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MN CDL Combination practice questions

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Question 1Combination vehicle inspection and coupling/uncoupling

When performing a pre-trip inspection on a combination vehicle, what is the correct order for checking the coupling system after the trailer is already connected?

  1. Check the fifth wheel jaws, then the kingpin, then the safety latch
  2. Inspect the air lines, then the electrical cord, then the fifth wheel
  3. Confirm the locking jaws are closed, then tug-test, then check the safety latch Correct answer.
  4. Verify the landing gear is raised, then check the coupling, then the lights

Explanation

After coupling, the driver must confirm the locking jaws have closed around the kingpin, perform a pull test (tug test) with the tractor to make sure the trailer will not separate, and then secure the safety latch. These three steps verify the coupling is locked before any road travel.

Question 2Combination vehicle inspection and coupling/uncoupling

What is the purpose of the tug test when coupling a tractor to a semi-trailer?

  1. To verify the trailer brakes release when the supply line is connected
  2. To seat the fifth wheel plate against the trailer apron
  3. To confirm the locking jaws have fully closed on the kingpin Correct answer.
  4. To stretch the air lines so they reach the trailer gladhands

Explanation

The tug test is performed by pulling forward gently with the trailer brakes set; if the jaws are not locked around the kingpin, the trailer will not move with the tractor and the coupling is faulty. It is the practical confirmation that the locking mechanism engaged.

Question 3Combination vehicle inspection and coupling/uncoupling

Before backing under a trailer to couple, the trailer height should be adjusted so that:

  1. The trailer apron is slightly lower than the fifth wheel plate
  2. The trailer kingpin is at the same height as the fifth wheel opening Correct answer.
  3. The landing gear is fully retracted to clear the frame
  4. The trailer is raised so the kingpin sits above the fifth wheel surface

Explanation

The trailer should be positioned so the kingpin will strike the fifth wheel at the correct height, roughly level with the fifth wheel opening. Too low and the tractor may strike the trailer; too high and the kingpin can ride over the fifth wheel instead of locking.

Question 4Combination vehicle inspection and coupling/uncoupling

After coupling and locking the fifth wheel, why must the landing gear be fully raised before moving?

  1. To keep the trailer level while driving
  2. To allow the trailer brakes to release automatically
  3. To prevent the landing gear from striking the ground or obstacles Correct answer.
  4. So the trailer air lines reach the gladhands

Explanation

Landing gear left down can catch on railroad tracks, pavement, or other obstacles and cause serious damage or a crash. It must be cranked fully up and the handle secured after the tractor supports the trailer weight.

Question 5Combination vehicle inspection and coupling/uncoupling

When uncoupling a trailer, which step should be performed first?

  1. Disconnect the air lines and electrical cord
  2. Pull the fifth wheel release handle
  3. Set the trailer parking brakes Correct answer.
  4. Lower the landing gear to the ground

Explanation

The trailer parking brakes must be set first so the trailer cannot roll once the tractor is removed. Only after the brakes are set should the landing gear be lowered, lines disconnected, and the fifth wheel released.

Question 6Combination vehicle inspection and coupling/uncoupling

What does the trailer hand valve (johnson bar) control on a combination vehicle?

  1. The tractor service brakes only
  2. The trailer spring brakes
  3. The trailer service brakes only Correct answer.
  4. The tractor parking brakes

Explanation

The trailer hand valve applies only the trailer service brakes and is used mainly to test the trailer brakes. It does not apply the tractor brakes or the spring (parking) brakes.

Question 7Combination vehicle inspection and coupling/uncoupling

A driver is checking the air line connections after coupling. Which gladhand color connects to the trailer emergency (supply) line?

  1. Black
  2. Red Correct answer.
  3. Blue
  4. Green

Explanation

The emergency (supply) line is red and the service line is blue. Correct color matching helps prevent cross-connection, which would cause brake problems.

