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California CDL Combination Vehicles Test study materials
California CDL Combination Vehicles Test study resources

California CDL Combination Vehicles Test practice questions

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Question 1Combination vehicle inspection and basic control skills

Before driving a combination vehicle, what is the first thing you should check during the walk-around inspection?

  1. That the radio and horn work
  2. That the trailer is fully loaded
  3. That the air lines and electrical cord are properly connected Correct answer.
  4. That the fuel tank is full

Explanation

A pre-trip inspection of a combination vehicle begins with confirming that the glad hands are locked and the electrical cord is secured, because a disconnected or leaking line can cause brake failure or loss of lights while driving. These connections are the foundation of safe tractor-trailer operation.

Question 2Combination vehicle inspection and basic control skills

When coupling a tractor to a trailer, how should the trailer be positioned relative to the fifth wheel?

  1. Directly in line with the tractor and slightly lower Correct answer.
  2. At an angle of about 45 degrees
  3. Far to the side of the tractor
  4. Raised above the fifth wheel

Explanation

The trailer should be centered in line with the tractor and its kingpin slightly lower than the fifth wheel so the tractor backs under it and the fifth wheel lifts the trailer. This alignment prevents the kingpin from striking the wrong part of the fifth wheel.

Question 3Combination vehicle inspection and basic control skills

After coupling, you should tug-test the connection. What does a proper tug test confirm?

  1. That the trailer lights are working
  2. That the fifth wheel jaws have locked around the kingpin Correct answer.
  3. That the landing gear is fully raised
  4. That the air tanks are full

Explanation

Pulling gently against the trailer with the trailer brakes set confirms the fifth wheel jaws are locked on the kingpin and will not release. If the trailer moves forward with the tractor, the coupling has failed and must be redone.

Question 4Combination vehicle inspection and basic control skills

What is the purpose of the trailer hand valve (trolley valve)?

  1. To test the trailer brakes while the vehicle is stopped Correct answer.
  2. To release the fifth wheel jaws
  3. To set the parking brakes on the tractor
  4. To raise and lower the landing gear

Explanation

The trailer hand valve applies only the trailer brakes and is used to check that trailer braking works when the vehicle is stationary. It should never be used to stop a moving vehicle because it can cause a jackknife.

Question 5Combination vehicle inspection and basic control skills

During a pre-trip inspection, you find that the trailer's spring brakes do not release when air pressure builds. What is the most likely cause?

  1. The electrical cord is unplugged
  2. The landing gear is too high
  3. The trailer is overloaded
  4. The emergency (supply) glad hand is not properly connected Correct answer.

Explanation

Spring brakes require air pressure delivered through the emergency supply line to release. If that glad hand is disconnected or leaking, the trailer brakes stay applied and the trailer cannot roll.

Question 6Combination vehicle inspection and basic control skills

What is the correct order for uncoupling a tractor from a trailer?

  1. Disconnect the lines, pull forward, then lower the landing gear
  2. Release the fifth wheel first, then lower the landing gear
  3. Set the trailer brakes, release the fifth wheel, then disconnect the lines
  4. Lower the landing gear, disconnect the lines, release the fifth wheel, then pull forward Correct answer.

Explanation

The trailer must be supported before the tractor leaves, so the landing gear is lowered first, then the air lines and electrical cord are disconnected, the fifth wheel is released, and the tractor is pulled clear. Following this order prevents the trailer from dropping.

Question 7Combination vehicle inspection and basic control skills

When backing a tractor-trailer, which technique helps you track the trailer's direction?

  1. Turn the steering wheel sharply and hold it
  2. Accelerate quickly to keep momentum
  3. Use the trailer's rear wheels as a guide point Correct answer.
  4. Watch only the tractor's side mirrors

Explanation

Watching the trailer's rear wheels gives a reliable reference for where the trailer is heading and how much to correct the steering. Small, smooth steering inputs keep the trailer tracking in the intended path.

Question 8Combination vehicle inspection and basic control skills

Why should you avoid sudden steering movements when driving a combination vehicle?

  1. They can cause the trailer to roll over or swing out of control Correct answer.
  2. They make the brakes apply automatically
  3. They reduce tire wear
  4. They improve fuel economy

Explanation

A combination vehicle has a high center of gravity and a long trailer, so sharp steering inputs can roll the trailer or cause it to swing into another lane. Smooth, gradual steering keeps the trailer stable.

