CDL Combination Vehicles Practice Test

25 practice questions

Choose an answer, then check it under Show answer.

1. Question 1 of 25Medium

   When you inspect the fifth wheel before coupling, where should the safety unlocking handle be?

   1. AA. Tied back so it cannot catch on anything
   2. BB. Removed from the fifth wheel
   3. CC. In the manual release position
   4. DD. In the automatic lock position
   Show answer to question 1

   Answer: D. In the automatic lock position

   The fifth wheel is in proper position for coupling when the jaws are open, the wheel is tilted down toward the rear, and the safety unlocking handle is in the automatic lock position. Leaving the handle in the release position or tying it back prevents the fifth wheel from locking properly.

2. Question 2 of 25Medium

   While checking that air flows to the rear of a combination vehicle, what should you do immediately after hearing air escape from the rear emergency line shut-off valve?

   1. AA. Open the service line shut-off valve
   2. BB. Push in the trailer air supply knob
   3. CC. Release the trailer handbrake control
   4. DD. Close the emergency line valve
   Show answer to question 2

   Answer: D. Close the emergency line valve

   Once you verify air is escaping from the open emergency line shut-off valve at the rear, you must close that valve before opening the service line valve. Opening the service line valve before closing the emergency valve would improperly vent the system.

3. Question 3 of 25Medium

   On an older vehicle with a lever-type trailer air supply control, what does the emergency position do?

   1. AA. It shuts off the air and puts on the trailer emergency brakes.
   2. BB. It opens the tractor protection valve to charge the trailer.
   3. CC. It increases service line pressure for a brake test.
   4. DD. It supplies air to the trailer for normal driving.
   Show answer to question 3

   Answer: A. It shuts off the air and puts on the trailer emergency brakes.

   The manual says the normal position is used for pulling a trailer, and the emergency position is used to shut the air off and put on the trailer emergency brakes. Supplying air for normal driving is the normal position, not the emergency position. The lever does not raise service line pressure for a test.

4. Question 4 of 25Easy

   What happens to a truck's center of gravity as more cargo is piled up on it?

   1. AA. The center of gravity moves higher above the road
   2. BB. The center of gravity moves lower toward the road
   3. CC. The center of gravity stays the same but the brakes get weaker
   4. DD. The center of gravity shifts rearward toward the trailer
   Show answer to question 4

   Answer: A. The center of gravity moves higher above the road

   Piling cargo higher raises the center of gravity, which makes the truck easier to turn over. That is why the manual says to keep cargo as close to the ground as possible, especially in a combination vehicle.

5. Question 5 of 25Medium

   When coupling a gooseneck hitch, what is an essential step to perform right after connecting the trailer light wiring?

   1. AA. Unhitch the coupler to verify latch engagement.
   2. BB. Remove the safety chains to check for slack.
   3. CC. Loosen the trailer jacks to drop the tongue suddenly.
   4. DD. Check all of your lights, including your brake lights.
   Show answer to question 5

   Answer: D. Check all of your lights, including your brake lights.

   After connecting the trailer light wiring to the vehicle's connector, the driver must check all lights, including the brake lights, to ensure proper electrical connection and signaling safety. Disconnecting chains or dropping tongue weight abruptly are dangerous and incorrect procedures.

6. Question 6 of 25Medium

   What does the warning that ABS will not let you drive faster, follow more closely, or drive less carefully mean?

   1. AA. It does not replace safe driving habits
   2. BB. It removes the need to monitor the trailer
   3. CC. It allows you to drive at higher speeds on wet roads
   4. DD. It lets you follow more closely in poor weather
   Show answer to question 6

   Answer: A. It does not replace safe driving habits

   The manual warns that ABS will not let you drive faster, follow more closely, or drive less carefully. It does not replace safe driving habits, and it does not remove the need to monitor the trailer.

7. Question 7 of 25Medium

   A driver has charged the trailer air-brake system and checked that the trailer rolls freely. To test the trailer emergency brakes, what should the driver do next?

