CDL Combination Vehicles Practice Test

All 24 questions with answers

For reading through. Tap "Show answer" under a question to see the answer and the reason.

1. In a set of triples, which trailer is most likely to turn over?

   1. The middle trailer
   2. The first trailer
   3. The last trailer
   4. All are equally likely
   Show answer

   C. The last trailer Rearward amplification: a quick steering move is amplified down the train, the "crack the whip" effect. The last trailer moves the most. (CDL manual)

2. Fully loaded rigs are about how many times more likely to roll over in a crash than empty ones?

   1. 5 times
   2. 10 times
   3. The same
   4. Twice
   Show answer

   B. 10 times About 10 times. Keep the cargo as low as possible and take turns and ramps slowly. (CDL manual)

3. Glad hands are color-coded. Which is which?

   1. Service yellow, emergency green
   2. Both black
   3. Service line red, emergency line blue
   4. Service line blue, emergency line red
   Show answer

   D. Service line blue, emergency line red Service (control) line blue, emergency (supply) line red. The coupler seals must be clean and not leaking. (CDL manual)

4. What happens if the emergency (supply) line comes apart while driving?

   1. The trailer emergency brakes come on
   2. Nothing until you brake
   3. The tractor brakes fail
   4. The trailer loses its lights
   Show answer

   A. The trailer emergency brakes come on Loss of pressure in the emergency line makes the tractor protection valve close and the trailer emergency (spring) brakes come on. (CDL manual)

5. The tractor protection valve closes automatically when air pressure drops to about:

   1. 60 to 80 psi
   2. 100 to 120 psi
   3. 20 to 45 psi
   4. 0 psi only
   Show answer

   C. 20 to 45 psi It closes in the 20 to 45 psi range, keeping air in the tractor if the trailer breaks away or leaks badly. (CDL manual)

6. The trailer air supply control (red, eight-sided knob) is pulled out to:

   1. Release the parking brake
   2. Sound the air horn
   3. Shut off the trailer air supply and put on the trailer emergency brakes
   4. Supply air to the trailer
   Show answer

   C. Shut off the trailer air supply and put on the trailer emergency brakes Push it in to supply the trailer with air; pull it out to shut off the air and apply the trailer emergency brakes. (CDL manual)

7. Should you use the trailer hand valve (trolley valve) to park?

   1. Only overnight
   2. Yes, it holds the trailer
   3. Only on level ground
   4. No, air could leak off and the trailer could roll
   Show answer

   D. No, air could leak off and the trailer could roll Never park with the trailer hand valve: if air leaks off, the brakes release. Use the parking brakes. Do not use it while driving either, because of the skid risk. (CDL manual)

8. Before backing under a trailer, the trailer should be at a height where:

   1. The landing gear is fully retracted
   2. The kingpin is well above the fifth wheel
   3. The trailer is raised slightly by the tractor as you back under
   4. The fifth wheel is higher than the trailer by a foot
   Show answer

   C. The trailer is raised slightly by the tractor as you back under The trailer should be low enough to be lifted slightly when the tractor backs under it. Too high and it may not couple correctly. (CDL manual)

9. After coupling, when you look at the fifth wheel you should see:

   1. No space between the upper and lower fifth wheel
   2. A small gap between the upper and lower fifth wheel
   3. The kingpin resting on top of the jaws
   4. The release arm pulled out
   Show answer

   A. No space between the upper and lower fifth wheel No gap. If there is space, something is wrong: the kingpin may be on top of the closed jaws and the trailer could come loose. (CDL manual)

10. The fifth wheel locking jaws should close around:

    1. The shank of the kingpin
    2. The apron of the trailer
    3. The head of the kingpin
    4. The landing gear
    Show answer

    A. The shank of the kingpin Around the shank, not the head. Use a flashlight and look under the trailer to check. (CDL manual)

11. How do you check the coupling is secure before driving off?

    1. With the trailer brakes locked, pull gently forward in low gear
    2. Back up hard into the trailer
    3. Check the glad hands only
    4. Rock the steering wheel
    Show answer

    A. With the trailer brakes locked, pull gently forward in low gear The "tug test": with the trailer brakes on, pull forward gently in low gear. The tractor should be held back by the trailer. (CDL manual)

12. Before driving away with a coupled trailer, the landing gear should be:

    1. Removed
    2. Left down in case of an emergency
    3. Raised just enough to clear the ground
    4. Fully raised, with the crank handle secured
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    D. Fully raised, with the crank handle secured Fully raised and the handle secured. Landing gear left partly down can catch on railroad tracks or bumps. (CDL manual)