Question 9Combination vehicle inspection and basic control skills

What happens if you apply the trailer hand valve to stop a moving combination vehicle?

  1. It stops the vehicle more quickly
  2. It engages the tractor parking brake
  3. It releases the trailer brakes
  4. It can cause the trailer to jackknife Correct answer.

Explanation

Applying only the trailer brakes while the tractor keeps rolling can cause the trailer wheels to lock and the trailer to swing around, producing a jackknife. The hand valve is for stationary brake checks only.

Question 10Combination vehicle inspection and basic control skills

When making a right turn in a combination vehicle, what should you do to avoid hitting the curb with the trailer?

  1. Stay close to the right curb throughout
  2. Turn sharply and accelerate
  3. Brake hard during the turn
  4. Swing wide to the left before turning Correct answer.

Explanation

Because the trailer cuts the corner, you must swing wide to the left before turning right so the trailer's rear wheels clear the curb. Check mirrors throughout the turn to confirm trailer clearance.

Question 11Combination vehicle inspection and basic control skills

What is the recommended way to check that the trailer brakes are working before driving?

  1. Drive at highway speed and apply the brakes
  2. Honk the horn and watch the lights
  3. With the vehicle stopped, apply the trailer hand valve and listen for air flow Correct answer.
  4. Release the parking brake and roll forward

Explanation

With the vehicle stationary and the tractor protected by its parking brake, applying the trailer hand valve lets you confirm air reaches the trailer brake chambers and that they apply. This stationary test avoids the danger of testing brakes at speed.

Question 12Combination vehicle inspection and basic control skills

Which of these is a sign of a brake air leak in a combination vehicle?

  1. The air pressure gauge drops steadily with the engine off Correct answer.
  2. The engine idles smoothly
  3. The landing gear cranks easily
  4. The trailer lights flash

Explanation

A steady drop in air pressure with the engine off and brakes applied indicates a leak in the air system. Low pressure can cause the spring brakes to apply unexpectedly, so the leak must be found and fixed before driving.

Question 13Combination vehicle inspection and basic control skills

What is the purpose of the safety latch on the fifth wheel?

  1. To raise the trailer during coupling
  2. To prevent the release handle from opening accidentally Correct answer.
  3. To keep the fifth wheel plate lubricated
  4. To connect the electrical cord

Explanation

The safety latch locks the release handle in place so the fifth wheel jaws cannot open while driving. Without it, vibration could release the trailer from the tractor.

Question 14Combination vehicle inspection and basic control skills

When should you use the trailer's spring brakes to hold the trailer during uncoupling?

  1. Only when the trailer is empty
  2. Before releasing the fifth wheel, with the trailer brakes set Correct answer.
  3. Only after the tractor is disconnected
  4. After the landing gear is raised

Explanation

The trailer parking (spring) brakes should be set before releasing the fifth wheel so the trailer cannot roll once the tractor pulls away. Setting them first is essential for a controlled uncoupling.

Question 15Combination vehicle inspection and basic control skills

What should you check about the landing gear before driving with an empty trailer?

  1. That the handle is removed
  2. That it touches the ground
  3. That it is fully raised and the crank is secured Correct answer.
  4. That it is greased with oil

Explanation

Landing gear must be fully raised and its crank secured so it clears the road surface and does not strike obstacles. A partially lowered leg can catch on pavement or railroad crossings.

Question 16Combination vehicle inspection and basic control skills

Why is it important to check the trailer's rear underride guard during a pre-trip inspection?

  1. It supports the landing gear
  2. It contains the air tanks
  3. It protects following vehicles from sliding under the trailer in a crash Correct answer.
  4. It holds the trailer brakes

Explanation

The rear underride guard is a safety barrier that prevents smaller vehicles from sliding underneath the trailer in a collision. It must be intact and properly attached to do its job.

Question 17Combination vehicle inspection and basic control skills

When driving a combination vehicle, how should you adjust your following distance compared to a passenger car?

  1. Follow at one second
  2. Match the car's distance exactly
  3. Keep a longer following distance Correct answer.
  4. Follow more closely to save fuel

Explanation

A loaded combination vehicle needs much more stopping distance than a passenger car, so you should increase your following distance to allow for the extra weight and braking time. One second per ten feet of vehicle length is a common guideline.

