Practice questions written from the official CDL manual. Correct answers and explanations are shown below each question. (Turn on JavaScript for the interactive practice and exam-simulation modes.)
1. Why is it important to keep cargo as low as possible when loading a combination vehicle?
- AIt makes the trailer easier to uncouple later
- BA lower load lowers the center of gravity and reduces the risk of rolloverCorrect
- CIt keeps the trailer air tanks from freezing
- DIt allows you to drive faster on curves
Why: Loading cargo high raises the vehicle's center of gravity, which makes it top-heavy and much more likely to tip over. Keeping the load low and centered lowers the center of gravity and reduces rollover risk, especially on curves and ramps.
Based on: CDL Manual §6.1 — Driving Combination Vehicles Safely
2. What is the "crack-the-whip" effect (rearward amplification) in a combination vehicle?
- AThe trailer brakes grab harder than the tractor brakes
- BA steering movement is magnified as it travels back through the rig, so the rear trailer swings much more than the cabCorrect
- CThe engine loses power when pulling two trailers
- DThe air pressure drops faster in a longer combination
Why: In a combination vehicle, a quick steering move by the driver gets larger as it passes toward the back of the rig. The last trailer can swing with several times the force of the cab, which can flip it over. This is why sudden steering is dangerous with trailers.
Based on: CDL Manual §6.1 — Rearward Amplification
3. How should you steer a combination vehicle to reduce the chance of a rollover from the crack-the-whip effect?
- ASteer with quick, sharp corrections
- BKeep both hands off the wheel on straight roads
- CBrake hard in every turn
- DSteer gently and smoothly, and make lane changes slowlyCorrect
Why: Because a fast steering movement is amplified toward the rear trailer, you should steer gently and smoothly and change lanes slowly. Sudden, jerky steering can throw the last trailer sideways or roll it over.
Based on: CDL Manual §6.1 — Driving Combination Vehicles Safely
4. If the trailer wheels lock up and the trailer begins to swing out in a skid, what should you do?
- APress the brakes harder to slow the trailer
- BRelease the brakes to let the trailer wheels roll again and regain tractionCorrect
- CApply the trailer hand valve fully
- DSteer sharply toward the skid
Why: A trailer skid usually happens because the trailer wheels are locked by too much braking. To stop the skid you must let the wheels roll again, so you release the brakes. Once the wheels grip, the trailer straightens back behind the tractor.
Based on: CDL Manual §6.1 — Trailer Skids
5. A tractor jackknife happens when which wheels lock up?
- AThe trailer wheels
- BThe front steering wheels
- CThe trailer's rearmost axle only
- DThe tractor's drive wheelsCorrect
Why: A tractor jackknife occurs when the drive wheels of the tractor lose traction and lock, letting the back of the tractor slide sideways and fold toward the trailer. Locked trailer wheels cause a trailer skid instead, which is a different problem.
Based on: CDL Manual §6.1 — Vehicle Skids
6. How can a driver best tell early that the trailer has started to skid?
- ABy seeing the trailer swing out of line in the side mirrorsCorrect
- BThe engine stalls
- CThe air pressure gauge rises quickly
- DThe steering wheel begins to shake
Why: A trailer skid is hard to feel, so drivers should watch the mirrors. When the trailer swings out to the side of the tractor, it has begun to skid. Catching it early in the mirror lets you release the brakes before it gets worse.
Based on: CDL Manual §6.1 — Trailer Skids
7. What are the two jobs of the trailer emergency (supply) air line?
- AIt cools the trailer brakes and inflates the tires
- BIt supplies air to the trailer air tanks and controls the trailer emergency brakesCorrect
- CIt powers the trailer lights and the ABS
- DIt applies the service brakes only during hard stops
Why: The emergency line, also called the supply line, does two things: it fills the trailer's air tanks with air, and it controls the emergency brakes. If pressure in this line is lost, the trailer's emergency brakes come on automatically.