   1. AA. Push in the red knob and step on the brake pedal several times
   2. BB. Pull out the trailer air supply control and pull gently with the tractor
   3. CC. Apply the trolley valve while moving the rig forward slowly
   4. DD. Open the service line valve at the rear of the last trailer
   Show answer to question 7

   Answer: B. Pull out the trailer air supply control and pull gently with the tractor

   To test the trailer emergency brakes, the driver pulls out the trailer air supply control and pulls gently with the tractor to see whether the brakes hold. Applying the trolley valve while moving forward is the test for the trailer service brakes, not the emergency brakes.

8. Question 8 of 25Hard

   On a newer combination vehicle, what occurs when a driver pulls out the red eight-sided trailer air supply knob?

   1. AA. Air to the trailer is shut off and trailer emergency brakes apply.
   2. BB. Tractor parking brakes set while trailer brakes remain released.
   3. CC. Service line pressure increases to equal tractor reservoir pressure.
   4. DD. Air is supplied to charge the trailer air tanks.
   Show answer to question 8

   Answer: A. Air to the trailer is shut off and trailer emergency brakes apply.

   Pulling out the red eight-sided trailer air supply control valve shuts the air off and applies the trailer emergency brakes. Pushing the knob in is what supplies air to the trailer system.

9. Question 9 of 25Hard

   Your trailer gets stuck on raised railroad tracks at a crossing. What should you do?

   1. AA. Walk along the tracks to signal any approaching train
   2. BB. Get out and away from the tracks, then call 911 with the crossing location
   3. CC. Stay in the cab and call 911 so you can move if a train comes
   4. DD. Rock the rig by backing and pulling forward until it breaks free
   Show answer to question 9

   Answer: B. Get out and away from the tracks, then call 911 with the crossing location

   The CDL manual says that if you get stuck on the tracks for any reason, get out of the vehicle and away from the tracks, then call 911 and give the location using landmarks such as the DOT number. Staying in the cab to keep the option of moving is the tempting mistake, but it leaves you in the path of a train.

10. Question 10 of 25Medium

    How does the location of the ABS on your combination vehicle change the way you brake?

    1. AA. You brake earlier when only the tractor has ABS
    2. BB. You brake harder when only the trailer has ABS
    3. CC. You brake the same way in every case
    4. DD. You pump the pedal when both units have ABS
    Show answer to question 10

    Answer: C. You brake the same way in every case

    You should brake the same way regardless of whether you have ABS on the tractor, the trailer, or both. Changing your braking based on where the ABS is, or pumping the pedal, is not part of the procedure.

11. Question 11 of 25Medium

    What is the main danger if a driver fails to drain water and oil from trailer air tanks?

    1. AA. The brakes may not work correctly.
    2. BB. The glad hand rubber seals will dissolve rapidly.
    3. CC. The dummy couplers will crack under pressure.
    4. DD. The tractor protection valve will lock open permanently.
    Show answer to question 11

    Answer: A. The brakes may not work correctly.

    The CDL manual states that allowing water and oil to build up in air tanks can cause the brakes to not work correctly. Fluid accumulation in tanks does not crack external dummy couplers or permanently lock the tractor protection valve.

12. Question 12 of 25Medium

    Why is it critical for a combination vehicle driver to look far ahead down the road while driving?

    1. AA. To keep the engine operating within the correct cruising RPM range
    2. BB. To avoid being surprised and having to make a sudden lane change
    3. CC. To allow the trailer air suspension to balance out road bumps
    4. DD. To prevent the trailer brakes from applying automatically at speed
    Show answer to question 12

    Answer: B. To avoid being surprised and having to make a sudden lane change

    Looking far ahead gives the driver enough time to react smoothly and avoid sudden lane changes or panic maneuvers that could tip the trailer over. Looking ahead does not affect suspension adjustments or prevent trailer brakes from applying.

13. Question 13 of 25Easy

    A driver is preparing to back a tractor under a semitrailer. What is the correct height relationship between the trailer and tractor before coupling?

