Illinois CDL Combination Vehicles Practice Test

The Illinois CDL Combination Vehicles test is a multiple-choice knowledge test you take for your commercial learner's permit. Illinois Secretary of State, Driver Services Department says you need 80% to pass (source).

Last updated Oct 15, 2026. FirstTry CDL is a CDL permit test prep site by Hive Asset Group, LLC.

By Scott Wilder, Editor

Last reviewed against the Illinois CDL manual on October 15, 2026

Scott Wilder on X | scottwilderhq.com

FirstTry CDL is a CDL permit test prep site by Hive Asset Group, LLC. It offers free exam-style practice tests built from the official CDL manual. FirstTry CDL is not affiliated with any DMV, FMCSA or government agency. Practice questions are exam-style questions built from the official CDL manual, not actual test questions.

Illinois CDL permit facts

What	What Illinois says	Source
Combination Vehicles pass mark	80%	Official page
Combination Vehicles test questions	20	Official page
CLP fee (adding a CLP to an Illinois non-CDL license, or renewing an original CLP)	$50	Official page
CLP fee (current CDL holder upgrading class or adding an endorsement)	$5	Official page
CLP validity	One year. An original CLP expires one year from the date it is first issued.	Official page
Test languages	CDL knowledge tests are given in English only. You can ask for the audio format, where the computer reads the test. After trying audio, you can ask facility management about an oral test read by staff.	Official page