Question 18Combination vehicle inspection and basic control skills

What is the correct way to check that the kingpin is fully locked in the fifth wheel?

  1. Look at the release handle and confirm the safety latch is engaged, then tug test Correct answer.
  2. Check the trailer lights
  3. Kick the tires
  4. Listen for the engine to idle down

Explanation

Visually confirming the release handle is in and the safety latch is engaged, combined with a gentle tug test, verifies the kingpin is locked. Both checks together give the strongest assurance the trailer is secure.

Question 19Combination vehicle inspection and basic control skills

What should you do if the trailer begins to sway while you are driving at highway speed?

  1. Accelerate hard to pull it straight
  2. Ease off the accelerator and steer gently to keep the tractor straight Correct answer.
  3. Brake hard immediately
  4. Apply the trailer hand valve

Explanation

Sway is best corrected by easing off the accelerator and holding the steering steady so the trailer settles behind the tractor. Hard braking or acceleration can worsen the sway and lead to a rollover.

Question 20Combination vehicle inspection and basic control skills

During a pre-trip inspection, you notice the trailer's air lines are crossed. What problem does this cause?

  1. The landing gear will not crank
  2. The fifth wheel will not lock
  3. The trailer lights will not work
  4. The trailer brakes may not release or apply correctly Correct answer.

Explanation

The emergency and service lines must be connected to their matching glad hands; if they are crossed, the trailer brakes will not release or apply as designed. The lines must be corrected before the vehicle is moved.

Question 21Combination vehicle inspection and basic control skills

What is the proper way to raise the landing gear after coupling?

  1. Raise it halfway
  2. Leave it slightly touching the ground
  3. Crank it up fully and secure the handle Correct answer.
  4. Remove the crank and leave the gear down

Explanation

The landing gear must be cranked fully up and the handle secured so the legs clear the road and cannot strike obstacles. Leaving any part down can damage the gear and cause a hazard.

Question 22Combination vehicle inspection and basic control skills

Why should you check the fifth wheel plate for proper lubrication?

  1. To raise the trailer higher
  2. To allow the trailer to pivot smoothly and reduce wear Correct answer.
  3. To keep the electrical cord connected
  4. To make the trailer brakes grip better

Explanation

A properly lubricated fifth wheel plate lets the trailer pivot smoothly and reduces wear on the plate and kingpin. Poor lubrication can cause binding, noise, and steering difficulty.

Question 23Combination vehicle inspection and basic control skills

What is the best practice for the initial pull-forward after coupling?

  1. Pull forward slowly and recheck the coupling Correct answer.
  2. Accelerate quickly to test the connection
  3. Release the trailer brakes and coast
  4. Immediately drive onto the highway

Explanation

After coupling and raising the landing gear, pull forward slowly and stop to recheck the connection and clearance. A slow initial pull gives you time to catch a failure before entering traffic.

Question 24Combination vehicle inspection and basic control skills

How does a heavy load affect the braking distance of a combination vehicle?

  1. It lengthens the braking distance Correct answer.
  2. It has no effect on braking distance
  3. It shortens the braking distance
  4. It only affects steering, not braking

Explanation

Extra weight increases the momentum the brakes must overcome, so a loaded combination vehicle needs a longer distance to stop. Drivers must plan ahead and increase following distance when loaded.

Question 25Combination vehicle inspection and basic control skills

During a pre-trip inspection, you find a cracked brake drum on the trailer. What should you do?

  1. Drive slowly and monitor it
  2. Tape the crack and continue
  3. Ignore it if the other brakes work
  4. Place the vehicle out of service until it is repaired Correct answer.

Explanation

A cracked brake drum is a serious defect that can cause brake failure, so the vehicle must be placed out of service until it is repaired. Continuing to drive with a known brake defect is unsafe and illegal.

Question 26Combination vehicle inspection and basic control skills

During a pre-trip inspection of a combination vehicle, what should you check about the trailer's air line couplers?

  1. That the supply line is connected to the service line
  2. That only the emergency line needs to be connected
  3. That the lines are taped together to prevent rubbing
  4. That both lines are securely locked with no audible leak Correct answer.

Explanation

Both the service and emergency (supply) lines must be firmly seated in their couplers and locked so they cannot pop out under pressure. Listening for leaks at the couplers confirms the seals are holding. Air lines that come loose in motion trigger a trailer emergency brake application.

