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Ohio CDL Combination study materials
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Ohio CDL Combination practice questions

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Question 1Combination Vehicle Knowledge

When you are coupling a tractor to a semitrailer, at what point should you back the tractor under the trailer?

  1. Until the trailer kingpin is just in front of the fifth wheel opening Correct answer.
  2. Until the tractor frame contacts the trailer landing gear
  3. Until the fifth wheel jaw locks on the kingpin
  4. Until the trailer apron touches the back of the cab

Explanation

Proper coupling requires stopping when the kingpin is just ahead of the fifth wheel opening, so you can check alignment before the final push. Backing too far before checking can damage the kingpin, the fifth wheel, or the trailer. The final rearward movement should seat the kingpin and lock the jaws.

Question 2Combination Vehicle Knowledge

After coupling, you tug-test the tractor forward with the trailer brakes set. Which condition confirms a secure connection?

  1. The trailer moves with the tractor
  2. The tractor does not move and the trailer stays put Correct answer.
  3. The landing gear lifts off the ground
  4. The tractor moves forward with the trailer

Explanation

The tug test is a pull against locked trailer brakes. If the coupling is secure, the tractor cannot move forward because the trailer holds it back. If the tractor creeps forward, the kingpin is not locked and the trailer may drop.

Question 3Combination Vehicle Knowledge

What is the purpose of the trailer air supply (red) control knob in a combination vehicle?

  1. To supply or cut off air to the trailer emergency line Correct answer.
  2. To release the tractor parking brakes only
  3. To charge the trailer air tanks during normal driving
  4. To operate the trailer hand valve for braking

Explanation

The red trailer air supply knob controls air to the trailer emergency (supply) line. Pulling it out closes the line and applies the trailer spring brakes; pushing it in charges the trailer and releases those brakes. It is not the same as the parking brake control on the tractor.

Question 4Combination Vehicle Knowledge

Which condition is required before you release the tractor parking brake control (yellow knob) after coupling?

  1. The trailer emergency line must be disconnected
  2. The trailer hand valve must be fully applied
  3. The trailer air supply knob must be pushed in and the system charged Correct answer.
  4. The landing gear must still be on the ground

Explanation

The yellow knob will not stay in unless the tractor air system is charged and the red trailer supply knob is pushed in. Releasing the tractor brakes without trailer air can allow the combination to roll uncontrolled. Always confirm air pressure and knob position first.

Question 5Combination Vehicle Knowledge

How does the total stopping distance of a loaded combination vehicle compare with that of a passenger car?

  1. It is shorter because of the extra weight pressing the tires down
  2. It is about the same at highway speeds
  3. It is longer, and the gap widens as speed increases Correct answer.
  4. It is longer only on wet pavement

Explanation

A heavy combination vehicle needs much more distance to stop than a car, and the difference grows with speed because kinetic energy rises with the square of velocity. Perception, reaction, and brake lag add to the total. Drivers must maintain a following distance that reflects this reality.

Question 6Combination Vehicle Knowledge

Which of these is the correct order for uncoupling a tractor-semitrailer?

  1. Disconnect air lines, release fifth wheel, pull tractor clear, then lower landing gear
  2. Lower landing gear, disconnect air lines, release fifth wheel, then pull tractor clear Correct answer.
  3. Release fifth wheel, disconnect air lines, lower landing gear, then pull tractor clear
  4. Lower landing gear, release fifth wheel, pull tractor clear, then disconnect air lines

Explanation

Among these steps, lower the landing gear first, then disconnect the air lines and electrical cord, and then release the fifth wheel before pulling clear. The landing gear supports the trailer, and disconnecting the lines and cord prevents them from tearing when the tractor moves away.

Question 7Combination Vehicle Knowledge

A combination vehicle with a trailer that has a shorter wheelbase relative to the tractor will tend to do what in a turn?

  1. Swing wide only when backing
  2. Off-track less than a longer-wheelbase trailer
  3. Track exactly with the tractor's rear wheels
  4. Off-track more than a longer-wheelbase trailer Correct answer.

Explanation

Off-tracking is the difference between the path of the front wheels and the path of the rear trailer wheels. A shorter trailer wheelbase produces more off-tracking, so the trailer cuts the corner more sharply. Drivers must swing wider to keep the trailer clear of curbs and other vehicles.