    1. AA. The trailer should be low enough that the tractor raises it slightly when backed under.
    2. BB. The trailer should be raised completely off the ground before backing under.
    3. CC. The trailer should be high enough to clear the fifth wheel without contacting it.
    4. DD. The trailer should be level with the tractor frame rails before touching the fifth wheel.
    Show answer to question 13

    Answer: A. The trailer should be low enough that the tractor raises it slightly when backed under.

    The trailer must be adjusted so that it is raised slightly by the tractor when the fifth wheel backs under it. If the trailer is positioned too low, the tractor can strike and damage the trailer nose, and if it is too high, it may fail to couple properly.

14. Question 14 of 25Easy

    Before driving a coupled tractor-trailer, a driver checks the landing gear. Which condition should the landing gear be in?

    1. AA. Lowered enough to support the trailer
    2. BB. Partly raised with no crank handle
    3. CC. Fully raised with the crank handle secured
    4. DD. Bent slightly to fit the frame
    Show answer to question 14

    Answer: C. Fully raised with the crank handle secured

    The landing gear check requires it to be fully raised, with no missing parts, not bent or damaged, and the crank handle in place and secured. A lowered or bent landing gear would not meet these checks.

15. Question 15 of 25Easy

    What color is the ABS malfunction lamp on a trailer?

    1. AA. Green
    2. BB. White
    3. CC. Red
    4. DD. Yellow
    Show answer to question 15

    Answer: D. Yellow

    Trailer ABS malfunction lamps are yellow and are located on the left side, either at the front or rear corner. A red lamp is not the trailer ABS malfunction indicator described.

16. Question 16 of 25Easy

    After supplying air to the trailer, a driver shuts off the engine to listen to the brakes. Why is the engine shut off for this check?

    1. AA. So the trailer cannot roll during the test.
    2. BB. So the trailer brakes can be heard.
    3. CC. So the tractor protection valve can reset.
    4. DD. So the trailer tanks can build air pressure.
    Show answer to question 16

    Answer: B. So the trailer brakes can be heard.

    The CDL manual says to shut the engine off so you can hear the brakes as they apply and release. A running engine masks the sound of the brake parts moving and air escaping. Chocking or spring brakes, not shutting off the engine, are what keep the trailer from rolling.

17. Question 17 of 25Easy

    How does a driver operate the cab trailer air supply control on a newer vehicle to supply air to the trailer?

    1. AA. Pump the hand lever back and forth three times.
    2. BB. Push in the red eight-sided knob.
    3. CC. Pull out the red eight-sided knob.
    4. DD. Turn the round knob clockwise one-quarter turn.
    Show answer to question 17

    Answer: B. Push in the red eight-sided knob.

    On newer vehicles, the trailer air supply control is a red eight-sided knob that the driver pushes in to supply the trailer with air. Pulling the knob outward shuts off air to the trailer and applies the emergency brakes.

18. Question 18 of 25Medium

    Your combination vehicle is 60 feet long and you are traveling at 55 mph. What is the minimum following distance you should keep?

    1. AA. 11 seconds
    2. BB. 4 seconds
    3. CC. 7 seconds
    4. DD. 6 seconds
    Show answer to question 18

    Answer: C. 7 seconds

    The CDL manual says to allow at least 1 second for each 10 feet of vehicle length, plus another second if you are going over 40 mph. For 60 feet that is 6 seconds, plus 1 second because you are over 40 mph, for a total of 7 seconds.

19. Question 19 of 25Medium

    A driver is checking the upper fifth wheel on the trailer. Which check belongs to this part?

    1. AA. The crank handle is in place and secured
    2. BB. The release arm is seated with its safety latch engaged
    3. CC. The locking jaws are closed around the kingpin shank
    4. DD. The glide plate is secure and the kingpin undamaged
    Show answer to question 19

    Answer: D. The glide plate is secure and the kingpin undamaged

    The upper fifth wheel check covers the glide plate being securely mounted to the trailer frame and the kingpin not being damaged. Locking jaws and the release arm are lower fifth wheel checks, and the crank handle belongs to the landing gear.

20. Question 20 of 25Hard

    When you brake a tractor-trailer that has ABS, which sequence of actions is correct?