Based on: CDL Manual §6.2 — Combination Vehicle Air Brakes
8. What does the trailer service air line do?
- AIt carries air that is controlled by the foot brake or the trailer hand valveCorrect
- BIt keeps the trailer emergency brakes released at all times
- CIt supplies constant full pressure to the trailer tanks
- DIt only works when the parking brake is set
Why: The service line, also called the control or signal line, carries air whose pressure changes depending on how hard you press the brake pedal or the trailer hand valve. This is what applies the trailer's normal service brakes as you drive.
Based on: CDL Manual §6.2 — Service Air Line
9. What is the purpose of the tractor protection valve?
- ATo warn the driver of low oil pressure
- BTo keep air in the tractor's brake system if the trailer breaks away or develops a bad leakCorrect
- CTo automatically raise the landing gear
- DTo lock the fifth wheel jaws
Why: The tractor protection valve keeps air in the tractor (or truck) brake system if the trailer breaks away or a bad leak drains the air. This saves enough air for the tractor to stop safely. It is controlled by the trailer air supply valve in the cab.
Based on: CDL Manual §6.2 — Tractor Protection Valve
10. On newer vehicles, what does the trailer air supply control usually look like, and what does pushing it in do?
- AA red eight-sided knob; pushing it in supplies air to the trailerCorrect
- BA green round knob; pushing it in sets the parking brake
- CA yellow diamond-shaped knob; pushing it in turns on the lights
- DA blue lever; pushing it in releases the fifth wheel
Why: The trailer air supply control on newer rigs is a red, eight-sided knob. You push it in to send air to the trailer so its brakes can release, and you pull it out to shut the air off and set the trailer emergency brakes.
Based on: CDL Manual §6.2 — Trailer Air Supply Control
11. How do you apply the trailer emergency (spring) brakes from the cab?
- APush in the red trailer air supply knob
- BPump the brake pedal three times
- CTurn on the ABS malfunction lamp
- DPull out the red trailer air supply knob to shut off the trailer airCorrect
Why: Pulling out the red trailer air supply knob closes the tractor protection valve and shuts off air to the trailer. With the air gone, the trailer's spring emergency brakes come on. Pushing it back in supplies air and releases them again.
Based on: CDL Manual §6.2 — Trailer Air Supply Control
12. At what air pressure will the tractor protection valve close (and the trailer air supply knob pop out) on its own?
- ABetween 60 and 80 psi
- BAt exactly 100 psi
- CBetween 20 and 45 psiCorrect
- DOnly when pressure reaches zero
Why: If air pressure drops into the 20 to 45 psi range, the tractor protection valve closes automatically and the trailer air supply knob pops out. This shuts air off to the trailer and protects enough air for the tractor to brake.
Based on: CDL Manual §6.2 — Tractor Protection Valve
13. What happens to the trailer when the tractor protection valve closes because of very low air pressure?
- AThe trailer service brakes release completely
- BThe trailer lights turn off
- CThe trailer emergency brakes come onCorrect
- DNothing happens until you press the pedal
Why: When the tractor protection valve closes, air is shut off to the trailer and air already in the emergency line escapes. Losing that air causes the trailer's emergency brakes to come on automatically, which helps stop a trailer that has broken away.
Based on: CDL Manual §6.2 — Tractor Protection Valve
14. When connecting glad hands, how must the air lines be joined?
- AService line to emergency line
- BEither line to either coupler; it does not matter
- CService to service and emergency to emergencyCorrect
- DBoth lines to the electrical socket
Why: Glad hands are the coupling devices that join the air lines. You must connect the service line to the service coupler and the emergency line to the emergency coupler. Crossing them sends air to the wrong place and the trailer brakes will not work correctly.
Based on: CDL Manual §6.2 — Glad Hands
15. How are glad hands and air lines commonly marked to help you avoid crossing them?
- AThey are all painted white
- BThey have flashing lights
- CThey are numbered 1 through 4
- DThey are color-coded, often blue for service and red for emergency, and may be labeledCorrect
Why: To prevent crossed connections, air lines and glad hands are often color-coded, with blue for the service line and red for the emergency line, and some are stamped "service" or "emergency." The couplers may also be shaped differently so they only fit the correct way.
