I85PREP
⌘ K
Alabama CDL Combination Vehicles study materials
Alabama CDL Combination Vehicles study resources

Alabama CDL Combination Vehicles practice questions

One question at a time. Build your Alabama CDL Combination Vehicles understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Driving Combination Vehicles Safely

When you couple a tractor to a semitrailer, the fifth wheel should be checked to confirm that it is:

  1. slightly tilted down toward the rear Correct answer.
  2. coated with fresh grease on the jaws
  3. locked in the fully forward position
  4. set at the highest mounting height

Explanation

The fifth wheel should be tilted down toward the rear so the trailer kingpin slides under it and the jaws can close around the pin. A level or upward-tilted plate can cause the pin to strike the plate instead of the jaws.

Question 2Driving Combination Vehicles Safely

Before backing under a trailer, you should check that the trailer's landing gear is:

  1. firmly on the ground and the trailer is at the right height Correct answer.
  2. lowered so the trailer rests on it
  3. cranked fully to the top stop
  4. raised enough to clear the tractor frame

Explanation

The trailer should be at a height where the fifth wheel will slide under the trailer apron and contact the kingpin properly, with the landing gear supporting the trailer on solid ground. Cranking the gear fully up or down misaligns the coupling height.

Question 3Driving Combination Vehicles Safely

After coupling, you should tug-test the connection by:

  1. pulling forward gently with the trailer brakes released
  2. backing up sharply against the kingpin
  3. pulling forward gently with the trailer brakes locked Correct answer.
  4. releasing the tractor parking brake only

Explanation

With the trailer brakes set, pull forward gently against them. If the coupling is secure, the tractor will not move; if the jaws are not locked, the trailer will not hold and you will feel the tractor pull free.

Question 4Driving Combination Vehicles Safely

You have coupled to a trailer and are about to connect the air lines. The trailer has a spring brake system. The emergency (supply) line should be connected:

  1. only after the trailer brakes are released
  2. last, after the service line
  3. to either gladhand, since they are interchangeable
  4. first, before the service line Correct answer.

Explanation

On trailers with spring brakes, connect the emergency line first so the trailer's spring brakes are charged and released, then connect the service line. Connecting in the wrong order can leave the trailer brakes locked or cause the tractor to be pushed.

Question 5Driving Combination Vehicles Safely

When you uncouple a trailer, you should first:

  1. shut off the engine and drain all air pressure
  2. pull the tractor forward to clear the kingpin
  3. lower the landing gear to the ground
  4. close the trailer air supply valve and set the trailer parking brakes Correct answer.

Explanation

Before uncoupling, close the trailer air supply valve and set the trailer parking brakes so the trailer stays put when the tractor is pulled away. Then lower the landing gear and disconnect the lines.

Question 6Driving Combination Vehicles Safely

While driving a combination vehicle, you should check your mirrors regularly to:

  1. confirm the trailer is tracking properly behind you Correct answer.
  2. verify the trailer lights are turned off
  3. watch for the trailer brakes to release
  4. check the air pressure gauge on the trailer

Explanation

Regular mirror checks let you see whether the trailer is following in line and whether anything is wrong, such as a sway or a tire problem. A trailer that is not tracking properly is an early warning of trouble.

Question 7Driving Combination Vehicles Safely

The most common cause of a jackknife in a combination vehicle is:

  1. driving too slowly on a downgrade
  2. using the trailer hand valve while driving
  3. over-braking or locking the drive wheels Correct answer.
  4. shifting to a lower gear too early

Explanation

Jackknifing usually happens when the drive wheels lock and lose traction, letting the trailer swing around the tractor. Smooth braking and avoiding wheel lock are the primary defenses.

Question 8Driving Combination Vehicles Safely

To prevent a trailer from swinging out during a hard stop, you should:

  1. brake hard and steer sharply
  2. release the brakes and steer to regain control
  3. apply the trailer hand valve firmly
  4. brake smoothly and avoid locking the wheels Correct answer.

Explanation

Smooth, controlled braking keeps the wheels rolling and preserves traction, which helps the trailer stay behind the tractor. Locked wheels are what allow the trailer to swing out.