Question 8Combination vehicle inspection and coupling/uncoupling

During a pre-trip inspection, the driver finds the trailer's spring brakes will not release after the supply line is connected. What is the most likely cause?

  1. The service line is connected to the emergency gladhand
  2. The trailer reservoir is fully charged
  3. The tractor protection valve is open
  4. The trailer parking brake control is still engaged Correct answer.

Explanation

If the trailer parking brake control (or tractor parking brake valve) is still set, air may not reach the trailer spring brake chambers to release them. The driver should confirm the parking brake valves are pushed in and the system is charged.

Question 9Combination vehicle inspection and coupling/uncoupling

What is the correct position of the tractor protection valve during normal driving?

  1. Open, so air can flow to the trailer Correct answer.
  2. Closed, to isolate the trailer
  3. Partially open, to limit trailer pressure
  4. It has no effect once the lines are connected

Explanation

The tractor protection valve must be open during normal operation so the trailer receives air for its brakes and reservoir. It closes automatically if trailer air pressure drops, protecting the tractor's air supply.

Question 10Combination vehicle inspection and coupling/uncoupling

When sliding the fifth wheel to adjust weight distribution, what must be done before the tractor is moved?

  1. The air lines must be disconnected
  2. The fifth wheel locking pins must be fully engaged Correct answer.
  3. The landing gear must be lowered
  4. The trailer brakes must be released

Explanation

After repositioning the fifth wheel, the locking pins must be fully seated to hold the fifth wheel in place. Moving with the pins not engaged can let the fifth wheel slide and cause loss of control.

Question 11Combination vehicle inspection and coupling/uncoupling

A driver is coupling to a trailer and notices the kingpin is visibly worn and the jaws do not close tightly. What should the driver do?

  1. Place the trailer out of service until repaired Correct answer.
  2. Couple anyway and monitor for looseness
  3. Add a shim to tighten the jaws
  4. Reduce speed to compensate for the play

Explanation

A worn kingpin or jaws that do not close securely means the coupling cannot be trusted, and the vehicle must not be operated until repaired. Loose coupling can allow the trailer to separate from the tractor.

Question 12Combination vehicle inspection and coupling/uncoupling

Which of the following best describes the correct procedure for the visual check of the fifth wheel after coupling?

  1. Check that the trailer apron rests on the tractor frame
  2. Confirm the air lines hang free below the fifth wheel
  3. Look under the trailer to confirm the jaws are closed around the kingpin and the release handle is in Correct answer.
  4. Verify the landing gear crank is stored horizontally

Explanation

The visual check requires getting under the trailer to see that the locking jaws are closed around the kingpin and the release handle is in the locked position. This confirms the mechanical lock before the tug test.

Question 13Combination vehicle inspection and coupling/uncoupling

When uncoupling on a level surface, the landing gear should be lowered until:

  1. The trailer apron clears the tractor frame
  2. The trailer is lifted off the fifth wheel
  3. The landing gear touches the ground and the trailer is supported Correct answer.
  4. The crank becomes hard to turn

Explanation

The landing gear must contact the ground and support the trailer's weight before the tractor is pulled out. This prevents the trailer from dropping when the fifth wheel is released.

Question 14Combination vehicle inspection and coupling/uncoupling

A combination vehicle's trailer has a front axle and rear tandem axles. When making a pre-trip inspection, the driver should check the trailer brakes at:

  1. Each wheel position on the trailer Correct answer.
  2. Only the front axle
  3. Only the rear tandem
  4. Only the wheels with visible slack adjusters

Explanation

Every wheel position on the trailer must be checked for brake condition, including drums, linings, and slack adjusters. Skipping any wheel leaves an uninspected brake that could fail.

Question 15Combination vehicle inspection and coupling/uncoupling

What is the primary reason for chocking the trailer wheels before uncoupling on a slope?