Question 8Combination Vehicle Knowledge

Why should you avoid using the trailer hand valve (trolley valve) to stop a combination vehicle during normal driving?

  1. It has no effect unless the trailer is fully loaded
  2. It releases the trailer spring brakes instead of applying them
  3. It applies only the trailer brakes and can cause the trailer to swing or jackknife Correct answer.
  4. It disables the tractor's service brakes permanently

Explanation

The hand valve applies only the trailer brakes. Using it as a service brake during normal driving can cause the trailer to skid or swing out and may lead to a jackknife. It is intended for checking that trailer brakes work, not for routine stopping.

Question 9Combination Vehicle Knowledge

You are backing a tractor-semitrailer and want to turn the trailer to the left. Which way should you turn the steering wheel?

  1. Left
  2. The wheel direction has no effect while backing
  3. Right Correct answer.
  4. First right, then left

Explanation

When backing a semitrailer, the trailer responds opposite to the steering input: turning the wheel right pushes the trailer's rear to the left. Steering left while backing swings the trailer right. This reversal is a common source of backing errors for new drivers.

Question 10Combination Vehicle Knowledge

What does the term "crack-the-whip" effect refer to in a combination vehicle?

  1. The vibration caused by an unbalanced trailer tire
  2. The tendency of the rear trailer to swing outward as the combination changes direction Correct answer.
  3. The sound made by slack adjusters during a brake test
  4. The whipping motion of loose air lines at highway speed

Explanation

Crack-the-whip effect describes how the rear trailer of a multi-trailer combination swings outward and can amplify the vehicle's motion as it changes direction. The rear trailer can move sideways with considerable force, especially at higher speeds. Drivers must make smooth, gradual steering and speed changes to control it.

Question 11Combination Vehicle Knowledge

When should you use the trailer hand valve to test trailer brakes?

  1. Only after the tractor parking brake is released on a hill
  2. While stopped or moving very slowly, after the trailer air system is charged Correct answer.
  3. While driving at highway speed to check brake balance
  4. Only when the trailer is disconnected from the tractor

Explanation

The hand valve is used to confirm the trailer brakes apply and hold while the vehicle is stopped or barely moving. Testing at speed risks a skid or jackknife because only the trailer brakes engage. The trailer air system must be charged before the test is meaningful.

Question 12Combination Vehicle Knowledge

What is the correct method for checking that the trailer brakes apply when the trailer air supply knob is pulled out?

  1. Check the trailer air gauge for a drop to zero
  2. Watch for the trailer to roll forward
  3. Feel for resistance by trying to pull the tractor forward slowly Correct answer.
  4. Listen for the compressor to cycle

Explanation

After pulling the red knob out, the trailer spring brakes should apply. A gentle pull with the tractor against the trailer brakes confirms they are holding. If the combination moves easily, the trailer brakes are not applying correctly.

Question 13Combination Vehicle Knowledge

What is the proper position of the trailer landing gear when the vehicle is ready to drive after being coupled?

  1. Partially lowered to aid braking
  2. Lowered only if the trailer is empty
  3. Resting lightly on the ground for stability
  4. Fully raised and secured Correct answer.

Explanation

Landing gear must be fully raised and secured before moving, or it can strike the ground and damage the trailer or cause a crash. Some systems require the handle to be secured so the gear does not creep down. Driving with the gear down is a serious safety hazard.

Question 14Combination Vehicle Knowledge

You are driving a combination vehicle and the trailer begins to jackknife. What is the recommended first response?

  1. Accelerate to straighten the trailer
  2. Apply the trailer hand valve firmly
  3. Release the brakes and steer to pull the trailer back in line Correct answer.
  4. Turn hard into the skid and hold the brakes

Explanation

Releasing the brakes restores wheel rotation and allows steering to regain control. Steering in the direction needed to align the trailer helps bring it back. Hard braking or using the hand valve tends to worsen a jackknife by locking wheels and increasing trailer swing.

Question 15Combination Vehicle Knowledge

Which factor most increases the risk of a rearward amplification (crack-the-whip) accident in a triple-trailer combination?

  1. Driving at low speed on a straight road
  2. Keeping the trailer air system fully charged
  3. Using the trailer hand valve while stopped
  4. Rapid lane changes at highway speed Correct answer.