Question 9Driving Combination Vehicles Safely

When making a right turn in a combination vehicle, you should:

  1. swing wide to the left before turning
  2. turn sharply and accelerate through the turn
  3. turn from the left lane
  4. keep the rear wheels close to the curb Correct answer.

Explanation

A combination vehicle off-tracks, so the rear wheels cut inside the path of the tractor. Keeping the rear wheels close to the curb prevents the trailer from running over the curb or striking objects.

Question 10Driving Combination Vehicles Safely

The main reason a combination vehicle needs more stopping distance than a single vehicle is that it:

  1. has a higher center of gravity
  2. has a longer total wheelbase
  3. uses smaller brake chambers
  4. carries more weight and has less braking efficiency Correct answer.

Explanation

A loaded combination is heavier and its brakes must stop far more mass, so stopping distance grows. Weight and brake efficiency, not wheelbase alone, are the key factors.

Question 11Driving Combination Vehicles Safely

When driving a combination vehicle on a curve, you should:

  1. slow down before the curve and maintain a steady speed Correct answer.
  2. coast with the clutch disengaged
  3. accelerate through the curve to stabilize the trailer
  4. brake hard while in the curve

Explanation

Speed should be reduced before entering a curve, then held steady through it. Braking or accelerating hard in a curve can cause the trailer to skid or roll.

Question 12Driving Combination Vehicles Safely

The trailer hand valve should be used:

  1. whenever you need to stop quickly
  2. to hold the vehicle at a stoplight
  3. to slow down on long downgrades
  4. only to test the trailer brakes Correct answer.

Explanation

The trailer hand valve is intended only for testing the trailer brakes. Using it for normal stopping or parking can cause the trailer to swing or the brakes to overheat.

Question 13Driving Combination Vehicles Safely

If your combination vehicle starts to skid, the first thing you should do is:

  1. release the brakes to get the wheels rolling again Correct answer.
  2. accelerate to pull out of the skid
  3. apply the brakes harder
  4. steer sharply in the direction of the skid

Explanation

Releasing the brakes lets the wheels roll again and regain traction, which is the first step in recovering from a skid. Steering and throttle inputs should follow only after traction returns.

Question 14Driving Combination Vehicles Safely

When backing a combination vehicle, you should:

  1. rely only on the mirrors
  2. use a helper or get out and check behind the trailer Correct answer.
  3. back with the trailer brakes partially applied
  4. back up quickly to keep the trailer straight

Explanation

Backing a combination vehicle has large blind spots, so a helper or a walk-around check behind the trailer is the safest approach. Mirrors alone do not show everything.

Question 15Driving Combination Vehicles Safely

The air line that supplies air to the trailer's spring brakes and charges the trailer system is the:

  1. service line
  2. gladhand seal
  3. emergency line Correct answer.
  4. electrical cord

Explanation

The emergency (supply) line charges the trailer air tanks and releases the spring brakes. The service line carries the signal that applies the trailer brakes.

Question 16Driving Combination Vehicles Safely

You are driving a combination vehicle and the trailer begins to sway. You should:

  1. ease off the accelerator and steer straight Correct answer.
  2. accelerate to pull the trailer straight
  3. steer sharply to correct the sway
  4. brake hard immediately

Explanation

Easing off the accelerator and holding the steering straight lets the vehicle slow and the sway settle. Hard braking or sharp steering can make the sway worse.

Question 17Driving Combination Vehicles Safely

When a combination vehicle's trailer is empty, the risk of a jackknife during braking is:

  1. greater because the trailer wheels can lock easily Correct answer.
  2. lower because the trailer is lighter
  3. eliminated by the trailer's spring brakes
  4. the same as when loaded

Explanation

An empty trailer has less weight on its wheels, so the trailer brakes can lock the tires easily and the trailer can swing. Lightly loaded brakes need to be used gently.

Question 18Driving Combination Vehicles Safely

The proper way to check that the trailer brakes are working after coupling is to:

  1. listen for air leaks at the gladhands
  2. release the trailer brakes and roll forward
  3. pull the tractor forward with the trailer brakes set Correct answer.
  4. apply the trailer hand valve while moving

Explanation

With the trailer brakes set, pulling forward tests whether they hold. If the trailer rolls, the brakes are not working and must be checked before driving.

