25 realistic Combination Vehicles practice questions for Illinois CDL applicants. Required for: Class A CDL (tractor-trailer combinations). Official test: 20 questions, 16 correct to pass.
Question 1 of 25 · Inspection
Which is true about gladhand seals?
-
A.
They should be replaced if cracked or damaged
-
B.
They never need replacement
-
C.
They are the same as electrical connections
-
D.
They are always made of metal
Correct answer: A.
Replace damaged or worn rubber seals to keep air from leaking at the gladhands.
Question 2 of 25 · Skids
How can drivers avoid jackknifing?
-
A.
Brake gently and look ahead
-
B.
Adjust speed for road and weather
-
C.
Use ABS where equipped
-
D.
All of the above
Correct answer: D.
Smooth braking, appropriate speed, and ABS all reduce jackknife risk.
Question 3 of 25 · Coupling
You should NEVER:
-
A.
Back under a trailer at an angle
-
B.
Inspect the fifth wheel
-
C.
Use the parking brake
-
D.
Tug-test the kingpin
Correct answer: A.
Backing at an angle can damage the kingpin or push the trailer sideways.
Question 4 of 25 · Combination Air
What should the tractor protection valve do?
-
A.
Keep tractor air supply if trailer breaks away or develops a major leak
-
B.
Set the trailer parking brake on a hill
-
C.
Apply trailer brakes during a normal stop
-
D.
Disconnect the trailer
Correct answer: A.
It protects the tractor air supply by closing if a major air loss occurs.
Question 5 of 25 · Combination Air
When the trailer breaks away or loses air pressure, the trailer brakes should:
-
A.
Release
-
B.
Apply automatically
-
C.
Vibrate
-
D.
Disengage
Correct answer: B.
A loss of air should automatically apply the trailer brakes via the emergency system.
Question 6 of 25 · Vehicle Dynamics
Which of these is true about the front trailer of a doubles or triples?
-
A.
It is less likely to overturn
-
B.
It is more stable
-
C.
It is more likely to overturn than the rear trailer
-
D.
It does not need brakes
Correct answer: A.
The front trailer in a combination is generally more stable than the rear trailer.
Question 7 of 25 · Inspection
When inspecting your trailer's wheels, what should you look for?
-
A.
Loose or missing nuts
-
B.
Cracked or broken wheel rims
-
C.
Bent or broken studs
-
D.
All of the above
Correct answer: D.
Inspect for loose nuts, cracks, bent studs, and other defects on every pre-trip.
Question 8 of 25 · Inspection
When checking the trailer air supply, you should:
-
A.
Charge the trailer with air, then disconnect the supply hose to test trailer emergency brakes
-
B.
Pump the brakes
-
C.
Disconnect the trailer power cord
-
D.
Open the trailer service line manually
Correct answer: A.
Disconnecting the supply line tests that the trailer emergency brakes apply automatically.
Question 9 of 25 · Inspection
You should inspect the rear of the trailer for:
-
A.
Cracked or missing lights
-
B.
Lights and reflectors are clean and operational
-
C.
Mud flap condition
-
D.
All of the above
Correct answer: D.
Check lights, reflectors, mud flaps, and license plate during pre-trip.
Question 10 of 25 · Inspection
What is the proper way to test the parking brakes after coupling?
-
A.
Apply trailer parking brake and try to gently pull forward
-
B.
Drive at normal speed
-
C.
Apply foot brake repeatedly
-
D.
Disconnect glad hands
Correct answer: A.
A gentle pull forward with parking brakes set confirms holding power.
Question 11 of 25 · Coupling
What are landing gear used for?
-
A.
Supporting the front of the trailer when uncoupled
-
B.
Steering the trailer
-
C.
Powering the trailer brakes
-
D.
Lighting the trailer
Correct answer: A.
Landing gear holds up the trailer when it is not connected to a tractor.
Question 12 of 25 · Skids
Tractor jackknife occurs when:
-
A.
Drive wheels lose traction and skid
-
B.
The trailer wheels lock
-
C.
The trailer slides forward
-
D.
Steering wheels lock
Correct answer: A.
Drive wheels skidding cause tractor jackknife; trailer wheels skidding cause trailer jackknife.
Question 13 of 25 · Coupling
When coupling, what should you do after the fifth wheel locks onto the kingpin?
-
A.
Pull the tractor forward gently to make sure the kingpin is locked
-
B.
Disconnect the air lines
-
C.
Drive away immediately
-
D.
Release the trailer parking brake first
Correct answer: A.
Tug-test by gently pulling forward with the trailer brakes set to confirm the lock.
Question 14 of 25 · Coupling
What is the proper position for the fifth wheel?
-
A.
Tilted down toward the rear of the tractor with jaws open
-
B.
Pointed straight up
-
C.
Locked and ready
-
D.
Removed for inspection
Correct answer: A.
Before coupling, the fifth wheel should tilt down toward the rear with the jaws open.
Question 15 of 25 · Coupling
When you finish coupling, the air lines should be:
-
A.
Connected to the proper glad hands and supported off the catwalk and the deck plate
-
B.