Explanation

Rearward amplification grows with speed and abrupt steering inputs. Rapid lane changes at highway speed cause the rear trailer to swing much farther sideways than the tractor. Smooth, gradual maneuvers and lower speeds reduce the risk.

Question 16Combination Vehicle Knowledge

What is the purpose of the glad hands on a combination vehicle?

  1. They lock the fifth wheel jaws around the kingpin
  2. They couple the air lines between tractor and trailer Correct answer.
  3. They connect the electrical wiring between tractor and trailer
  4. They secure the landing gear in the raised position

Explanation

Glad hands are the couplers that join the tractor and trailer air lines, usually color-coded for the emergency and service lines. Properly connected glad hands allow air to charge the trailer and operate its brakes. Crossed or loose connections cause brake failure or drag.

Question 17Combination Vehicle Knowledge

After coupling, the trailer air supply knob will not stay pushed in. What is the most likely cause?

  1. The trailer emergency line is not connected or is leaking Correct answer.
  2. The landing gear is fully raised
  3. The trailer spring brakes are released
  4. The tractor parking brake is set

Explanation

The red knob will pop back out if the trailer emergency line is not supplying air, often because it is disconnected, crossed, or leaking. Without emergency line pressure, the trailer brakes stay applied. Check glad hand connections and air line condition.

Question 18Combination Vehicle Knowledge

You must make a right turn in a combination vehicle. What is the safest technique?

  1. Swing the tractor into the oncoming lane to give the trailer room
  2. Turn wide enough to keep the trailer's rear wheels clear of the curb, staying in your lane as much as possible Correct answer.
  3. Stop in the intersection and back the trailer around the corner
  4. Turn from the left lane and cut sharply across the right lane

Explanation

Right turns require enough room for the trailer to clear the curb without swinging into other lanes. The correct approach is to turn wide but remain in your lane as much as possible. Cutting the corner or swinging into oncoming traffic creates a collision risk.

Question 19Combination Vehicle Knowledge

Why must the trailer air system be fully charged before you begin a trip?

  1. So the fifth wheel jaws will lock
  2. So the electrical cord will stay connected
  3. So the landing gear can be raised
  4. So the trailer spring brakes can release and the service brakes can function Correct answer.

Explanation

Trailer spring brakes are held applied by springs and released by air pressure. If the system is not charged, the trailer brakes will not release and service braking will be inadequate. Charging the trailer air system is a required step before moving.

Question 20Combination Vehicle Knowledge

A driver is coupling and finds the trailer kingpin is slightly high relative to the fifth wheel. What is the correct action?

  1. Adjust the trailer height with the landing gear so the kingpin enters the fifth wheel properly Correct answer.
  2. Raise the fifth wheel with the tractor suspension
  3. Back under at an angle to force the kingpin down
  4. Pull forward and try coupling at higher speed

Explanation

The trailer height should be adjusted with the landing gear until the kingpin aligns with the fifth wheel opening. Forcing a misaligned coupling can damage the kingpin, fifth wheel, or tractor. Never use speed or angle to overcome a height mismatch.

Question 21Combination Vehicle Knowledge

What is the primary reason for checking the fifth wheel jaws after coupling?

  1. To verify the trailer air lines are connected
  2. To check that the landing gear is raised
  3. To confirm the electrical cord is plugged in
  4. To confirm the jaws have closed and locked around the kingpin Correct answer.

Explanation

Visual inspection confirms the jaws are closed and locked around the kingpin. A partially closed jaw can allow the trailer to separate from the tractor while driving. The tug test and visual check together provide the strongest assurance of a secure coupling.

Question 22Combination Vehicle Knowledge

You are driving a loaded combination on a downgrade and the trailer begins to push the tractor. Which action best helps maintain control?

  1. Use a lower gear and controlled braking to keep the combination from gaining speed Correct answer.
  2. Shift to a higher gear and coast
  3. Apply the trailer hand valve fully
  4. Release the brakes and let the combination roll

Explanation

Descending a grade requires selecting a gear low enough that engine braking controls speed, combined with controlled service braking. Coasting or relying on brakes alone can lead to overheating and loss of braking. The trailer hand valve is not for controlling speed on a downgrade.

Question 23Combination Vehicle Knowledge

In a combination vehicle, why does a lightly loaded trailer behind a heavily loaded tractor increase the risk of trailer swing during braking?

