CDL Combination Vehicles Practice Test (Free)

Pick an answer for each question. You’ll see right away if it’s correct and why.

1. Before backing under a trailer, how should the fifth wheel be set? A. Tilted down toward the rear of the tractor, jaws open B. Level, with the jaws closed C. Tilted up toward the cab, jaws closed D. Any position, as long as it's greased
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Answer: A. Tilted down toward the rear of the tractor, jaws open The fifth wheel should tilt down toward the back of the tractor with the jaws open and the release handle in the automatic lock position, so the kingpin can slide in and lock.
2. How high should the trailer nose be when you back the tractor under it? A. High enough that the kingpin drops onto the fifth wheel B. Just low enough that the tractor lifts it slightly C. Resting on the ground with the landing gear up D. Height doesn't matter with a sliding fifth wheel
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Answer: B. Just low enough that the tractor lifts it slightly The trailer should sit slightly low so the tractor raises it a bit as it backs under. Too low can damage the trailer nose; too high and it may not couple right.
3. The tractor protection valve closes on its own when air pressure drops to about what range? A. 90 to 100 psi B. 60 to 80 psi C. 20 to 45 psi D. 0 to 5 psi
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Answer: C. 20 to 45 psi The manual says the tractor protection valve closes automatically when pressure falls into the 20 to 45 psi range, protecting the tractor's air if the trailer side leaks or breaks away.
4. What happens when you pull out the red eight-sided trailer air supply knob? A. The trailer brakes release B. Air goes only to the service line C. The tractor spring brakes release D. Air to the trailer shuts off and the trailer emergency brakes come on
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Answer: D. Air to the trailer shuts off and the trailer emergency brakes come on Push the knob in to send air to the trailer; pull it out to shut off the air and set the trailer's emergency brakes.
5. When the glad hands are color-coded, the service line is usually what color? A. Blue B. Red C. Yellow D. Green
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Answer: A. Blue Blue marks the service line and red marks the emergency (supply) line. Some rigs use metal tags stamped 'service' and 'emergency' instead.
6. What does the emergency (supply) line do? A. Carries air only when you press the brake pedal B. Fills the trailer air tanks and controls the trailer emergency brakes C. Powers the trailer lights D. Controls only the tractor's parking brake
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Answer: B. Fills the trailer air tanks and controls the trailer emergency brakes The emergency line keeps the trailer tanks charged. If it loses pressure, the trailer emergency brakes come on. The service line is the one that responds to the brake pedal.
7. You hooked up a trailer that has spring brakes, but crossed the air lines. What is likely to happen? A. The trailer brakes will be stronger than normal B. Nothing; the lines work either way C. The trailer spring brakes won't release D. The trailer lights will stop working
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Answer: C. The trailer spring brakes won't release With crossed lines, supply air goes to the service line instead of filling the trailer tanks, so the spring brakes stay on. Always test the trailer brakes before you drive.
8. What is the trailer hand valve (trolley valve) meant for? A. Parking the trailer overnight B. Straightening out a jackknife C. Slowing down on long grades D. Testing the trailer brakes
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Answer: D. Testing the trailer brakes Use the hand valve only to test trailer brakes. Using it while driving can skid the trailer, and it should never be used to park because air can leak off.
9. After coupling, you look into the back of the fifth wheel. What should you see? A. Jaws closed around the shank of the kingpin B. The kingpin resting on top of the closed jaws C. A small gap between the upper and lower fifth wheel D. The release handle in the open position
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Answer: A. Jaws closed around the shank of the kingpin The jaws must close around the kingpin shank, with the locking lever in the lock position and the safety latch over it.
10. You see a gap between the upper and lower fifth wheel plates. What does it mean? A. It's normal and closes once you start driving B. Something is wrong, possibly the kingpin is on top of the jaws; don't drive C. The trailer is overloaded D. The air pressure is too low
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Answer: B. Something is wrong, possibly the kingpin is on top of the jaws; don't drive There should be no space between the plates. A gap can mean the kingpin is sitting on the closed jaws, and the trailer could come loose.
11. What should you do with the landing gear once the trailer is coupled? A. Leave it partly down for extra support B. Raise it halfway so it clears speed bumps C. Raise it all the way and secure the crank handle D. Remove the crank handle and leave it in the cab
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Answer: C. Raise it all the way and secure the crank handle Raise the landing gear fully and secure the crank handle. Driving with it partly raised can catch it on railroad tracks or other objects.
12. Compared with an empty rig, how much more likely is a fully loaded rig to roll over in a crash? A. About the same B. Twice as likely C. Less likely, because it's heavier D. Ten times as likely
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Answer: D. Ten times as likely The manual says fully loaded rigs are ten times more likely to roll over in a crash than empty ones. Keep cargo low and centered and slow down for curves and ramps.
13. What is off-tracking? A. The rear wheels following a shorter path than the front wheels in a turn B. The trailer swinging wider than the tractor in a turn C. The trailer sliding sideways on ice D. Drifting out of your lane on a straight road
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Answer: A. The rear wheels following a shorter path than the front wheels in a turn In a turn the rear wheels cut inside the path of the front wheels. Longer rigs off-track more, so turn wide enough to keep the rear clear of curbs and people.
14. Your trailer starts to skid during braking. What should you do? A. Apply the trailer hand brake to straighten it B. Release the brakes so the wheels can grip again C. Brake harder until the rig stops D. Turn sharply away from the skid
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Answer: B. Release the brakes so the wheels can grip again Releasing the brakes lets the trailer wheels regain traction. Don't use the trailer hand brake to straighten the rig; locked trailer wheels are what caused the skid.
15. While uncoupling, where should you stop after pulling the tractor partly clear? A. Fully clear of the trailer, so you can check the kingpin B. With the fifth wheel still locked to the kingpin C. With the tractor frame still under the trailer D. Anywhere, as long as the trailer is chocked
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Answer: C. With the tractor frame still under the trailer Stop with the frame still under the trailer. If the landing gear fails or sinks, the trailer drops onto the frame instead of the ground.

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About this test

If you're testing for a Class A CDL, you'll take the combination vehicles knowledge test on top of general knowledge and air brakes. It covers coupling and uncoupling, the trailer air system (emergency and service lines, glad hands, the tractor protection valve), and how a tractor-trailer handles: rollover risk, crack-the-whip, off-tracking and trailer skids.

These are practice questions, not the official test. Exact test questions vary by state, so study your state's CDL manual too. Every answer below was checked against the AAMVA-based state CDL manual.

More CDL practice tests

Official sources

This page is general information, not legal or tax advice. Rules change; confirm with FMCSA, the IRS or your state before you file.

Questions carriers ask

Who needs the combination vehicles test?

Drivers applying for a Class A CDL, which covers tractor-trailers and other combinations where the towed unit is over 10,000 lbs GVWR.

Is the combination test a skills test or a written test?

It's a written knowledge test. The Class A skills test (pre-trip, basic control and road test) is taken separately in a combination vehicle.

Are these the real test questions?

No. They are original practice questions based on the state CDL manual. Use them to check what you know, then study your state's manual.