Question 19Driving Combination Vehicles Safely

A combination vehicle's rearward amplification means that during a quick lane change:

  1. the trailer brakes apply sooner
  2. the tractor turns more sharply than the trailer
  3. the trailer can swing wider than the tractor Correct answer.
  4. the steering becomes lighter

Explanation

Rearward amplification causes the rear of the combination to swing farther than the tractor during sudden maneuvers, increasing rollover risk. Smooth, gradual lane changes reduce this effect.

Question 20Driving Combination Vehicles Safely

Before driving a combination vehicle, you should check that the coupling is secure by:

  1. looking at the fifth wheel from the cab
  2. confirming the trailer lights work
  3. checking that the air lines are connected
  4. verifying the locking lever is in the locked position and the pin is seated Correct answer.

Explanation

The locking lever should be in the locked position and a visual check should confirm the kingpin is seated in the jaws. This is the direct check that the coupling is secure.

Question 21Driving Combination Vehicles Safely

When descending a long, steep grade with a combination vehicle, you should:

  1. use a low gear and the service brakes lightly Correct answer.
  2. ride the brakes continuously
  3. use the trailer hand valve to control speed
  4. coast in neutral to save fuel

Explanation

A low gear uses engine braking to hold speed and light service-brake use prevents overheating. Coasting or riding the brakes can lead to brake fade and loss of control.

Question 22Driving Combination Vehicles Safely

A combination vehicle is most likely to roll over:

  1. when the trailer is empty
  2. when the brakes are applied gently
  3. when turning at low speed
  4. during a sudden maneuver at high speed Correct answer.

Explanation

High speed combined with a sudden maneuver produces strong lateral forces that can tip a combination vehicle. Reducing speed before maneuvers is the key defense.

Question 23Driving Combination Vehicles Safely

You are driving a combination vehicle and the trailer starts to pass you on the left during a stop. This is a sign that:

  1. the tractor is accelerating
  2. the trailer is lighter than the tractor
  3. the trailer has lost its air supply
  4. the trailer brakes are stronger than the tractor brakes Correct answer.

Explanation

If the trailer swings out during braking, its brakes may be applying harder than the tractor's or the drive wheels have locked. The correct response is to release the brakes to regain traction.

Question 24Driving Combination Vehicles Safely

When you must stop on a railroad crossing with a combination vehicle, you should:

  1. stop only if a train is visible
  2. drive around the gates if they are down
  3. stop before the crossing and never on it Correct answer.
  4. stop on the tracks and check both ways

Explanation

A combination vehicle can be long enough to straddle the tracks, so you should never stop on a crossing. Stop before the tracks and proceed only when you can clear them completely.

Question 25Driving Combination Vehicles Safely

The safest way to handle a combination vehicle's tendency to off-track on a left turn is to:

  1. swing wide to the right before turning Correct answer.
  2. keep the rear wheels close to the curb
  3. accelerate through the turn
  4. turn from the right lane

Explanation

On a left turn, the rear wheels cut inside the path, so swinging wide to the right before the turn keeps the trailer from running over the curb or striking objects.

Question 26Driving Combination Vehicles Safely

On a combination vehicle, what does the red (emergency) gladhand supply to the trailer?

  1. Air that releases the trailer spring brakes and charges the trailer tanks Correct answer.
  2. Electrical power for the trailer marker lights
  3. Vacuum to assist the trailer brake booster
  4. Air that applies the trailer service brakes only

Explanation

The emergency (red) line charges the trailer air tanks and supplies the air that holds the spring brakes released. If it loses pressure or separates, the trailer spring brakes apply automatically. The service (blue) line carries the signal that applies the trailer brakes during normal braking.

Question 27Driving Combination Vehicles Safely

When you test the trailer brakes with the trailer hand valve while parked, what should happen?