  1. The air system pressure is higher with a light trailer
  2. The fifth wheel cannot hold a light trailer
  3. The trailer has less weight on its tires, so it locks more easily and slides sideways Correct answer.
  4. The tractor brakes do not engage when the trailer is light

Explanation

A lightly loaded trailer has less tire-to-road friction, so its wheels lock more readily under braking. Locked trailer wheels lose directional stability and can swing out, especially on slippery surfaces. Balanced braking and smooth inputs reduce this risk.

Question 24Combination Vehicle Knowledge

Which combination of conditions most directly causes off-tracking to increase?

  1. Longer trailer wheelbase and slower turns
  2. Higher tire pressure and gentle turns
  3. Shorter trailer wheelbase and sharper turns Correct answer.
  4. Heavier trailer load and straight roads

Explanation

Off-tracking increases when the trailer wheelbase is short and the turn is sharp, because the rear wheels cut a tighter path than the front wheels. Drivers must allow extra room in tight urban turns and around obstacles. Long wheelbases and gentle turns reduce the effect.

Question 25Combination Vehicle Knowledge

A driver reports that the trailer brakes drag after the air system is charged. What is the most likely explanation?

  1. The trailer spring brakes are working normally
  2. The trailer service line is connected to the emergency glad hand Correct answer.
  3. The tractor parking brake is released
  4. The trailer supply line is not connected

Explanation

Crossing the service and emergency lines can cause the trailer brakes to apply or drag because the wrong line receives pressure. Correct color-coded connections prevent this. Dragging brakes also overheat and reduce braking performance.

Question 26Combination Vehicle Knowledge

In a combination vehicle, what does the term "off-tracking" describe?

  1. The rear wheels of the trailer follow a path inside the path of the front wheels during a turn Correct answer.
  2. The trailer wheels wander side to side at highway speed because of slack in the coupling
  3. The trailer brakes apply slightly later than the tractor brakes during a hard stop
  4. The tractor's front axle tracks wider than the drive axles when the vehicle is loaded

Explanation

Off-tracking is the tendency of a trailer's rear axles to cut inside the curve traced by the tractor's steering axle, so the trailer takes a shorter path through the turn. The effect grows with wheelbase length and with sharper turns, which is why a driver must swing wide and watch the trailer's rear wheels in the mirrors.

Question 27Combination Vehicle Knowledge

What is the recommended way to adjust the trailer height when coupling so the fifth wheel will pick up the trailer properly?

  1. Set the landing gear so the trailer apron is lower than the fifth wheel and back under it
  2. Raise the landing gear so the trailer apron is well above the fifth wheel
  3. Leave the landing gear fully retracted and let the trailer rest on the frame
  4. Lower the landing gear until the trailer is just slightly higher than the fifth wheel Correct answer.

Explanation

The trailer should sit just a little high so the fifth wheel slides under the apron and the kingpin rides up into the jaws as you back in. If the trailer is too low the kingpin strikes the fifth wheel and can damage it; if it is too high the tractor can pass under without a solid lock.

Question 28Combination Vehicle Knowledge

Why does a combination vehicle require a longer stopping distance than a single straight truck of similar weight?

  1. The added weight and the push of the trailer lengthen the distance needed to stop Correct answer.
  2. The fifth wheel connection prevents the tractor brakes from reaching full pressure
  3. The trailer's air brakes are designed to engage only after the tractor has slowed
  4. Combination vehicles are limited to lower speeds, which reduces brake effectiveness

Explanation

Total weight is greater and the trailer's mass keeps pushing forward when the brakes are applied, so more distance is needed to stop. A driver must also allow for the lag while trailer brakes receive air, which adds to the total stopping distance.

Question 29Combination Vehicle Knowledge

During a pre-trip inspection you find that the trailer's spring brakes do not release after the air system is fully charged. Which condition is most consistent with this symptom?

  1. The service line glad hand is capped off with no trailer attached
  2. The tractor's low-air warning buzzer is set too high
  3. The trailer's relay valve is working normally and the brakes are simply cold
  4. The trailer supply line is restricted or disconnected, so air never reaches the trailer reservoir Correct answer.

Explanation

Spring brakes release only when enough air pressure reaches the trailer's brake chambers, so a blocked or disconnected supply line leaves the trailer brakes applied. The supply line is the one that charges the trailer reservoir and signals the tractor protection valve.

