CDL Combination Test Questions and Answers 2025

CDL Combination Test Questions and Answers 2025: Our CDL combination vehicles practice test will help you to pass the tests for combination vehicles Endorsements written test (tractor-trailer, doubles, triples, straight truck and trailer). The Combination Test Questions and Answers is only to give you the minimum knowledge needed for a written test of combination vehicles.

Combination vehicles are usually heavier, longer, and require more driving skills than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles. In this chapter, we talk about some important safety factors that apply specifically to combination vehicles.

CDL Combination Test Questions and Answers

Test Name	CDL Practice Test 2025
Language	English
License	Endorsements Permit
Total Questions	30 sample questions
Test For	Commercial Driver License
Test Preparation	Combination vehicles Endorsements
Country	USA
Explanation	No

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CDL Combination Vehicles Test

1 / 30

The earliest and best way to recognize that your trailer has started to skid is by

A) periodically applying the brakes to check for traction.

B) seeing it in your mirrors.

C) feeling the rig “pull” to the right or left.

D) carefully watching how other drivers are reacting to your vehicle.

2 / 30

When the wheels of a trailer lock up and the trailer begins to swing around ______ is likely to occur.

A) offtracking

B) a rear wheel braking skid

C) trailer jackknife

D) rollover

3 / 30

The standard following distance behind other vehicles is at least

A) one second for each ten feet of vehicle length plus another second if going over 40 mph.

B) two seconds for each ten feet of vehicle length plus one second if going over 40 mph.

C) two seconds for each ten feet of vehicle length.

D) one second for each twenty feet of vehicle length.

4 / 30

The “normal” position of the trailer air supply control is used for

A) putting on the trailer emergency brake.

B) shutting the air off.

C) pulling a trailer.

D) parking a trailer.

5 / 30

Failure to keep the _________ lubricated could cause steering problems because of friction between tractor and trailer.

A) trailer protection valve

B) glad hands

C) air supply knob

D) fifth wheel plate

6 / 30

If the service and emergency air lines are crossed when hooking up an old trailer, what will happen?

A) The trailer spring brake will release.

B) You will be able to drive away, but you won’t have trailer brakes.

C) Water and dirt will enter the air lines.

D) Nothing will happen.

7 / 30

Which of the following is a coupling device used to connect the service and emergency air lines from the truck or tractor to the trailer?

A) Glad hands

B) The tractor protection valve

C) The trailer air supply control

D) The relay valve

8 / 30

To supply air to the trailer air tanks in a combination vehicle, you must

A) pull out the trailer air supply control.

B) press the trailer hand brake.

C) push in the trailer supply control.

D) open the relay valve.

9 / 30

Your trailer has begun to skid. You should

A) turn hard in the direction of the skid.

B) apply the foot brake hard, and come to a full stop.

C) release the foot brake to get traction back.

D) use the trailer hand brake to straighten out the rig.

10 / 30

Why should the trailer glad hands be locked together or attached to “dead end,” or dummy, couplers when not in use?

A) To keep air pressure normal

B) To keep dirt out of the lines

C) To prevent air from escaping

D) To keep the service and emergency glad hands from getting mixed up

11 / 30

Which of the following reduces the risk of rollover?

A) Keeping the cargo as close to the ground as possible

B) Changing lanes quickly

C) Loading the cargo on one side of the truck only

D) None of the above

12 / 30

When coupling a tractor to a semitrailer, what should you hook up before backing under the trailer?

A) The tractor protection valve

B) The trailer air supply control

C) The emergency and service air lines

D) Nothing; you are ready to back up

13 / 30

The air leakage rate for a combination vehicle (engine off, brake off) should be less than _________ psi per minute

A) 2

B) 4

C) 3

D) 1

14 / 30

Fully loaded rigs are __________ times more likely to rollover in a crash than empty rigs

A) 8

B) 20

C) 10

D) 5

15 / 30

The tractor protection valve will close automatically if air pressure is low or in the range of

A) 46 psi or above

B) 11-19 psi

C) 20-45 psi

D) 0-10 psi

16 / 30

When parking trailers without spring brakes, use _________ to safely secure the vehicle.