  1. The trailer brakes set and hold while the tractor stays free to move Correct answer.
  2. The trailer spring brakes release and the wheels roll
  3. Both tractor and trailer brakes release fully
  4. The tractor brakes apply and the trailer stays free

Explanation

The trailer hand valve applies only the trailer service brakes, so a proper test shows the trailer holding while the tractor is unaffected. This confirms that the trailer brake signal is reaching the trailer. If nothing happens, the trailer brakes or the service line are faulty.

Question 28Driving Combination Vehicles Safely

When you inspect a coupling, the gap between the upper and lower fifth wheel plates should be:

  1. Wider at the front than at the rear
  2. About one inch at the rear of the plates
  3. Open enough to see the kingpin
  4. Closed, with no visible space between the plates Correct answer.

Explanation

A properly locked fifth wheel has no gap between the upper and lower plates. Any visible gap means the kingpin is not fully seated and the trailer could separate. You should also tug-test with the trailer brakes set to confirm the lock.

Question 29Driving Combination Vehicles Safely

Why must you avoid making sudden steering movements in a combination vehicle at highway speed?

  1. The tractor's front axle will lose its steering alignment
  2. The trailer brakes will apply automatically
  3. The trailer can swing wide and cause a rollover or loss of control Correct answer.
  4. The air lines will disconnect from the trailer

Explanation

Quick steering inputs are magnified at the rear of a combination, so the trailer can swing far to the side and tip over. Smooth, gradual steering keeps the trailer tracking behind the tractor. Rearward amplification makes this risk greater the faster you drive.

Question 30Driving Combination Vehicles Safely

You are driving a combination vehicle and the trailer begins to drift sideways while you brake on a wet road. What is the best immediate response?

  1. Press harder on the brake pedal to stop quickly
  2. Apply the trailer hand valve fully
  3. Accelerate hard to pull the trailer straight
  4. Release the brakes to let the wheels roll and steer to keep the rig straight Correct answer.

Explanation

A trailer sliding sideways under braking is starting to jackknife, and releasing the brakes lets the wheels roll so the trailer can straighten behind the tractor. Hard braking with locked wheels is what allows the trailer to swing out. Steering to stay aligned keeps the rig from folding.

Question 31Driving Combination Vehicles Safely

When turning at an intersection, why must a combination vehicle driver swing wider than a car driver?

  1. The trailer is longer than the tractor
  2. The trailer brakes lock during turns
  3. The rear wheels of the trailer track inside the path of the front wheels Correct answer.
  4. The steering axle cannot turn as sharply

Explanation

Off-tracking means the rear axles follow a shorter path than the front wheels, so the trailer cuts the corner. Swinging wide keeps the trailer tires off the curb and away from pedestrians. The effect is greatest at slow speed in tight turns.

Question 32Driving Combination Vehicles Safely

You are backing a tractor-trailer and the trailer starts to turn toward the driver's side in your mirrors. To correct, you should:

  1. Turn the steering wheel toward the side the trailer is drifting Correct answer.
  2. Hold the wheel steady and continue backing
  3. Accelerate to straighten the trailer
  4. Turn the steering wheel away from the drift

Explanation

When backing, turn the top of the steering wheel toward the direction you want the trailer to go. If the trailer drifts to the driver's side, steering toward that side brings it back. Small corrections made early keep the rig from folding.

Question 33Driving Combination Vehicles Safely

What is the correct way to set the tractor protection valve before uncoupling a trailer?

  1. Open both gladhands and leave the engine running
  2. Disconnect the electrical cord first, then release all brakes
  3. Close the trailer air supply valve and set the trailer parking brakes Correct answer.
  4. Release the trailer spring brakes and pull the air lines

Explanation

Before uncoupling, you close the trailer air supply valve and set the trailer parking brakes so the trailer stays put and the lines can be separated safely. This also protects the tractor air system from a sudden loss of pressure. Only then do you disconnect the lines.

Question 34Driving Combination Vehicles Safely

You are descending a steep grade with a heavy load and your combination vehicle's speed keeps climbing despite gearing down. What should you do?

  1. Use the brakes firmly enough to reduce speed, then release them to cool Correct answer.
  2. Shift to neutral to save fuel and coast
  3. Ride the brakes lightly all the way down the grade
  4. Apply the trailer hand valve and hold it down the grade

Explanation

Snubbing the brakes, applying them firmly to cut speed and then releasing them, lets the brakes cool between applications. Continuous light braking overheats the drums and can lead to brake fade. The trailer hand valve is not for controlling speed on a grade.