  1. To keep the trailer from rolling if the parking brakes fail Correct answer.
  2. To keep the landing gear from sinking into soft ground
  3. To prevent the air lines from dragging
  4. To help align the kingpin for recoupling

Explanation

Wheel chocks provide a physical backup in case the trailer parking brakes do not hold, especially on a grade. They prevent the trailer from rolling away during uncoupling.

Question 16Combination vehicle inspection and coupling/uncoupling

During a pre-trip inspection, the driver notices the trailer's electrical cord is chafed and the insulation is worn through. What is the correct action?

  1. Tape the cord and continue the trip
  2. Replace or repair the cord before operating Correct answer.
  3. Route the cord inside the cab
  4. Connect only the good side of the plug

Explanation

A chafed cord with exposed insulation can cause short circuits and loss of trailer lights, so it must be repaired or replaced before the vehicle is operated. Taping over damaged insulation is not an adequate repair.

Question 17Combination vehicle inspection and coupling/uncoupling

When testing the trailer brakes with the hand valve while parked, what should the driver observe?

  1. The tractor brakes should lock
  2. The trailer brakes should apply and hold the trailer Correct answer.
  3. The trailer should roll forward slightly
  4. The air pressure should drop to zero

Explanation

Applying the trailer hand valve should engage the trailer service brakes and hold the trailer. This confirms the trailer brake system is receiving air and functioning.

Question 18Combination vehicle inspection and coupling/uncoupling

What is the function of the trailer supply (emergency) line in a combination vehicle?

  1. It applies the trailer service brakes when the foot pedal is pressed
  2. It controls the tractor's parking brake
  3. It charges the trailer air tanks and releases the spring brakes Correct answer.
  4. It carries electrical signals to the trailer lights

Explanation

The emergency or supply line delivers air to charge the trailer reservoir and holds the spring brakes off. If it loses pressure, the trailer spring brakes apply automatically.

Question 19Combination vehicle inspection and coupling/uncoupling

A driver is preparing to couple at night and cannot clearly see the fifth wheel jaws. What is the safest course of action?

  1. Rely on the tug test alone and skip the visual check
  2. Assume the jaws locked if the handle moved
  3. Use a flashlight to visually confirm the jaws are locked Correct answer.
  4. Ask a passenger to listen for the click

Explanation

Adequate lighting is required to see that the jaws are closed around the kingpin and the release handle is in. Without a visual confirmation, the driver cannot be sure the coupling is secure.

Question 20Combination vehicle inspection and coupling/uncoupling

When uncoupling, in what order should the air lines be disconnected?

  1. Emergency line first, then service line Correct answer.
  2. Any order, since the brakes are already set
  3. Both at the same time
  4. Service line first, then emergency line

Explanation

Disconnecting the emergency (supply) line first causes the trailer spring brakes to apply, so the trailer stays put while the service line is removed. This order adds a safety margin during uncoupling.

Question 21Combination vehicle inspection and coupling/uncoupling

What does the red knob on the dash control in a combination vehicle?

  1. The trailer hand valve
  2. The tractor parking brakes and trailer emergency air Correct answer.
  3. The engine brake
  4. The trailer service brakes

Explanation

The red (trailer air supply) knob controls air to the trailer emergency line and also relates to the tractor parking brake system. Pulling it out closes the supply and applies the trailer spring brakes.

Question 22Combination vehicle inspection and coupling/uncoupling

During a pre-trip inspection, the driver finds the fifth wheel mounting bolts are loose. What is the correct action?

  1. Drive at reduced speed to the next stop
  2. Ignore them if the fifth wheel still locks
  3. Place the vehicle out of service until repaired Correct answer.
  4. Tighten them later at the terminal

Explanation

Loose fifth wheel mounting bolts can allow the fifth wheel to shift or separate from the frame, which is a serious safety defect. The vehicle must not be operated until the mounting is repaired and torqued to specification.

