, if necessary, adjust the trailer height?
  57. While inspecting a completed coupling with a flashlight, a driver looks into the back of the fifth wheel. What should the fifth wheel jaws be closed around?
  58. In the AAMVA 16-step tractor-semitrailer coupling procedure, which of these actions is performed last?
  59. During uncoupling, after the fifth wheel is unlocked, the AAMVA procedure says to pull the tractor forward only until it is clear of the trailer, stopping with the tractor frame still underneath it. Why?
  60. When uncoupling a loaded semitrailer, after the landing gear makes firm contact with the ground, why does the AAMVA procedure call for cranking the landing gear a few extra turns in low gear?
  61. After disconnecting the electrical cable from the trailer during uncoupling, how should the driver hang it on the tractor?
  62. When coupling a trailer to a pintle hook, how can the driver tell the latch is correctly locked?
  63. Before extending the trailer jack to raise the coupler off a tow vehicle's pintle hook, what must the driver check?
  64. When coupling a drawbar (ball-and-cup) connection, after reinstalling the safety lock screw and self-locking nut, to what torque should they be tightened?
  65. When coupling a gooseneck hitch, why does the AAMVA manual instruct the driver to attach safety chains as one of the required steps?
  66. During the walk-around inspection of a combination vehicle's lower fifth wheel, where should the locking jaws be closed?
  67. While inspecting a sliding fifth wheel during a combination-vehicle walk-around, why does the driver check that it is not positioned too far forward?
  68. What should a driver confirm about the landing gear during the combination-vehicle walk-around inspection?
  69. During the "check that air flows to all trailers" test on a combination vehicle, you charge the system, go to the rear, and open the emergency line shut-off valve at the back of the last trailer — but you hear no air escaping. What should you do?
  70. While testing the tractor protection valve, you charge the system, shut off the engine, and pump the brake pedal repeatedly to bleed down tank pressure. At what point should the trailer air supply control pop out (or move to "emergency")?
  71. To test the trailer emergency brakes, after charging the system and confirming the trailer rolls freely, what should the driver do next?
  72. During the trailer service brake test, the driver moves forward slowly and applies the brakes using only the hand control (trolley valve). Why does the procedure call for the hand valve here instead of the foot pedal?
Other practice categories:
General Knowledge Air Brakes Hazmat (H) Endorsement Tanker (N) Endorsement Doubles/Triples (T) Endorsement Passenger (P) Endorsement School Bus (S) Endorsement