Coiled tightly on top of the trailer
-
C.
Loose so they can move freely
-
D.
Disconnected
Correct answer: A.
Air lines must be connected and supported off the deck so they don't snag or wear.
Question 16 of 25 · Speed Management
What is the most common cause of fatal crashes for combination vehicles?
-
A.
Loss of control due to speed too fast for conditions
-
B.
Engine failure
-
C.
Tire blowouts
-
D.
Mechanical defects only
Correct answer: A.
Driving too fast for conditions is the leading cause of serious crashes.
Question 17 of 25 · Backing
Which is correct about backing a tractor-trailer?
-
A.
Steer in the same direction you want the trailer to go
-
B.
Steer in the opposite direction you want the trailer to go
-
C.
Use the throttle to back
-
D.
Back as fast as possible
Correct answer: B.
When backing, turn the steering wheel opposite to the direction you want the trailer to move.
Question 18 of 25 · Coupling
Which is true about pulling away after coupling?
-
A.
Test that the kingpin is locked by gently pulling forward against the trailer brakes
-
B.
Pull away as quickly as possible
-
C.
Lower the landing gear before pulling away
-
D.
Disconnect the air lines first
Correct answer: A.
A pull-test confirms the kingpin is locked before driving away.
Question 19 of 25 · Vehicle Dynamics
When you turn suddenly with a high center of gravity, the trailer can:
-
A.
Roll over
-
B.
Stop more quickly
-
C.
Track better
-
D.
Reduce fuel use
Correct answer: A.
High centers of gravity make trailers prone to rollover during sharp turns.
Question 20 of 25 · Following Distance
What is "rear-end collision" risk for combination vehicles?
-
A.
It is reduced when carrying long cargo
-
B.
They have a higher risk because of long stopping distances and rear blind spots
-
C.
They have no greater risk
-
D.
Only for empty rigs
Correct answer: B.
Long stopping distance and rear blind spots increase rear-end collision risk.
Question 21 of 25 · Coupling
What should you do before connecting air lines?
-
A.
Clean the rubber seals on the glad hands
-
B.
Lubricate them with oil
-
C.
Replace the seals every trip
-
D.
Heat them up
Correct answer: A.
Clean glad hand seals before connecting to ensure a good air seal.
Question 22 of 25 · Skids
What is the result of a "trailer jackknife"?
-
A.
Trailer continues straight ahead
-
B.
Trailer swings out and tractor pivots
-
C.
Trailer comes to a quick safe stop
-
D.
Trailer disengages
Correct answer: B.
In a trailer jackknife, the trailer swings out, often into adjacent lanes.
Question 23 of 25 · Skids
What can cause a trailer skid in a combination vehicle?
-
A.
Locked trailer wheels and over-application of brakes
-
B.
Light loads on the trailer
-
C.
Tight steering at low speed
-
D.
High pressure in steering tires
Correct answer: A.
Locked trailer wheels cause trailer skid; reduce brake pressure to recover.
Question 24 of 25 · Coupling
How can you keep a stiff coupling system from causing problems?
-
A.
Lubricate the fifth wheel
-
B.
Avoid backing under fast
-
C.
Inspect mounting bolts
-
D.
All of the above
Correct answer: D.
Lubricate, back slowly, and inspect mounting bolts to keep the coupling working safely.
Question 25 of 25 · Skids
What should you do to recover from a trailer skid?
-
A.
Apply more brake pressure
-
B.
Release the brakes to allow trailer wheels to roll again
-
C.
Steer hard right
-
D.
Accelerate forward
Correct answer: B.
Release brakes to let trailer wheels roll, then steer to recover direction.
About the Combination Vehicles Test
The Combination Vehicles test covers driving combination vehicles safely, combination vehicle air brakes, antilock brake systems, coupling and uncoupling, and inspecting a combination vehicle. Combination vehicles are usually heavier, longer, and require more driving skill than single commercial vehicles. They are also more likely to roll over and jackknife.
The Illinois Secretary of State — Driver Services administers this knowledge test as part of its commercial driver license program. To earn the Combination Vehicles on your Illinois CDL, you need to score at least 16 out of 20 on the official version.
Topics covered on this practice test
- Coupling: 8 questions
- Inspection: 5 questions
- Skids: 5 questions
- Combination Air: 2 questions
- Vehicle Dynamics: 2 questions
- Speed Management: 1 question
- Backing: 1 question
- Following Distance: 1 question
How to use this practice test
- Read each question carefully. CDL questions are written precisely — small wording differences matter.
- Click "Show Answers & Explanations" only after answering every question. Don't peek mid-test.
- Read the explanation for every question, even ones you got right. The reasoning is more important than the answer.
- Repeat the test until you score 100%. The questions are deterministic per state, so you can track your improvement over multiple sessions.
- Move on to the other endorsement tests for Illinois until you're ready for the official exam.
Tips specific to the Combination Vehicles test
Combination Vehicles questions cover off-tracking, jackknifing, coupling and uncoupling, the fifth wheel, kingpin, glad hands, landing gear, and the special air-brake considerations for tractor-trailer combinations. Expect detailed step-by-step coupling questions.