Question 35Driving Combination Vehicles Safely

What does the tractor protection valve do when air pressure in the tractor system drops too low?

  1. It increases air pressure to the trailer to keep brakes released
  2. It releases the tractor's spring brakes so you can steer
  3. It disconnects the electrical cord from the trailer
  4. It closes off air to the trailer and applies the trailer spring brakes Correct answer.

Explanation

The tractor protection valve guards against a major air loss by cutting off the trailer supply, which causes the trailer spring brakes to apply. This keeps a trailer from rolling free if a line breaks. The low pressure warning should also alert you before this happens.

Question 36Driving Combination Vehicles Safely

You are driving a combination vehicle at night and the trailer marker lights go out while the tractor lights stay on. What is the most likely cause?

  1. Low coolant in the tractor engine
  2. A loss of air pressure in the emergency line
  3. A worn fifth wheel locking jaw
  4. A fault in the electrical cord or its connection to the trailer Correct answer.

Explanation

Trailer lights are powered through the electrical cord, so a loose plug or damaged cord cuts power to the trailer lights only. Air pressure and fifth wheel condition affect braking and coupling, not lighting. You should check the cord connection before continuing.

Question 37Driving Combination Vehicles Safely

What is the purpose of the trailer spring brakes when you park a combination vehicle?

  1. They release automatically when the parking brake is set
  2. They hold the trailer only while the engine is running
  3. They apply automatically when air pressure is released, holding the trailer Correct answer.
  4. They apply only when the foot pedal is pressed

Explanation

Spring brakes are held applied by heavy springs and are released by air pressure, so they set whenever air is drained or the parking valve is pulled. This gives a fail-safe parking brake for the trailer. They do not depend on the engine running.

Question 38Driving Combination Vehicles Safely

A combination vehicle with a lightly loaded trailer is traveling on a slick road. Why is this configuration more prone to jackknifing under braking?

  1. The light trailer has less traction, so its wheels lock and slide more easily Correct answer.
  2. The trailer's spring brakes release during braking
  3. The trailer brakes apply later than the tractor brakes
  4. The tractor's drive axles carry too little weight to brake

Explanation

An empty or lightly loaded trailer has little weight on its tires, so the wheels lock and slide easily when the brakes are applied. A sliding trailer axle swings out and folds the rig. Smooth braking and reduced speed on slick roads help prevent this.

Question 39Driving Combination Vehicles Safely

You are checking a combination vehicle before a trip and find the trailer's spring brakes will not release even with normal air pressure. What is the most likely explanation?

  1. The service line is connected but the foot pedal is stuck
  2. The emergency line is not properly connected or is blocked Correct answer.
  3. The fifth wheel is greased too heavily
  4. The electrical cord is plugged in backwards

Explanation

Spring brakes release only when air reaches the trailer through the emergency line, so a bad connection or blockage there leaves them applied. The service line only signals the brakes to apply during normal braking. Checking the emergency gladhand connection is the first step.

Question 40Driving Combination Vehicles Safely

Why does a combination vehicle need a longer following distance than a single vehicle at the same speed?

  1. Its mirrors show a wider view behind the trailer
  2. Its greater weight and multiple air-braked units extend the time needed to stop Correct answer.
  3. Its trailer tires are larger than tractor tires
  4. Its engine brakes work only at low speed

Explanation

A loaded combination carries more momentum and its air brakes take time to build pressure at each wheel, so total stopping distance grows. Extra following distance gives room for that longer stop. Mirror view and tire size do not change stopping distance.

Question 41Air Brakes and Brake System Checks

In a dual air brake system, what is the purpose of the two separate circuits?

  1. To increase the top speed the vehicle can reach
  2. To let the driver choose between front and rear brakes while driving
  3. To reduce the number of brake chambers needed
  4. To provide a backup so one circuit can still stop the vehicle if the other fails Correct answer.