A) the parking brake

B) the emergency brakes

C) wheel chocks

D) the trailer hand brake

17 / 30

In combination vehicles, the trailer service brake should be tested with the ________, but controlled in normal operation with the _________.

A) foot brake; hand valve

B) trailer emergency valve; foot brake

C) hand valve; foot brake

D) trailer air supply control; hand valve

18 / 30

Which of the following may happen if the tractor protection valve doesn’t work right?

A) The engine may shut off.

B) An air hose or trailer brake lead could drain all the air from the trailer and possibly cause a loss of control in the vehicle.

C) Nothing will happen.

D) The pressure may rise in the trailer.

19 / 30

What will happen if the service air line comes apart while you are driving a combination vehicle?

A) the emergency trailer brakes will come on

B) nothing is likely to happen until you put on your brakes

C) nothing at all is going to happen

D) the trailer’s air tank will exhaust through the open line

20 / 30

Emergency lines on combination vehicles are often color coded to keep them from getting mixed up. The service lines are coded with the color __________, and the emergency lines are coded with the color __________ .

A) blue . . . yellow

B) red . . . black

C) blue . . . red

D) red . . . blue

21 / 30

When lowering the landing gear of a loaded trailer,

A) lift the trailer off the fifth wheel.

B) turn the crank in low gear a few extra turns after the landing gear makes firm contact with ground.

C) lower the gear until it makes firm contact with ground.

D) urn the crank in high gear a few extra turns after the landing gear makes firm contact with ground.

22 / 30

How much space should be between the upper and lower fifth wheel after coupling?

A) No space

B) 6 inches

C) 1 foot

D) More than 1 foot

23 / 30

When coupling a tractor and semitrailer, you must lock the kingpin into the fifth wheel. Which is the best way to check the connection?

A) Back up and unlock the trailer brake.

B) Drive forward.

C) Lock the trailer brakes and pull the tractor gently forward.

D) Lock the trailer brakes and pull the tractor quickly forward.

24 / 30

After the tractor has coupled with the semitrailer, in what position should you put the front trailer supports, or landing gear, before driving away?

A) Partway up without the crank handle secured

B) On the ground

C) Fully raised with the crank handle safely secured

D) Halfway up

25 / 30

When coupling, the fifth wheel jaws should close around the __________________ of the kingpin.

A) head

B) middle

C) bottom

D) shank

26 / 30

When coupling with a semitrailer, back the tractor up

A) at an angle to the trailer.

B) slightly off the center of the trailer.

C) directly in front of the trailer.

D) in front of the right wheel.

27 / 30

Which of the following is the correct definition of offtracking?

A) When a vehicle is turning the corner, the front wheels and rear wheels follow the same path.

B) When a vehicle is turning the corner, the front wheels lead the rear wheels around the turn.

C) Keeping the front of the rig as close to the curb as possible to avoid entering other lanes.

D) When a vehicle is turning the corner, the rear wheels follow a different path than the front wheels.

28 / 30

What are the two purposes of the emergency air line on combination vehicles?

A) To supply air to the trailer air tanks and control the emergency brakes

B) To control the emergency brakes and carry air controlled by the foot brake

C) To carry air, which is controlled by the foot brake only

D) To supply air to the trailer air tanks and to connect the trailer air tank to the trailer air brakes

29 / 30

When should the trailer hand valve be used?

A) Only in an emergency

B) To test the trailer brake

C) To park the trailer

D) To drive the trailer

30 / 30

You are coupling a tractor and semitrailer, and ready to back your trailer under a semi-trailer. When is the trailer at the right height?

A) When the kingpin is lower than the fifth wheel

B) When the trailer is raised high above the trailer as the tractor is backed under it

C) When the trailer is raised slightly by the tractor as the tractor is backed under it

D) When the tractor and semitrailer are even

Your score is

The average score is 80%

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