Commercial Driver’s License (CDL) · Knowledge Test Overview
Combination Vehicles
On this page 6 sections
In 30 seconds
The Combination Vehicles knowledge test has 20 questions, and 16 correct, or 80 percent, passes. It is required only for Class A, because Class A is the only license that covers a tractor pulling a trailer over 10,000 pounds. Class B and Class C applicants skip it entirely. The questions come from the high-priority Combination chapter: coupling and uncoupling, the trailer-specific inspection items, and driving dynamics such as off-tracking and trailer skids.
Why this matters
A Class A license is a license to pull a heavy trailer. The Combination Vehicles test is the state's way of confirming, before you ever hook one up, that you understand coupling, the trailer air system, and how a long articulated vehicle behaves differently from a straight truck.
The college version
In normal terms
- Twenty questions, sixteen right, only if you are going for Class A.
- If you will never pull a trailer, you never take this test.
- It sits on top of General Knowledge and Air Brakes, so a Class A applicant takes three tests, not one.
- The coupling sequence and the air-line colors are the heart of it.
Concepts in this outline
- Combination Vehicles: 20 questions — A short, narrow test drawn from the high-priority Combination chapter on coupling, trailer inspection, off-tracking, and trailer handling.
- Passing score: 16 (80%) — Sixteen correct out of twenty pass, allowing four misses; the test is scored separately from General Knowledge and Air Brakes.
- Required for: Class A — Only the license that covers a towed unit over 10,000 pounds requires it; Class B and Class C applicants do not take it.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture a bike pulling a small wagon. The whole thing bends in the middle, swings on turns, and can fold up. A tractor-trailer is that bike and wagon, only much heavier. Class A means a combination rated at 26,001 pounds or more. The towed part must weigh over 10,000 pounds. That is the tractor-trailer. Class B is one heavy vehicle that may tow only a trailer of 10,000 pounds or less. Only Class A involves a heavy trailer, so only Class A applicants take the Combination Vehicles test. A Class B bus or dump-truck driver never sees it.
The test has 20 questions, and the passing score is 16, which is 80 percent. Four misses are allowed. It is the shortest core test, and nearly every question comes from one high-priority chapter.
You will be asked about coupling, which means connecting the tractor to the trailer. The fifth wheel is a flat plate on the tractor with a slot. The kingpin is a steel peg under the trailer that fits that slot. The locking jaws must close around the kingpin. You connect the glad hands, which are the air hose connectors. The blue one is the service line and the red one is the emergency line. Then you push in the red eight-sided trailer air supply valve to fill the trailer with air. You will be asked why you chock the trailer wheels, meaning block them so they cannot roll. You will be asked what the tractor protection valve does when the trailer air line breaks. Uncoupling is the reverse. You also inspect trailer-only items, like the landing gear and the fifth-wheel gap.
On a turn, a trailer's rear wheels follow a shorter path than the tractor's front wheels. That is called off-tracking, and it is why a tractor-trailer swings wide. An empty trailer stops worse than a loaded one, because its brakes were built for a load. A trailer can jackknife, or fold up, if its wheels lock and slide. So you release the brakes and let the wheels roll. The knowledge test wants these as facts, and the skills test later wants you to show them.
Test day is usually a computer test at the licensing office, scored right away. It is scored on its own. A fail here does not undo a pass on General Knowledge or Air Brakes. You come back for Combination alone. Retake waiting periods and fees are in your state's CDL manual. Most Class A applicants take all three core tests in one sitting, about 95 questions. So plan on a morning, not a lunch break.
Worked example
Priya Raman is switching from a Class B delivery job to a Class A over-the-road position. She already holds a Class B, so General Knowledge is behind her, but she still needs to pass the 20-question Combination Vehicles test to upgrade. She studies the coupling sequence out loud until she can say it without looking: chock the trailer wheels, back until the fifth wheel touches the trailer, connect blue service and red emergency glad hands, push in the red valve, back under until the jaws lock, tug test, check the gap, raise the landing gear, pull the chocks.
At the office she takes the test on the computer. One question asks which line supplies air to the trailer service brakes, and she picks the blue line. Another asks what happens at the tractor protection valve when trailer pressure is lost, and she answers that it closes to keep air in the tractor. She finishes with 18 out of 20, two above the 16 required, and walks out with a Class A learner's permit and a skills test date 14 days out.
Study tools & related lessonsRelated
Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.