Question 30Combination Vehicle Knowledge

When should the trailer hand valve be used during a pre-trip inspection?

  1. To release the trailer spring brakes before backing
  2. To supply air to the trailer reservoir while the engine is off
  3. To hold the vehicle on a hill while the driver checks the mirrors
  4. To test that the trailer service brakes apply and release Correct answer.

Explanation

The hand valve applies only the trailer service brakes, which makes it useful for confirming that the trailer brakes work and release during the pre-trip inspection. It should not be used for normal stopping because it can cause the trailer to swing or jackknife.

Question 31Combination Vehicle Knowledge

A driver must make a left turn in a tractor-semitrailer. Which practice best keeps the trailer's rear wheels from striking the curb or a vehicle on the inside of the turn?

  1. Delay the turn until the trailer's rear wheels pass the stop line, then steer straight through
  2. Begin the turn from the far right lane and cut sharply once the tractor clears the corner
  3. Keep the tractor close to the inside curb so the trailer has room on the outside
  4. Turn from the proper lane and swing wide enough that the trailer's rear wheels clear the inside corner Correct answer.

Explanation

Because of off-tracking, the trailer's rear wheels cut inside the tractor's path, so the turn must be set up from the correct lane with enough room for the trailer to clear the inside corner. Watching the trailer wheels in the mirrors confirms the clearance before completing the turn.

Question 32Combination Vehicle Knowledge

What is the function of the tractor protection valve?

  1. It regulates the trailer's spring brake release pressure during coupling
  2. It limits tractor brake pressure so the trailer brakes apply first
  3. It closes the air lines to the trailer when pressure drops, protecting the tractor's air supply Correct answer.
  4. It balances air pressure between the tractor and trailer reservoirs at all times

Explanation

The tractor protection valve senses low air pressure and closes off the trailer lines so the tractor retains enough air to stop. This is why a serious leak can trigger the trailer spring brakes while leaving the tractor brakes usable.

Question 33Combination Vehicle Knowledge

You are driving a triple-trailer combination and feel the rear trailers swinging side to side. Which action is the safest response?

  1. Ease off the accelerator and slow gradually while keeping the steering steady Correct answer.
  2. Steer sharply in the direction the rear trailer is swinging
  3. Accelerate slightly to pull the trailers into line
  4. Apply the brakes firmly to straighten the trailers

Explanation

Side-to-side movement of the rear trailers calls for a smooth, gentle reduction in speed rather than abrupt braking or steering, which can amplify the swing. Keeping the steering steady and slowing gradually lets the trailers settle into line.

Question 34Combination Vehicle Knowledge

What is the correct position of the tractor protection valve control (trailer air supply) when you are ready to drive after coupling?

  1. Pushed in, so the trailer air lines are open and the trailer brakes are released Correct answer.
  2. Pulled out, so the trailer spring brakes stay applied during the trip
  3. Set halfway to allow partial air flow to the trailer
  4. Left in whichever position it was in before coupling

Explanation

With the system charged, pushing in the trailer air supply opens the lines and releases the trailer spring brakes so the combination can move. Pulling it out applies the trailer spring brakes and is used for parking or an emergency.

Question 35Combination Vehicle Knowledge

A driver is preparing to couple to a trailer and notices the trailer is not chocked and the spring brakes are released. What is the risk?

  1. The trailer could roll when the kingpin contacts the fifth wheel Correct answer.
  2. The trailer lights could fail during coupling
  3. The landing gear could retract on its own
  4. The fifth wheel jaws could open too quickly

Explanation

An unchocked trailer with released spring brakes can roll forward or backward when the tractor pushes against it during coupling. The trailer should be secured with chocks or applied spring brakes before the tractor backs under it.

Question 36Combination Vehicle Knowledge

What does the red octagonal-shaped trailer air line glad hand normally connect to?

  1. The electrical cord, which powers the trailer marker lights
  2. The emergency line, which releases the trailer spring brakes only
  3. The service line, which carries the signal to apply the trailer brakes
  4. The supply line, which charges the trailer's air reservoir Correct answer.

Explanation

The emergency (supply) line is color-coded red and charges the trailer reservoir while also controlling the trailer spring brakes through the tractor protection system. The blue service line carries the signal that applies the trailer service brakes.