    1. AA. Use the force needed to stop safely, brake the same way, then ease off as you slow
    2. BB. Apply the trailer brakes first, then the tractor brakes, and hold
    3. CC. Pump the pedal firmly, then hold it down until you stop
    4. DD. Brake late and hard, then release when you feel the pedal pulse
    Show answer to question 20

    Answer: A. Use the force needed to stop safely, brake the same way, then ease off as you slow

    With ABS you use only the braking force necessary to stop safely and stay in control, brake the same way regardless of where the ABS is, and as you slow down back off the brakes to stay in control. Pumping the pedal or releasing it when it pulses is not part of this procedure.

21. Question 21 of 25Easy

    Why must a driver allow extra room on the inside of a corner for the rear of the rig?

    1. AA. The steering axle cannot turn far enough at low speed
    2. BB. The rear wheels follow a different path than the front wheels
    3. CC. The trailer brakes drag on the inside wheels during a turn
    4. DD. The cargo shifts to the outside of the turn
    Show answer to question 21

    Answer: B. The rear wheels follow a different path than the front wheels

    When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels, which is called off-tracking. This is why the front end must be steered wide enough that the rear does not run over the curb or pedestrians. Brake drag, steering limits and cargo shift are not the reason for allowing the extra room.

22. Question 22 of 25Hard

    When testing the trailer service brakes, what is the correct order of operational steps?

    1. AA. Apply the trailer hand control, release parking brakes, move forward slowly, and pump the foot pedal
    2. BB. Check air pressure, engage the tractor protection valve, stop the vehicle, and apply the trolley valve
    3. CC. Check air pressure, release parking brakes, move forward slowly, and apply the trailer hand control
    4. DD. Release parking brakes, accelerate to road speed, check air pressure, and apply the service pedal
    Show answer to question 22

    Answer: C. Check air pressure, release parking brakes, move forward slowly, and apply the trailer hand control

    The proper sequence is to check for normal air pressure, release the parking brakes, move forward slowly, and apply the trailer brakes with the hand control to ensure they engage. Applying the hand control before the vehicle is moving or pumping the foot pedal does not follow the service brake test procedure.

23. Question 23 of 25Hard

    When inspecting the fifth wheel mechanism before coupling, which position confirms that it is ready to receive the trailer?

    1. AA. The fifth wheel jaws are closed with the safety latch engaged.
    2. BB. The safety unlocking handle is placed into the manual release position.
    3. CC. The fifth wheel plate is tilted down toward the front of the tractor.
    4. DD. The fifth wheel plate is tilted down toward the rear of the tractor.
    Show answer to question 23

    Answer: D. The fifth wheel plate is tilted down toward the rear of the tractor.

    A fifth wheel is properly positioned for coupling when the plate is tilted down toward the rear of the tractor so the trailer apron can slide onto it. Tilting the plate down toward the front of the tractor is incorrect and would prevent the trailer from mounting the fifth wheel.

24. Question 24 of 25Hard

    While traveling on the highway, a combination vehicle suffers a complete trailer separation that tears the air lines apart. Which device closes to keep air inside the tractor brake system?

    1. AA. The service relay valve
    2. BB. The trailer hand valve
    3. CC. The tractor protection valve
    4. DD. The foot brake valve
    Show answer to question 24

    Answer: C. The tractor protection valve

    The tractor protection valve is designed to keep air in the tractor or truck brake system should the trailer break away or develop a bad leak. The trailer hand valve is a cab control used solely to test trailer brakes and does not protect tractor air during a separation.

25. Question 25 of 25Medium

    What mistake can a driver make when braking a tractor-trailer with ABS?

    1. AA. Monitoring the tractor and trailer while slowing
    2. BB. Braking the same way with or without ABS
    3. CC. Backing off the brakes if control is lost
    4. DD. Using more braking force than needed to stop safely
    Show answer to question 25

    Answer: D. Using more braking force than needed to stop safely

    With ABS, you should brake as you always have and use only the braking force necessary to stop safely and stay in control. Using too much force can work against staying in control, while the other choices are recommended actions.

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The full CDL bank has 262 Combination Vehicles questions. See plans