Based on: CDL Manual §6.2 — Glad Hands
16. Why is it dangerous to accidentally cross the service and emergency air line connections?
- AYou could drive away with no working trailer brakes and not know itCorrect
- BThe trailer tires will deflate
- CThe engine will not start
- DThe landing gear will not raise
Why: If the air lines are crossed, air goes to the wrong parts of the system and the trailer service brakes may not respond when you press the pedal. You could pull out with no trailer braking. That is why you test the brakes after coupling and check the connections.
Based on: CDL Manual §6.2 — Glad Hands
17. Before backing the tractor under a trailer, how should the trailer height be set?
- AHigh enough that the tractor passes underneath without touching
- BExactly level with the fifth wheel
- CAs low as the landing gear will go
- DLow enough that the trailer will be raised slightly when the tractor backs under itCorrect
Why: The trailer should be a little low so that the fifth wheel lifts it slightly as the tractor backs under. If the trailer is too low the tractor may strike it and miss the kingpin; if it is too high the coupling can be incomplete. Adjust with the landing gear.
Based on: CDL Manual §6.3 — Coupling and Uncoupling
18. After you supply air to the trailer during coupling, what test should you do before backing fully under the trailer?
- ARev the engine to full speed
- BRaise and lower the landing gear five times
- CPush and pull the trailer air supply knob to check that the trailer brakes apply and release, listening for the brakesCorrect
- DHonk the horn twice
Why: Once air reaches the trailer, you test the brake connection by applying and releasing the trailer brakes and listening for them to work. This checks that the air lines are not crossed and that the trailer brakes will hold before you complete the coupling.
Based on: CDL Manual §6.3 — Supply Air to the Trailer
19. When backing under the trailer to complete the coupling, you should back until what happens?
- AThe kingpin locks into the fifth wheel jawsCorrect
- BThe tires touch the curb
- CThe landing gear lifts off the ground on its own
- DThe trailer lights come on
Why: Using the lowest reverse gear, back slowly and straight until the kingpin is caught and locked by the fifth wheel jaws. You should feel and hear the jaws close. Backing too fast can hit the kingpin hard or cause a bad connection.
Based on: CDL Manual §6.3 — Back Under the Trailer
20. How do you test that the fifth wheel is locked onto the kingpin after coupling?
- ARock the trailer side to side by hand
- BRaise the landing gear slightly, then pull the tractor forward gently with the trailer brakes lockedCorrect
- CBounce on the rear bumper
- DPush in the air supply knob and wait five minutes
Why: With the trailer brakes locked and the landing gear raised a little, gently pull the tractor forward. If the coupling is locked, the trailer will not come loose. This pull test confirms the kingpin is caught before you drive.
Based on: CDL Manual §6.3 — Check Connection for Security
21. When you inspect the coupling with a flashlight, what should you see between the upper and lower fifth wheel?
- AA gap of about two inches
- BA pool of grease filling a large space
- CNo gap at all between the two platesCorrect
- DThe kingpin resting on top of the closed jaws
Why: There should be no space between the upper and lower fifth wheel plates. A gap means the kingpin may be sitting on top of the closed jaws instead of locked inside them, and the trailer could come loose. Get under and look with a flashlight to be sure.
Based on: CDL Manual §6.3 — Inspect Coupling
22. For a correct coupling, the fifth wheel locking jaws must close around what part of the kingpin?
- AThe tip of the kingpin only
- BThe trailer's landing gear leg
- CThe glide plate
- DThe shank of the kingpinCorrect
Why: The locking jaws must close around the shank (the narrow neck) of the kingpin, not around the wide base or catch only the tip. Jaws that grip the wrong part make a weak connection that can pull apart.
Based on: CDL Manual §6.3 — Inspect Coupling
23. After coupling, where must the fifth wheel safety catch (safety latch) be?
- ARemoved and stored in the cab
- BHanging loose beside the fifth wheel
- COnly used on the trailer, not the tractor
- DOver the locking lever so it cannot come openCorrect
Why: The safety catch must be in place over the locking lever. It keeps the release handle from moving and the jaws from opening on their own while you drive. On some fifth wheels you must close the safety catch by hand.