Question 37Combination Vehicle Knowledge

Why should you avoid backing a combination vehicle with the trailer at a sharp angle to the tractor for more than a short distance?

  1. The trailer lights stop working when the combination is folded
  2. The angle can damage the coupling and the trailer can tip or the tractor can be pushed sideways Correct answer.
  3. The steering axle loses its ability to turn once the angle is reached
  4. The trailer brakes automatically lock whenever the angle exceeds 45 degrees

Explanation

Backing at a sharp angle puts severe side loads on the fifth wheel and trailer frame and can tip the trailer or shove the tractor sideways. If a sharp angle is unavoidable, it should be corrected with a pull-up rather than continued backing.

Question 38Combination Vehicle Knowledge

After coupling, you pull the tractor gently forward with the trailer brakes applied. What should happen if the coupling is secure?

  1. The tractor should not move because the trailer holds it Correct answer.
  2. The tractor should stall immediately
  3. The fifth wheel should release the kingpin
  4. The trailer should slide backward

Explanation

This is the pull test: with the trailer spring brakes applied, a gentle pull in a low gear should not move the tractor if the kingpin is locked in the fifth wheel. If the tractor moves, the coupling is not secure and must be corrected.

Question 39Combination Vehicle Knowledge

What is the correct sequence for releasing the trailer spring brakes after coupling and charging the system?

  1. Pull out the trailer air supply, then check that the trailer brakes release
  2. Open the service line glad hand to bleed air into the trailer
  3. Release the tractor parking brake before charging the trailer
  4. Push in the trailer air supply, then check that the trailer brakes release Correct answer.

Explanation

Once the system is charged and the tractor protection valve has opened, pushing in the trailer air supply releases the trailer spring brakes. The driver should confirm the release by watching for the trailer to roll or by checking the wheels before moving.

Question 40Combination Vehicle Knowledge

A combination vehicle's trailer is much lighter than the tractor's load. Why does this raise the risk of trailer swing during braking?

  1. The light trailer's ABS is disabled by the tractor
  2. The trailer's spring brakes apply automatically during hard braking
  3. The light trailer has less traction and can be pushed sideways by the heavier tractor Correct answer.
  4. The light trailer's brakes grab harder than the tractor's brakes

Explanation

A lightly loaded trailer has less weight on its tires, so it has less grip and can be pushed out of line by the heavier tractor during hard braking. Smooth braking and adequate following distance reduce the chance of the trailer swinging out.

Question 41Combination Vehicle Knowledge

What is the purpose of the trailer's spring brake priority valve?

  1. To increase trailer brake pressure beyond the tractor's setting
  2. To route air from the service line into the trailer reservoir
  3. To keep the trailer spring brakes applied until the trailer reservoir has enough pressure to release them safely Correct answer.
  4. To ensure the trailer parking brakes release before the service brakes can be applied

Explanation

The priority valve holds the trailer spring brakes applied until the trailer reservoir reaches a safe pressure, preventing the trailer from rolling with insufficient air for stopping. Once pressure is sufficient, the spring brakes release and normal operation begins.

Question 42Combination Vehicle Knowledge

How does the pivot point of a tractor-semitrailer affect its turning behavior compared with a straight truck?

  1. The articulation at the fifth wheel lets the trailer cut the corner more sharply than a straight truck Correct answer.
  2. The pivot point keeps the trailer on the same path as the tractor's rear axles
  3. The articulation makes the combination turn like a single rigid unit
  4. The pivot point prevents the trailer from off-tracking at low speed

Explanation

Because the trailer pivots at the fifth wheel, its rear wheels follow a tighter path than the tractor's, so the combination cuts corners more than a straight truck. Drivers must allow extra room on the inside of turns and watch the trailer wheels.

Question 43Combination Vehicle Knowledge

You are inspecting a combination vehicle and notice that the trailer service line glad hand is connected to the tractor's supply line glad hand. What will result?

  1. The tractor protection valve will automatically swap the lines
  2. The trailer brakes will apply continuously and the trailer reservoir will not charge properly Correct answer.
  3. Only the trailer marker lights will fail to work
  4. The system will operate normally because the lines are interchangeable

Explanation

The supply and service lines serve different functions and must be connected to their matching glad hands, which are color-coded to prevent mistakes. Cross-connecting them prevents proper charging of the trailer reservoir and causes incorrect brake behavior.