13. Pulling doubles, which trailer should be right behind the tractor?

    1. The heavier one
    2. The lighter one
    3. It does not matter
    4. The shorter one
    Show answer

    A. The heavier one The heavier trailer goes first, behind the tractor. The lighter one is at the rear. (CDL manual)

14. On doubles and triples, the air shut-off valves at the rear of the trailers should be:

    1. Open on all but the last trailer, closed on the last
    2. Open on every trailer
    3. Closed on all trailers
    4. Closed on the first trailer only
    Show answer

    A. Open on all but the last trailer, closed on the last Open so air can reach every trailer behind, and closed at the very back so air does not escape from the last trailer. (CDL manual)

15. When a combination vehicle turns, the rear wheels:

    1. Follow a shorter path than the front wheels (off-tracking)
    2. Stay straight
    3. Follow a longer path
    4. Follow the front wheels exactly
    Show answer

    A. Follow a shorter path than the front wheels (off-tracking) Off-tracking: the longer the vehicle, the more the rear wheels cut inside the path of the front ones. Plan turns wide enough. (CDL manual)

16. A trailer jackknife usually starts when:

    1. The load is too light
    2. The trailer wheels lock and skid
    3. The fifth wheel is greased
    4. The tractor goes too slowly
    Show answer

    B. The trailer wheels lock and skid Locked trailer wheels lose grip and the trailer swings around. Releasing the brakes lets the wheels grip again. (CDL manual)

17. The posted speed on an exit ramp is:

    1. The minimum speed
    2. Always safe for a loaded truck
    3. Possibly too fast for a loaded truck
    4. Only a guide for motorcycles
    Show answer

    C. Possibly too fast for a loaded truck Ramp speed limits are set for cars. With a high center of gravity, a loaded truck can roll over at or below the posted speed. (CDL manual)

18. Before coupling, the fifth wheel should be:

    1. Tilted up toward the cab
    2. Tilted down toward the rear of the tractor, jaws open
    3. Level, with the jaws closed
    4. Locked
    Show answer

    B. Tilted down toward the rear of the tractor, jaws open Tilted down toward the rear, jaws open and the safety unlocking handle in the automatic lock position. (CDL manual)

19. Before unlocking the fifth wheel when uncoupling, you should:

    1. Ease pressure on the locking jaws by backing gently with the trailer brakes locked
    2. Disconnect the tractor brakes
    3. Drive forward quickly
    4. Lower the landing gear only after unlocking
    Show answer

    A. Ease pressure on the locking jaws by backing gently with the trailer brakes locked Shut off the trailer air supply to lock the trailer brakes, then back up gently to take pressure off the jaws so the release handle moves easily. (CDL manual)

20. You uncouple a trailer that has no spring brakes. You must:

    1. Chock the trailer wheels
    2. Leave the air lines connected
    3. Park it with the hand valve applied
    4. Leave it on a slope
    Show answer

    A. Chock the trailer wheels Without spring brakes, nothing holds the trailer once the tractor leaves. Chock the wheels. (CDL manual)

21. Why check that the air lines are not crossed when coupling?

    1. The trailer brakes may not work, or its spring brakes may not release
    2. Crossed lines only affect the lights
    3. It wastes fuel
    4. The horn will not work
    Show answer

    A. The trailer brakes may not work, or its spring brakes may not release With crossed lines, supply air goes to the service line. A trailer with spring brakes will not release them; one without may have no working brakes at all. (CDL manual)

22. Low-slung units can have a special problem at:

    1. Gas stations
    2. Weigh stations
    3. Raised railroad crossings, where they can get stuck
    4. Toll booths
    Show answer

    C. Raised railroad crossings, where they can get stuck A low-slung trailer, or one with the landing gear not fully raised, can hang up on a raised crossing. Never stop on the tracks. (CDL manual)

23. Steering a combination vehicle in an emergency, you should steer:

    1. Toward the shoulder always
    2. Only with the trailer brakes applied
    3. Sharply, to get out of the way fast
    4. Gently and smoothly
    Show answer

    D. Gently and smoothly Sudden steering can roll the rig over or cause a jackknife, and rearward amplification makes the rear trailer swing even more. (CDL manual)

24. To test the trailer emergency brakes, you:

    1. Use the trailer hand valve at speed
    2. Pull the trailer air supply control out, then pull forward gently to check the trailer holds
    3. Press the brake pedal hard at speed
    4. Disconnect the service line while driving
    Show answer

    B. Pull the trailer air supply control out, then pull forward gently to check the trailer holds Charge the trailer air system, pull out the trailer air supply control (or disconnect the emergency line), and pull gently forward: the trailer brakes should hold it. (CDL manual)