Question 23Combination vehicle inspection and coupling/uncoupling

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

  1. To secure the electrical cord
  2. To keep the release handle from moving out of the locked position Correct answer.
  3. To hold the air lines in place
  4. To lock the landing gear crank

Explanation

The safety latch secures the release handle so vibration or road shock cannot move it and accidentally release the locking jaws. It is a secondary lock that must be engaged after coupling.

Question 24Combination vehicle inspection and coupling/uncoupling

A driver couples to a loaded trailer and the tractor appears to squat excessively at the rear after the trailer weight is transferred. What should the driver check?

  1. The tractor suspension and fifth wheel height adjustment Correct answer.
  2. The trailer hand valve setting
  3. The electrical cord routing
  4. The trailer light circuit

Explanation

Excessive squatting after weight transfer suggests the tractor suspension is overloaded or the fifth wheel height is incorrect. The driver should verify the suspension and fifth wheel adjustment before operating.

Question 25Combination vehicle inspection and coupling/uncoupling

When performing the pre-trip inspection, which item confirms the trailer's spring brakes will apply if the trailer breaks away from the tractor?

  1. The landing gear crank
  2. The service line gladhand seal
  3. The emergency line and its automatic application Correct answer.
  4. The electrical cord connection

Explanation

The emergency (supply) line is designed so that if it separates, air pressure is lost and the trailer spring brakes apply automatically. This breakaway protection is a key safety feature to verify.

Question 26Combination vehicle inspection and coupling/uncoupling

Before backing under a trailer to couple, what should the trailer height be set to relative to the fifth wheel plate?

  1. Slightly lower than the fifth wheel plate so the trailer lifts it
  2. As high as the landing gear will extend
  3. Level with the top of the tractor frame rails regardless of the fifth wheel
  4. High enough that the kingpin clears the fifth wheel plate but the trailer apron touches the plate Correct answer.

Explanation

The trailer should sit so the kingpin passes over the fifth wheel plate and the trailer apron contacts the plate, allowing the plate to lift the trailer slightly as the tractor backs under. Too low, and the kingpin strikes the plate; too high, and the tractor may pass under without latching.

Question 27Combination vehicle inspection and coupling/uncoupling

After the fifth wheel jaws lock on the kingpin, what is the correct next step in the coupling sequence?

  1. Connect the air lines and electrical cord
  2. Tug-test the tractor gently forward Correct answer.
  3. Release the trailer spring brakes
  4. Raise the landing gear

Explanation

A tug test with a slight forward pull confirms the jaws are actually closed around the kingpin before anything else is done. Only after the coupling holds should the driver connect lines, raise the gear, and release brakes.

Question 28Combination vehicle inspection and coupling/uncoupling

What is the purpose of the trailer air supply (red) knob being pushed in during normal driving?

  1. It vents trailer air to atmosphere to test the relay valve
  2. It charges the trailer tanks and keeps the trailer spring brakes released Correct answer.
  3. It supplies air only to the trailer service brakes
  4. It locks the fifth wheel jaws to prevent accidental release

Explanation

With the red knob pushed in, supply air flows through the emergency line to charge the trailer tanks and hold off the trailer spring brakes. Pulling it out cuts supply, which applies the trailer spring brakes.

Question 29Combination vehicle inspection and coupling/uncoupling

When uncoupling a trailer, why should the tractor be pulled forward slightly after lowering the landing gear but before releasing the fifth wheel handle?

  1. To check that the trailer brakes hold on the grade
  2. To align the kingpin with the jaws for the next coupling
  3. To stretch the air lines so they disconnect cleanly
  4. To relieve pressure on the fifth wheel jaws so the release handle moves easily Correct answer.

Explanation

Pulling forward a small amount takes the load off the jaws, letting the release handle pull out without forcing it. Forcing the handle while the tractor is still pressed against the kingpin can damage the latch.

Question 30Combination vehicle inspection and coupling/uncoupling

What color is the gladhand for the trailer service brake line on a standard combination vehicle?