Question 44Combination Vehicle Knowledge

What is the main reason to check that the trailer's landing gear is fully raised before driving?

  1. To prevent the landing gear from striking the road or railroad crossings Correct answer.
  2. To improve the trailer's brake balance
  3. To reduce the trailer's off-tracking in turns
  4. To keep the trailer's ABS functioning correctly

Explanation

Landing gear left down can catch on pavement, railroad crossings, or debris and cause serious damage or loss of control. The pre-trip routine includes confirming the gear is fully retracted and the crank handle is secured.

Question 45Combination Vehicle Knowledge

A driver is descending a long grade with a loaded semitrailer and wants to avoid the trailer pushing the tractor. Which technique is most appropriate?

  1. Apply the trailer hand valve to hold back the trailer on the grade
  2. Coast in neutral to save fuel and rely on the trailer brakes
  3. Select a lower gear before the descent and use light, steady braking to keep the combination under control Correct answer.
  4. Brake hard at the top of the grade to build a speed cushion

Explanation

Selecting a lower gear before the grade uses engine braking to control speed and reduces reliance on the service brakes, which helps prevent the trailer from pushing the tractor. Light, steady brake applications keep the combination stable and avoid overheating the brakes.

Question 46Combination Vehicle Knowledge

What does the term "rearward amplification" describe in a multi-trailer combination?

  1. The rise in air pressure in the rear trailer's reservoir
  2. The increase in brake pressure toward the rear of the combination
  3. The widening of the trailer's track at highway speed
  4. The tendency of the rear trailer to swing more than the tractor during a rapid steering maneuver Correct answer.

Explanation

Rearward amplification means each successive trailer in a multi-trailer combination swings farther than the one ahead during a quick lane change or steering input. The effect is greatest at higher speeds and with more trailers, so smooth steering and moderate speed are essential.

Question 47Combination Vehicle Knowledge

When coupling, after the fifth wheel has locked onto the kingpin, what should you do before raising the landing gear?

  1. Pull forward gently with the trailer brakes applied to confirm the lock Correct answer.
  2. Disconnect the air lines to check for leaks
  3. Release the tractor parking brake and drive a short distance
  4. Lower the landing gear until it touches the ground

Explanation

The pull test with the trailer spring brakes applied confirms that the kingpin is locked in the fifth wheel before the trailer's weight is transferred. Only after a successful test should the landing gear be raised and the air lines and electrical cord connected.

Question 48Combination Vehicle Knowledge

Why is it important to inspect the fifth wheel mounting bolts and frame during a pre-trip inspection?

  1. Loose or cracked mounts can let the fifth wheel shift or separate under load Correct answer.
  2. The bolts adjust the trailer's ride height automatically
  3. The mounts determine the trailer's spring brake release pressure
  4. The bolts control the trailer's brake balance

Explanation

The fifth wheel carries the trailer's weight and transmits pulling and braking forces, so loose or cracked mounting hardware can allow it to shift or fail. A visual check for missing bolts, cracks, and secure mounting is part of the combination vehicle inspection.

Question 49Combination Vehicle Knowledge

A driver is backing a tractor-semitrailer into a dock on the left side and the trailer is drifting the wrong way. What is the safest correction?

  1. Pull forward to straighten the combination and start the maneuver again Correct answer.
  2. Accelerate to straighten the trailer quickly
  3. Apply the trailer hand valve to pivot the trailer
  4. Turn the steering wheel harder in the same direction

Explanation

When a backing maneuver starts to go wrong, pulling forward to straighten out and restarting is safer than forcing the correction. Continuing to back while the trailer drifts the wrong way can lead to a jackknife or contact with the dock.

Question 50Combination Vehicle Knowledge

What is the correct action if the low-air warning buzzer and light come on while driving a combination vehicle?

  1. Increase engine speed to build air pressure while continuing the trip
  2. Continue driving until you reach the next scheduled stop
  3. Pull the trailer air supply knob out and keep driving
  4. Move to a safe location and stop as soon as possible to find the leak Correct answer.

Explanation

A low-air warning means the system is losing pressure and the spring brakes may apply soon, so the driver should stop in a safe place and investigate. Continuing to drive risks a loss of braking and an uncontrolled stop.