20 question Combination Vehicles practice test

1. What do relay valves do for the trailer brakes?
1. A. They let the trailer brakes apply more quickly
2. B. They lock the trailer brakes for parking
3. C. They fill the trailer tanks
4. D. They close the air-lines
Show answer
Answer: A. They let the trailer brakes apply more quickly
Relay valves on the service line let the trailer brakes apply faster.
From the official CDL manual, section 6.2.4.
2. A trailer without spring brakes is parked with its emergency brakes on. Why is this not enough?
1. A. The brakes hold only as long as the tank has air, and the air eventually leaks away
2. B. The brakes wear out from sitting on
3. C. The emergency brakes cannot hold a loaded trailer
4. D. The tractor protection valve will reopen
Show answer
Answer: A. The brakes hold only as long as the tank has air, and the air eventually leaks away
Once the air leaks away, the brakes let go.
From the official CDL manual, section 6.2.8.
3. What do you do with the wheel chocks in the last step of coupling?
1. A. Leave them under the trailer wheels
2. B. Throw them on the ground behind the trailer
3. C. Hang them on the landing gear
4. D. Remove them and store them in a safe place
Show answer
Answer: D. Remove them and store them in a safe place
The last step is to remove the wheel chocks and store them in a safe place.
From the official CDL manual, section 6.4.1.
4. After backing under, how do you check that the trailer is locked onto the tractor?
1. A. Pull forward fast with the trailer brakes released
2. B. Pull gently forward with the trailer brakes still locked
3. C. Look at the landing gear only
4. D. Check the air gauge for normal pressure
Show answer
Answer: B. Pull gently forward with the trailer brakes still locked
This tug test shows the trailer is held by the fifth wheel.
From the official CDL manual, section 6.4.1.
5. Which event can make the emergency line lose air pressure?
1. A. Pressing the foot brake lightly
2. B. A trailer breaking loose and tearing the emergency hose
3. C. Pushing in the trailer air supply control
Show answer
Answer: B. A trailer breaking loose and tearing the emergency hose
A trailer that breaks loose can tear the emergency hose and let the air out.
From the official CDL manual, section 6.2.4.
6. On a sliding fifth wheel, what should you check about its position?
1. A. That it is not so far forward that the frame hits the landing gear or the cab hits the trailer in turns
2. B. That it is as far forward as it will go
3. C. That it is all the way to the rear
4. D. That it is unlocked so it can move in turns
Show answer
Answer: A. That it is not so far forward that the frame hits the landing gear or the cab hits the trailer in turns
Too far forward can cause damage during turns.
From the official CDL manual, section 6.5.1.
7. After coupling, you see a gap between the upper and lower fifth wheel. What does this mean?
1. A. Something is wrong; the kingpin may be on top of the closed jaws
2. B. The coupling is normal, since the plates should have some room
3. C. The fifth wheel needs only more grease
4. D. The landing gear is too high
Show answer
Answer: A. Something is wrong; the kingpin may be on top of the closed jaws
The trailer could come loose very easily.
From the official CDL manual, section 6.4.1.
8. You drive a tractor-trailer with ABS. How should you brake?
1. A. The same way you always have
2. B. Pump the brake pedal quickly
3. C. Brake lightly so ABS takes over
4. D. Use only the trailer hand valve
Show answer
Answer: A. The same way you always have
Use only the braking force needed to stop safely and stay in control.
From the official CDL manual, section 6.3.2.
9. When is a trailer jackknife more likely to happen?
1. A. When the trailer is fully loaded
2. B. Only when the road is wet
3. C. Only when driving a bobtail tractor
4. D. When the trailer is empty or lightly loaded
Show answer
Answer: D. When the trailer is empty or lightly loaded
Locked wheels on a trailer that is empty or lightly loaded make the trailer swing around.
From the official CDL manual, section 6.1.5.
10. The trailer air supply knob pops out by itself. What does this tell you?
1. A. The trailer brakes are fully released
2. B. The trailer tanks are overfilled
3. C. The glad hands are crossed
4. D. Air pressure has dropped into the 20 to 45 psi range and the valve has closed
Show answer
Answer: D. Air pressure has dropped into the 20 to 45 psi range and the valve has closed
The knob pops out as the tractor protection valve closes at low pressure.
From the official CDL manual, section 6.2.3.
11. What causes the crack-the-whip effect in a tractor-trailer?
1. A. Off-tracking
2. B. Trailer brake lag
3. C. Rearward amplification
4. D. Bobtailing
Show answer
Answer: C. Rearward amplification
The manual says rearward amplification causes the crack-the-whip effect.
From the official CDL manual, section 6.1.2.
12. When should you slow down for a turn in a tractor-trailer?
1. A. Halfway through the turn
2. B. Before you go into the turn
3. C. After the turn is finished
4. D. Only if the road is wet
Show answer
Answer: B. Before you go into the turn
You must slow to a safe speed before you enter the turn.
From the official CDL manual, section 6.1.2.
13. Triples have a rearward amplification of 3.5. How easily can the last trailer roll, compared with a five-axle tractor?
1. A. 1.5 times as easily
2. B. 3.5 times as easily
3. C. 2 times as easily
4. D. 10 times as easily
Show answer
Answer: B. 3.5 times as easily
The manual says the last trailer of triples can roll 3.5 times as easily as a five-axle tractor.
From the official CDL manual, section 6.1.2.
14. When you begin to raise the landing gear, which gear range do you use?
1. A. Low gear range, then high gear range once free of weight
2. B. High gear range the whole time
3. C. Low gear range the whole time
4. D. High range first, then low range
Show answer
Answer: A. Low gear range, then high gear range once free of weight
Start in low range, then switch to high range once the gear is free of weight.
From the official CDL manual, section 6.4.1.
15. Which pair of actions best helps prevent a rollover in a combination vehicle?
1. A. Stack cargo high to spread the weight and drive slowly around turns
2. B. Keep cargo as low as possible and drive slowly around turns
3. C. Keep cargo low and hold your speed steady through turns
4. D. Load to one side and drive slowly around turns
Show answer
Answer: B. Keep cargo as low as possible and drive slowly around turns
Low cargo and a slow speed in turns are the two things the manual names for preventing rollover.
From the official CDL manual, section 6.1.1.
16. Which two jobs does the emergency (supply) line do?
1. A. Carries the foot brake signal and controls the hand valve
2. B. Supplies air to the trailer air tanks and controls the emergency brakes
3. C. Supplies air to the trailer tanks and carries the foot brake signal
4. D. Controls the parking valve and drains the tanks
Show answer
Answer: B. Supplies air to the trailer air tanks and controls the emergency brakes
These are the two purposes the manual gives.
From the official CDL manual, section 6.2.4.
17. Before coupling, where should the safety unlocking handle of the fifth wheel be?
1. A. In the open position
2. B. In the automatic lock position
3. C. Removed from the fifth wheel
4. D. In the release position
Show answer
Answer: B. In the automatic lock position
The fifth wheel must be ready for coupling with the handle in the automatic lock position.
From the official CDL manual, section 6.4.1.
18. You must back a trailer along a curved path. Which way should you curve?
1. A. To the passenger's side
2. B. To the driver's side
3. C. Either side; it makes no difference
4. D. Away from the nearest wall, no matter which side it is on
Show answer
Answer: B. To the driver's side
Backing toward the driver's side lets you see the path.
From the official CDL manual, section 6.1.7.
19. Before releasing the fifth wheel, how do you ease pressure on the locking jaws?
1. A. Pull forward gently with the trailer brakes released
2. B. Back up gently, then set the parking brakes while the tractor pushes against the kingpin
3. C. Release the parking brakes and idle in neutral
4. D. Raise the landing gear fully
Show answer
Answer: B. Back up gently, then set the parking brakes while the tractor pushes against the kingpin
This helps you release the fifth wheel locking lever.
From the official CDL manual, section 6.4.2.
20. Cargo is loaded toward one side and makes the trailer lean. What is the result?
1. A. The trailer tracks better in turns
2. B. A rollover is more likely
3. C. Braking distance gets shorter
4. D. Nothing, if the weight is within legal limits
Show answer
Answer: B. A rollover is more likely
A load off to one side makes the trailer lean, which makes a rollover more likely.
From the official CDL manual, section 6.1.1.

Illinois CDL Combination Vehicles questions

What score do you need to pass the Illinois CDL Combination Vehicles test?

Illinois says you need 80% to pass the Combination Vehicles test.

Is the Illinois CDL Combination Vehicles test the same as other states?

The Combination Vehicles test is built on the same federal knowledge standards in every state, so the topics match the CDL manual. Illinois uses its own handbook: https://www.ilsos.gov/content/dam/publications/pdf_publications/dsd_cdl10.pdf

How much does a CDL permit cost in Illinois?

CLP fee (adding a CLP to an Illinois non-CDL license, or renewing an original CLP): $50.

What languages is the Illinois CDL knowledge test offered in?

CDL knowledge tests are given in English only. You can ask for the audio format, where the computer reads the test. After trying audio, you can ask facility management about an oral test read by staff.

Sources

https://www.ilsos.gov/content/dam/publications/pdf_publications/dsd_cdl10.pdf
https://www.ilsos.gov/departments/drivers/basicfees.html
https://www.ilsos.gov/departments/drivers/drivers-license/cdl/cdl.html
Manual sections used in this test: 6.1.1; 6.1.2; 6.1.5; 6.1.7; 6.2.3; 6.2.4; 6.2.8; 6.3.2; 6.4.1; 6.4.2; 6.5.1

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