  1. Blue Correct answer.
  2. Red
  3. Green
  4. Black

Explanation

The service brake line uses blue gladhands, while the emergency/supply line uses red. Matching colors helps prevent cross-connection when hooking up.

Question 31Combination vehicle inspection and coupling/uncoupling

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

  1. That the plate is greased on both the top and bottom surfaces
  2. That the release handle is wired in the open position
  3. That the mounting bolts are tight and the plate is not cracked or bent Correct answer.
  4. That the jaws are packed with grease

Explanation

The fifth wheel must be securely bolted to the frame with no cracks or deformation, since a loose or damaged mount can let the trailer separate. Grease belongs on the top surface and jaw area, not the mounting hardware.

Question 32Combination vehicle inspection and coupling/uncoupling

What is the correct way to check that the trailer spring brakes apply when supply air is lost?

  1. Push in the red knob and check that the trailer rolls freely
  2. Open the trailer tank drain and watch the gauge drop to zero
  3. Shut off the engine and watch the trailer wheels for movement
  4. Pull the red knob out with the engine running and listen for the relay valve to open Correct answer.

Explanation

Pulling the red supply knob out simulates loss of supply air, and the trailer spring brakes should set with an audible exhaust. This confirms the emergency braking function works before driving.

Question 33Combination vehicle inspection and coupling/uncoupling

When sliding the trailer tandems to adjust weight distribution, what should be done with the trailer brakes and wheels first?

  1. Disconnect the electrical cord to prevent shorts
  2. Set the trailer parking brakes and chock the wheels Correct answer.
  3. Release the trailer spring brakes so the axles roll freely
  4. Lower the landing gear to take weight off the tandems

Explanation

The trailer must be secured with its parking brakes set and wheels chocked before the tandem locking pins are released. Otherwise the trailer can roll when the pins disengage.

Question 34Combination vehicle inspection and coupling/uncoupling

After coupling and connecting the air lines, the driver hears a continuous leak at the gladhand. What is the most likely cause?

  1. The gladhand coupler is not fully seated or the seal is damaged Correct answer.
  2. The trailer spring brakes are released
  3. The trailer tank is overcharged
  4. The tractor protection valve is in the closed position

Explanation

A steady leak right at the gladhand connection points to a coupler that is not locked on fully or a worn or missing rubber seal. Reseating the couplers or replacing the seal corrects it.

Question 35Combination vehicle inspection and coupling/uncoupling

A driver backs under a trailer, hears the jaws close, and sees the release handle snap into place. Why is a visual check underneath still required?

  1. To confirm the jaws are closed around the kingpin and the latch is over the pin Correct answer.
  2. To check that the air lines are not crossed
  3. To verify the trailer lights are working
  4. To make sure the landing gear is fully raised

Explanation

Sound alone does not prove the jaws closed around the kingpin rather than in front of it. A physical look underneath confirms the jaws are wrapped around the pin and the safety latch is engaged.

Question 36Combination vehicle inspection and coupling/uncoupling

What does the tractor protection valve do when air pressure in the tractor system drops below about 20 to 45 psi?

  1. It increases trailer supply pressure to maintain braking
  2. It opens the service line to vent excess pressure
  3. It closes the trailer supply line and applies the trailer spring brakes Correct answer.
  4. It bypasses the trailer relay valve to speed release

Explanation

The tractor protection valve closes off the trailer supply line when tractor pressure falls too low, which sets the trailer spring brakes and preserves remaining tractor air for its own brakes. This prevents a tractor leak from draining the trailer system.

Question 37Combination vehicle inspection and coupling/uncoupling

A driver is coupling in cold weather and finds the trailer gladhand seal stiff and cracked. What should be done?

  1. Leave the line disconnected until the engine warms up
  2. Coat the seal with grease and connect it anyway
  3. Connect the lines and accept a small leak
  4. Replace the seal before connecting the lines Correct answer.

