How many questions are on the Illinois CDL knowledge test?

The Illinois CDL general knowledge test has 30 multiple-choice questions, and you need 80% (24 correct) to pass. The Combination test is shorter. Most states do not publish its question count, but the 80% pass mark is federal under 49 CFR 383.135 and applies to every CDL knowledge test.

What's the minimum age for a CDL in Illinois?

In Illinois, the minimum age for a CDL is 18 (intrastate); 21 (interstate). Federal law requires drivers to be 21+ to drive a CMV across state lines or to transport hazardous materials, regardless of state minimum age.

Why does Class A require Combination?

Because Class A is defined by towing. The class covers a combination with a gross combination weight rating of 26,001 pounds or more where the towed unit is over 10,000 pounds, so coupling, trailer handling and the failure modes that come with an articulated vehicle are the whole point of the class rather than an add-on.

What is the difference between a jackknife and a trailer skid?

A jackknife occurs when the tractor drive wheels lose traction and the rear of the tractor swings toward the trailer, forming a V-shape. A trailer skid occurs when the trailer wheels lock up and the trailer swings out to the side. In a jackknife, the tractor pivots; in a trailer skid, the trailer pivots. Both are dangerous and can be caused by sudden braking on slippery surfaces.

What is off-tracking and why does it matter?

Off-tracking is the tendency of a trailer's rear wheels to follow a path closer to the inside of a turn than the tractor's front wheels. The longer the vehicle, the greater the off-tracking. This means combination vehicles need wider turns and drivers must swing wide enough to prevent the trailer from running over curbs, signs, or other vehicles.

Glad hands are the coupling devices that connect the air lines between the tractor and the trailer. There are two glad hand connections: one for the service (foot brake) line and one for the emergency (supply) line. They are typically color-coded, blue for service and red for emergency. Crossed glad hands can cause the brakes to malfunction.

How do I check the fifth wheel during a pre-trip inspection?

During a pre-trip, check that the fifth wheel is securely mounted to the tractor frame with no missing or damaged parts. Verify the locking jaws are closed around the kingpin and the release handle is in the locked position. Check for visible space between the upper and lower fifth wheel, there should be none. Also inspect the fifth wheel skid plate for damage and proper lubrication.

What is the tractor protection valve?

The tractor protection valve keeps air in the tractor's brake system if the trailer breaks away or develops a severe air leak. It automatically closes when air pressure drops to around 20-45 PSI, disconnecting the tractor's air supply from the trailer. This prevents all air from draining out of the tractor so you can still use the tractor's brakes to stop safely.

Illinois CDL combination practice test

Updated for 2026 · 50 combination questions of 480 across the full IL CDL path · Aligned to the commercial driver manual · Part of the IL CDL path

Start with general knowledge and air brakes first. We recommend completing them before drilling combination questions.

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Required for Class A, and the only section covering coupling and uncoupling. Also handles trailer behavior, including what causes a jackknife and how off-tracking pulls the trailer inside your turn.

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About the Illinois Combination test

Required for Class A, and a separate written exam at the federal 80% pass mark. Most states do not publish a question count for it. Questions cover coupling and uncoupling tractor-trailers, rollover prevention, jackknife causes and prevention, off-tracking, braking with a trailer, air brake connections (glad hands), proper use of the trailer hand valve, weight distribution, and the specific inspection procedures for fifth wheels, kingpins, and trailer connections. This is a core test for anyone seeking a Class A CDL. This is a separate knowledge test from General Knowledge, required in every state under 49 CFR 383.71, and the Secretary of State publishes the Illinois Commercial Driver's License Study Guide as the study source for it.