Question 27Combination vehicle inspection and basic control skills

You are coupling a tractor to a trailer and the fifth wheel jaw will not close around the kingpin. What is the most likely cause?

  1. The kingpin is too far forward of the fifth wheel
  2. The fifth wheel jaw is open and the kingpin is not seated Correct answer.
  3. The trailer is too heavily loaded for the tractor
  4. The trailer brakes are still released

Explanation

The locking jaws only close when the kingpin sits fully inside the throat of the fifth wheel. If the jaws stay open, the kingpin is not seated, often because the trailer was positioned too high or too far off center. You must reposition and try again rather than force it.

Question 28Combination vehicle inspection and basic control skills

What is the purpose of a tractor protection valve in a combination vehicle?

  1. To release the trailer spring brakes automatically
  2. To balance air between the front and rear tractor axles
  3. To increase air pressure to the trailer brakes
  4. To close the trailer air lines if the tractor loses air pressure Correct answer.

Explanation

The tractor protection valve guards the tractor's air supply by shutting off flow to the trailer lines when pressure drops too low. This keeps enough air for the tractor's own brakes and triggers the trailer emergency brakes. It is a safety device, not a pressure booster.

Question 29Combination vehicle inspection and basic control skills

When performing a tug test after coupling, what are you confirming?

  1. That the fifth wheel jaws have locked on the kingpin Correct answer.
  2. That the trailer lights are wired correctly
  3. That the glad hands are sealed
  4. That the landing gear is fully raised

Explanation

A tug test means pulling gently forward with the trailer brakes set to see whether the trailer holds. If the jaws are locked on the kingpin, the trailer will resist and the tractor will not pull free. If the tractor moves out from under the trailer, the coupling is not secure.

Question 30Combination vehicle inspection and basic control skills

How should you position the trailer axles when loading a combination vehicle to help maintain steering control?

  1. Put more weight on the rear trailer axles than the tractor drive axles
  2. Keep the load centered so the steer axle stays within its weight limit Correct answer.
  3. Load the trailer as far back as possible to lighten the drive axles
  4. Place all cargo weight directly over the tractor steer axle

Explanation

Steering control depends on keeping enough weight on the front steer axle while staying within its rating. A centered load that respects axle limits preserves steering traction and avoids overloading any axle. Too little weight on the steer axle makes steering feel light and unsafe.

Question 31Combination vehicle inspection and basic control skills

Before backing a tractor-trailer, why should you get out and look at the area behind the trailer?

  1. To confirm the trailer doors are locked
  2. To verify the landing gear is down
  3. To identify obstacles and plan the path you cannot see from the cab Correct answer.
  4. To check tire pressure on the trailer

Explanation

A large blind spot sits directly behind and beside the trailer, so you cannot rely on mirrors alone. Walking the area lets you spot obstacles, people, and clearance issues and plan the maneuver. This step reduces the risk of backing over something or someone.

Question 32Combination vehicle inspection and basic control skills

When backing a tractor-trailer, which mirror setup gives you the best view of the trailer's path?

  1. Rely on the rearview mirror inside the cab
  2. Use only the driver-side mirror for all backing
  3. Point the mirrors up to watch for overhead clearance
  4. Adjust both mirrors to see the trailer wheels and the area beside the trailer Correct answer.

Explanation

Backing requires watching the trailer's rear corners and wheels to track its direction. Both mirrors should be set so you can see the trailer sides and the ground beside it. Checking both sides helps you catch drift before it becomes a jackknife.

Question 33Combination vehicle inspection and basic control skills

What is the correct first step when starting to uncouple a tractor from a trailer?

  1. Disconnect the air lines
  2. Pull the tractor forward slowly
  3. Release the trailer spring brakes
  4. Lower the landing gear to the ground Correct answer.

Explanation

Before any separation, the trailer must be supported, so lowering the landing gear to the ground comes first. This prevents the trailer from dropping when the tractor is removed. Air lines and electrical cord are disconnected only after the trailer is stable.

Question 34Combination vehicle inspection and basic control skills

When driving a combination vehicle through a curve, what should you do about your speed?

  1. Accelerate slightly to pull the trailer through
  2. Brake firmly while in the middle of the curve
  3. Maintain highway speed because the trailer tracks the tractor
  4. Slow down before entering the curve Correct answer.