Explanation

A cracked or hardened seal will leak and can fail completely, so it must be replaced before the lines are connected. Greasing a damaged seal does not restore it and can contaminate the air system.

Question 38Combination vehicle inspection and coupling/uncoupling

When uncoupling, in what position should the trailer spring brakes be before the air lines are disconnected?

  1. Released only if the ground is level
  2. Applied only if the landing gear is raised
  3. Released, so the trailer can be repositioned
  4. Applied, so the trailer stays parked when supply air is removed Correct answer.

Explanation

The trailer parking brakes should be set so the trailer stays put once the supply line is disconnected and the spring brakes take over. Disconnecting lines from an unsecured trailer can let it roll.

Question 39Combination vehicle inspection and coupling/uncoupling

What is the purpose of the trailer relay valve in a combination vehicle?

  1. To switch the trailer lights between running and brake modes
  2. To apply trailer brakes quickly using air stored at the trailer Correct answer.
  3. To release the fifth wheel jaws when supply air is lost
  4. To limit tractor air pressure to the trailer tanks

Explanation

The relay valve uses a small control signal from the service line to send trailer tank air to the brake chambers, speeding up brake application and release on a long trailer. It is a pneumatic relay, not a pressure limiter or latch control.

Question 40Combination vehicle inspection and coupling/uncoupling

After sliding the fifth wheel on the tractor to shift weight, what must be verified before driving?

  1. That the landing gear is still on the ground
  2. That the trailer apron clears the tractor frame
  3. That the air lines reach without tension
  4. That the locking pins are fully seated and the plate is secured Correct answer.

Explanation

The fifth wheel locking pins must be fully engaged in the slide track so the plate cannot shift under load. Driving with partially seated pins can let the fifth wheel move and cause loss of control.

Question 41Combination vehicle inspection and coupling/uncoupling

A driver connects the electrical cord before the air lines during coupling. What problem can this cause?

  1. The cord can be pinched or damaged as the tractor settles when air bags inflate Correct answer.
  2. The ABS system will not initialize
  3. The trailer lights may burn out from voltage spikes
  4. The gladhands will not seal properly

Explanation

Connecting the cord first leaves it vulnerable to being crushed or stretched when the tractor suspension settles as trailer weight comes onto the fifth wheel. Air lines are normally connected first, then the electrical cord, with slack checked.

Question 42Combination vehicle inspection and coupling/uncoupling

During a pre-trip inspection, the driver finds the trailer apron bent so the kingpin area is distorted. What is the correct action?

  1. Couple normally and monitor for vibration
  2. Raise the trailer higher to clear the bent area
  3. Report the damage and do not use the trailer until repaired Correct answer.
  4. Straighten the apron with a hammer and couple

Explanation

A bent apron can prevent the fifth wheel from seating properly and may indicate kingpin or structural damage, so the trailer must be taken out of service until repaired. Coupling to a distorted trailer risks an incomplete latch and separation.

Question 43Combination vehicle inspection and coupling/uncoupling

What should a driver check about the trailer landing gear during a pre-trip inspection?

  1. That the gear is fully extended and touching the ground
  2. That the gear is greased on the outside of the legs
  3. That the crank turns freely, the legs are not bent, and the sand shoes are present
  4. That the gear is locked in the raised position with the handle secured Correct answer.

Explanation

In normal operation the landing gear is fully retracted and the crank handle is secured so it cannot swing out and strike the ground or a person. Bent legs, a stiff crank, and missing sand shoes are defects to note when the gear is used.

Question 44Combination vehicle inspection and coupling/uncoupling

A driver pulls the red supply knob out and the trailer spring brakes do not set. What is the most likely explanation?

  1. The service line gladhand is cross-connected
  2. The trailer relay valve is stuck open
  3. The tractor governor is set too high
  4. The trailer supply line is blocked or the spring brakes are mechanically released (caged) Correct answer.

