CDL Combination Practice Test

Combination vehicles knowledge is tested as part of the Class A CDL general knowledge exam. It covers the operation of tractor-trailer rigs and other combination vehicles where a power unit pulls one or more trailers. Understanding coupling, uncoupling, jackknifing, and the unique handling characteristics of articulated vehicles is essential for any Class A CDL driver.

Requirements

Applying for or holding a Class A CDL
Pass the Combination Vehicles knowledge test at your state DMV (often part of the general knowledge exam)
Pass the Class A CDL skills test with a combination vehicle
Complete Entry-Level Driver Training (ELDT) for Class A CDL
Be at least 18 for intrastate or 21 for interstate operation

What's on the Test

The Combination Vehicles knowledge test has 20 to 30 multiple-choice questions with an 80% passing score. 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.

Study Tips

Master the coupling and uncoupling procedure step by step -the test asks about the correct sequence and common errors that can lead to separation

Understand what causes a jackknife and how to prevent it: avoid hard braking on slippery surfaces and brake before curves, not during them

Know what off-tracking means -the rear wheels of a trailer follow a shorter path than the front wheels during turns

Study the trailer air supply system: how the tractor-trailer connection works, what a tractor protection valve does, and what happens when air pressure drops

Learn how to check the fifth-wheel coupling during a pre-trip inspection, including looking for proper locking jaw engagement

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Combination FAQ

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.

What are glad hands?

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.

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Doubles/Triples(T)

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