Explanation

Combination vehicles have a high center of gravity and can roll over if taken too fast. Slowing before the curve keeps forces low and lets you maintain control. Braking hard mid-curve can upset the trailer and cause a rollover.

Question 35Combination vehicle inspection and basic control skills

You must make a right turn in a combination vehicle. What is the safest technique to keep the trailer off the curb?

  1. Turn sharply at the last moment
  2. Swing wide to the left before turning right Correct answer.
  3. Turn from the far right lane without adjusting
  4. Keep the tractor close to the right curb throughout

Explanation

Because the trailer off-tracks to the inside of a turn, you need extra room on the right. Swinging wide to the left before turning right gives the trailer space to clear the curb. Turning too tightly causes the rear trailer wheels to ride over the curb or a pedestrian area.

Question 36Combination vehicle inspection and basic control skills

What is off-tracking in a combination vehicle?

  1. The tendency of the trailer to sway at high speed
  2. The delay in trailer brake application
  3. The trailer wheels following a tighter path than the tractor wheels in a turn Correct answer.
  4. The difference in tire pressure between axles

Explanation

Off-tracking happens because the rear wheels of a long combination cut the corner more sharply than the front wheels. The trailer's path is tighter than the tractor's, which is why turns require extra room. Drivers must account for this when turning and changing lanes.

Question 37Combination vehicle inspection and basic control skills

When changing lanes in a combination vehicle, why must you check your mirrors carefully and signal early?

  1. The steering wheel turns more slowly than in a car
  2. The trailer lights are dimmer than the tractor lights
  3. The trailer blocks part of your view and takes time to follow Correct answer.
  4. The trailer brakes respond faster than the tractor brakes

Explanation

The trailer creates large blind spots and needs more time and space to complete a lane change. Signaling early warns other drivers while you confirm the lane is clear in the mirrors. Cutting in too soon can clip a vehicle hidden in the blind spot.

Question 38Combination vehicle inspection and basic control skills

During a pre-trip inspection, you find the trailer's spring brake chambers will not release. What is the most likely cause?

  1. The trailer is overloaded
  2. The trailer lights are not connected
  3. The air pressure is too low to release the spring brakes Correct answer.
  4. The landing gear is still down

Explanation

Spring brakes apply when air pressure is lost and require adequate pressure to release. If they will not release, insufficient air supply or a restriction in the line is the likely cause. The system should be checked before the vehicle is moved.

Question 39Combination vehicle inspection and basic control skills

What does the trailer hand valve (trolley valve) do?

  1. It sets the tractor parking brake
  2. It applies the trailer brakes independently Correct answer.
  3. It releases the trailer spring brakes
  4. It increases engine braking

Explanation

The trailer hand valve lets the driver apply the trailer brakes alone, which is useful for testing them. It should not be used to stop a moving combination vehicle because it can cause a skid or jackknife. Its main safe use is checking trailer brake function.

Question 40Combination vehicle inspection and basic control skills

Why should you avoid using the trailer hand valve to stop a moving combination vehicle?

  1. It applies only the tractor brakes
  2. It can cause the trailer to skid and jackknife Correct answer.
  3. It disables the tractor's service brakes
  4. It drains the coolant system

Explanation

Applying only the trailer brakes while the tractor keeps rolling can make the trailer skid and swing out. This loss of traction can lead to a jackknife. The foot brake applies all brakes together and is the safe way to stop.

Question 41Combination vehicle inspection and basic control skills

What is the recommended way to test the trailer service brakes during a pre-trip inspection?

  1. Use the trailer hand valve while the vehicle is stopped Correct answer.
  2. Turn on the hazard lights and watch the dash
  3. Release the parking brake and roll forward
  4. Pump the clutch several times

Explanation

With the vehicle stopped and the wheels chocked or secured, applying the trailer hand valve lets you confirm the trailer brakes engage. You can watch for air pressure drop and listen for the brake chambers. This verifies the trailer service brakes function before driving.

Question 42Combination vehicle inspection and basic control skills

When driving a combination vehicle down a long steep grade, what is the best practice?

  1. Turn off the engine to coast
  2. Shift to neutral to save fuel
  3. Ride the brakes lightly the whole way
  4. Use a low gear and controlled braking Correct answer.

Explanation

Selecting a low gear lets engine braking help control speed on a downgrade. Applying the brakes in short, firm applications prevents overheating. Coasting or riding the brakes can lead to brake fade and loss of control.