Explanation

If pulling the red knob does not set the trailer spring brakes, either supply air is not reaching the trailer or the springs have been manually caged. Both conditions mean the trailer has no emergency braking and must not be driven.

Question 45Combination vehicle inspection and coupling/uncoupling

What is the proper way to secure the trailer when uncoupling on a slight grade?

  1. Leave the tractor coupled until the landing gear is fully raised
  2. Rely on the kingpin to hold the trailer in place
  3. Set the trailer brakes only, since spring brakes hold on any grade
  4. Chock the wheels in addition to setting the trailer brakes Correct answer.

Explanation

On a grade, chocks supplement the trailer parking brakes in case they slip or fail, and they hold the trailer while the tractor is pulled away. Relying on brakes alone on a slope is not sufficient.

Question 46Combination vehicle inspection and coupling/uncoupling

A driver has coupled, raised the landing gear, and is about to drive off when the trailer low-air warning sounds. What should be done?

  1. Pull the red knob out and continue
  2. Drive slowly to build trailer pressure
  3. Stop, check the supply line connection and trailer tank pressure, and correct the problem before moving Correct answer.
  4. Release the tractor parking brake and let the system catch up

Explanation

A low-air warning means the trailer is not being charged properly, possibly from a disconnected or leaking supply line, and the spring brakes may apply unexpectedly. The driver must stop and correct the cause before moving.

Question 47Combination vehicle inspection and coupling/uncoupling

During coupling, the driver notices the tractor is not aligned squarely with the trailer and the kingpin approaches the edge of the jaws. What is the best action?

  1. Lower the trailer onto the fifth wheel to force alignment
  2. Have a spotter push the trailer into line
  3. Stop, pull forward, and realign the tractor with the trailer centerline before backing again Correct answer.
  4. Continue backing slowly so the jaws guide the kingpin in

Explanation

Backing in at an angle can bend the jaws or let the kingpin miss the latch entirely, so the driver should pull forward and realign squarely. The jaws are not designed to guide an off-center kingpin into place.

Question 48Combination vehicle inspection and coupling/uncoupling

A driver finds that after coupling, the trailer sits noticeably nose-high and the tractor rear axle is overloaded. Which adjustment addresses both issues?

  1. Lower the landing gear and recheck the height
  2. Increase tractor tire pressure to raise the load rating
  3. Slide the fifth wheel forward to move weight off the tractor drive axles
  4. Slide the trailer tandems rearward to shift weight to the trailer axles Correct answer.

Explanation

Moving the trailer tandems rearward shifts weight from the tractor drive axles onto the trailer axles and lowers the trailer nose. This is the standard weight-distribution adjustment for an overloaded tractor rear axle.

Question 49Combination vehicle inspection and coupling/uncoupling

After a complete coupling, the driver does a tug test and the trailer moves with the tractor but the release handle is loose and will not stay in the locked position. What does this indicate?

  1. The latch is not fully engaged and the coupling must be corrected before driving Correct answer.
  2. The fifth wheel needs more grease on the jaws
  3. The trailer spring brakes are released
  4. Normal condition; the handle always has play

Explanation

A release handle that will not stay locked indicates the latch mechanism is not fully engaged, even if the tug test seemed to hold. The driver must not drive until the latch is properly secured, since the jaws could open under load.

Question 50Combination vehicle inspection and coupling/uncoupling

A driver is preparing to uncouple and the trailer is not equipped with spring brakes. What precaution becomes especially important?

  1. Pulling the red knob out before setting the trailer brakes
  2. Raising the landing gear before releasing the fifth wheel
  3. Connecting the electrical cord last
  4. Chocking the wheels before disconnecting the supply line Correct answer.

Explanation

Without spring brakes, the trailer has no automatic parking brake when supply air is lost, so chocks are the primary means of holding it. The wheels must be chocked before the supply line is disconnected.