Based on: CDL Manual §6.3 — Inspect Coupling
24. Before backing under a trailer, in what condition should the fifth wheel be?
- APlate dry and clean with the jaws closed
- BPlate greased, jaws open, and the release handle in the automatic (unlocked) positionCorrect
- CTilted fully forward with the safety catch locked
- DCovered so no dirt gets on it
Why: To couple correctly the fifth wheel plate should be greased, the locking jaws open, and the release arm set in the automatic or unlocked position. This lets the jaws snap shut around the kingpin when you back under. A dry plate or closed jaws will keep the coupling from working.
Based on: CDL Manual §6.3 — Inspect Fifth Wheel
25. When uncoupling, how far should you lower the landing gear before disconnecting?
- AOnly halfway down, leaving it well above the ground
- BAs high as it will go
- CUntil it makes firm contact with the ground, but not so far that it lifts the trailer off the fifth wheelCorrect
- DIt should not be lowered at all until the tractor is gone
Why: Lower the landing gear until it just touches the ground firmly so it will support the trailer, but do not crank it so far that it raises the trailer off the fifth wheel. Lifting the trailer makes the coupling hard to release and can damage parts.
Based on: CDL Manual §6.3 — Lower the Landing Gear
26. After unlocking the fifth wheel during uncoupling, why do you pull the tractor only partly clear of the trailer at first?
- ATo let the air lines drain
- BSo the tractor frame is still under the trailer to hold it up if the landing gear should collapseCorrect
- CTo warm up the trailer brakes
- DTo make the electrical cord reach
Why: You pull forward just until the fifth wheel comes out from under the trailer, keeping the tractor frame still under the trailer. If the landing gear were to fail, the tractor frame would catch the trailer instead of letting it crash to the ground.
Based on: CDL Manual §6.3 — Pull Tractor Partially Clear
27. After disconnecting the air lines during uncoupling, what should you do with the glad hands?
- AStore them on the dummy couplers or lock them together to keep dirt and water outCorrect
- BLeave them dragging on the ground
- CTape them to the exhaust pipe
- DToss them into the trailer
Why: Once the air lines are disconnected, hang the glad hands on the dummy (dead-end) couplers or lock them together. This keeps dirt and water out of the lines so the brakes will not be fouled the next time you connect them.
Based on: CDL Manual §6.3 — Disconnect Air Lines
28. When should the trailer hand valve (also called the trolley or Johnson bar) be used?
- AOnly to test the trailer brakes, not for regular driving or parkingCorrect
- BTo park the vehicle overnight
- CTo slow down normally while driving
- DWhenever you go downhill to save the foot brake
Why: The trailer hand valve applies only the trailer brakes and should be used just to test them. Using it while driving can cause a trailer skid, and never use it for parking because air can leak out and let the brakes release.
Based on: CDL Manual §6.2 — Trailer Hand Valve
29. Where is the ABS malfunction lamp located on a trailer equipped with antilock brakes, and what color is it?
- AA yellow lamp on the left side of the trailerCorrect
- BA green lamp on the front of the trailer
- CA red lamp on the rear doors
- DA blue lamp near the kingpin
Why: Trailers with ABS have a yellow malfunction lamp on the left (driver's) side. If the lamp stays on, the trailer ABS is not working. Trailers built on or after March 1, 1998 are required to have ABS.
Based on: CDL Manual §6.4 — Antilock Brake Systems on Trailers
30. What does antilock braking (ABS) on a trailer actually do for you?
- AIt lets you drive faster and follow more closely
- BIt always shortens your stopping distance by a large amount
- CIt keeps the wheels from locking up so you can keep control while braking hardCorrect
- DIt replaces the need to inspect the brakes
Why: ABS keeps the wheels from locking during hard or slippery braking, which helps you keep steering control and stay in your lane. It does not automatically shorten stopping distance, and it is not a reason to drive faster or follow closer.
Based on: CDL Manual §6.4 — Antilock Brake Systems on Trailers