Explanation

A dual air brake system splits the brakes into two independent circuits, each with its own reservoir. If one circuit loses pressure, the other still provides braking capability. This redundancy is the main safety advantage of dual systems.

Question 42Air Brakes and Brake System Checks

How is the air compressor normally driven on a heavy truck?

  1. By a hydraulic pump
  2. By a gear or belt from the engine Correct answer.
  3. By exhaust gas pressure
  4. By an electric motor powered by the battery

Explanation

The air compressor is engine-driven, typically through a gear or belt, so it builds air pressure whenever the engine runs. It pumps air into the reservoirs until the governor cuts it out at the proper pressure.

Question 43Air Brakes and Brake System Checks

What does the governor do in an air brake system?

  1. It drains moisture from the tanks
  2. It cuts the compressor in and out to control reservoir pressure Correct answer.
  3. It adjusts slack adjusters automatically
  4. It limits the vehicle's road speed

Explanation

The governor senses reservoir pressure and cuts the compressor out when the system reaches the cut-out pressure, then cuts it back in at the cut-in pressure. This keeps air pressure within the designed operating range.

Question 44Air Brakes and Brake System Checks

What is the function of the safety valve on an air reservoir?

  1. It keeps the compressor running continuously
  2. It balances pressure between the two circuits
  3. It releases air if pressure rises above a safe level Correct answer.
  4. It lets the driver manually release air for a faster stop

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure climbs above a safe preset level, protecting the system from over-pressurization. It is a backup protection in case the governor fails.

Question 45Air Brakes and Brake System Checks

What is the purpose of the alcohol evaporator or air dryer in an air brake system?

  1. To remove moisture that can freeze or corrode brake components Correct answer.
  2. To lubricate the brake shoes
  3. To cool the compressor
  4. To increase the pressure in the reservoirs

Explanation

Moisture in the air system can freeze in cold weather and cause brake failure, and it promotes corrosion. An alcohol evaporator or air dryer removes moisture from the air.

Question 46Air Brakes and Brake System Checks

During a pre-trip inspection, when should the air compressor build pressure from 85 to 100 psi?

  1. Only after the parking brake is released
  2. Within about 5 minutes at idle
  3. Within about 45 seconds at a fast idle Correct answer.
  4. Only while the vehicle is moving

Explanation

A properly functioning compressor should raise pressure from 85 to 100 psi within roughly 45 seconds at a fast idle. If it takes much longer, the system may have a leak or compressor problem.

Question 47Air Brakes and Brake System Checks

You perform a static leak test and the air pressure drops from 100 to 90 psi in one minute with the engine off and brakes released. What does this indicate?

  1. That the safety valve is stuck open by design
  2. An air leak that should be repaired before driving Correct answer.
  3. That the compressor is overcharging
  4. Normal operation for a dual system

Explanation

With the engine off and brakes released, pressure loss should be no more than about 3 psi per minute. A 10 psi drop in one minute indicates a leak that must be found and fixed before driving.

Question 48Air Brakes and Brake System Checks

When you apply and hold the foot brake during a static leak test with the engine off, what is the maximum allowable pressure loss per minute?

  1. No loss at all is allowed
  2. About 4 psi Correct answer.
  3. About 25 psi
  4. About 15 psi

Explanation

With the brakes fully applied and held, the allowable leak rate is about 4 psi per minute. This is slightly higher than the released-brake limit because more of the system is pressurized.

Question 49Air Brakes and Brake System Checks

What is the purpose of the low pressure warning signal?

  1. To warn the driver before air pressure drops too low for safe braking Correct answer.
  2. To signal that the air dryer needs service
  3. To indicate that the parking brake is set
  4. To show that the compressor is building air

Explanation

The low pressure warning device alerts the driver when system pressure falls below a safe level, typically around 60 psi. It gives warning so the driver can stop safely before braking is lost.

Question 50Air Brakes and Brake System Checks

At approximately what pressure should the low pressure warning signal activate?

  1. 120 psi
  2. 100 psi
  3. 60 psi or below Correct answer.
  4. 20 psi

Explanation

The low pressure warning must activate at or before pressure drops to about 60 psi. This gives the driver time to react before braking capability is seriously reduced.