Question 43Combination vehicle inspection and basic control skills

What should you check about the trailer's mud flaps and splash guards during a pre-trip inspection?

  1. That they are painted to match the tractor
  2. That they are secure and not damaged or missing Correct answer.
  3. That they touch the ground at all times
  4. That they are removed for better airflow

Explanation

Mud flaps and splash guards must be present, securely mounted, and in good condition to control spray and debris. Missing or damaged flaps can violate regulations and reduce safety for following traffic. They should hang clear of the tires, not drag on the ground.

Question 44Combination vehicle inspection and basic control skills

During a pre-trip inspection you notice a large crack in the trailer's frame rail near a suspension hanger. What should you do?

  1. Reduce the load and continue the trip
  2. Weld it yourself before departure
  3. Remove the vehicle from service until repaired Correct answer.
  4. Note it and drive if the trailer is empty

Explanation

A cracked frame rail weakens the trailer structure and can fail under load, causing a serious crash. Structural cracks require professional repair before the vehicle is used. The safe action is to place the vehicle out of service until it is fixed.

Question 45Combination vehicle inspection and basic control skills

What is the purpose of checking the trailer's sliding tandem locking pins before driving?

  1. To check the air pressure in the tires
  2. To confirm the axles are locked in position and will not slide Correct answer.
  3. To verify the trailer lights are grounded
  4. To measure the trailer's total weight

Explanation

Sliding tandem axles are held by locking pins that must be fully engaged in the frame. If a pin is not locked, the axles can shift while driving and cause loss of control. Confirming the pins are seated is a key pre-trip check.

Question 46Combination vehicle inspection and basic control skills

When coupling, what should you check about the gap between the tractor frame and the trailer apron?

  1. The gap should be measured with a tape for accuracy
  2. There should be a large gap for ventilation
  3. The gap should be small enough that the kingpin is fully seated Correct answer.
  4. The gap does not matter as long as the lights work

Explanation

After coupling, the trailer apron should sit close to the fifth wheel with little or no gap. A visible gap often means the kingpin is not fully seated in the jaws. Checking this gap helps confirm a secure connection before you drive.

Question 47Combination vehicle inspection and basic control skills

What is the recommended action if the trailer starts to sway while you are driving at highway speed?

  1. Apply the trailer hand valve firmly
  2. Steer sharply to correct the sway
  3. Ease off the accelerator and steer gently Correct answer.
  4. Accelerate hard to pull it straight

Explanation

Sway is best corrected by gradually reducing speed and making small, smooth steering inputs. Hard acceleration, sharp steering, or applying only the trailer brakes can worsen the sway. Controlled deceleration lets the trailer settle.

Question 48Combination vehicle inspection and basic control skills

During a pre-trip inspection, what should you check about the fifth wheel mounting to the tractor frame?

  1. That the mounting bolts are tight and the plate is secure Correct answer.
  2. That the plate is painted a bright color
  3. That the plate can move freely side to side
  4. That the plate is greased on top only

Explanation

The fifth wheel must be firmly bolted to the tractor frame so it can carry the trailer load safely. Loose or missing mounting bolts can let the fifth wheel shift or fail. This is a critical structural check during pre-trip inspection.

Question 49Combination vehicle inspection and basic control skills

Why is it important to check the trailer's electrical cord and connector during a pre-trip inspection?

  1. It carries signals for lights and antilock brakes Correct answer.
  2. It holds the trailer to the tractor
  3. It measures trailer weight
  4. It powers the trailer's air compressor

Explanation

The electrical cord carries power and signals for trailer lights and the antilock brake system. A damaged cord or loose connector can cause lights to fail or ABS to malfunction. Inspecting it helps ensure other drivers can see your signals and brakes work properly.

Question 50Combination vehicle inspection and basic control skills

You are backing a tractor-trailer into a tight dock and the trailer begins to turn too sharply toward a jackknife. What should you do?

  1. Continue backing and steer harder to correct
  2. Stop, pull forward to straighten, and restart the maneuver Correct answer.
  3. Apply the trailer hand valve to pivot the trailer
  4. Accelerate to push through the turn

Explanation

When the angle between tractor and trailer gets too sharp, continuing to back makes a jackknife more likely. Stopping and pulling forward straightens the combination so you can realign. Restarting with a better setup reduces the risk of damage or a jackknife.