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Iowa CDL Combination Vehicles Test study materials
Iowa CDL Combination Vehicles Test study resources

Iowa CDL Combination Vehicles Test practice questions

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Question 1Vehicle Inspection, Coupling and Uncoupling

On a combination vehicle, which color identifies the emergency (supply) air line running from the tractor to the trailer?

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

Explanation

Under the standard color code, the emergency or supply line is blue and the service line is red. Reversing the two keeps the trailer brakes from releasing and applying as designed, so confirm colors at the gladhands before coupling.

Question 2Vehicle Inspection, Coupling and Uncoupling

After backing under a trailer and hearing the fifth wheel lock, how do you perform the tug test?

  1. Set the tractor parking brake and tug on the air lines
  2. Push backward against the trailer with the tractor
  3. Set the trailer brakes, pull forward gently, and feel for resistance Correct answer.
  4. Have a helper watch the kingpin from behind the trailer

Explanation

The tug test confirms the closed jaws have captured the kingpin. With the trailer parking brake set, pull forward slowly in low gear; if the trailer holds the tractor back, the coupling is locked. Finish with a visual check of the jaws and release arm.

Question 3Vehicle Inspection, Coupling and Uncoupling

Once the fifth wheel has locked onto the kingpin, where should the release arm be?

  1. Wired open to prevent it from locking by accident
  2. Removed and stored in the cab while driving
  3. Fully in the locked position with the safety latch engaged Correct answer.
  4. Forward, so the trailer can pivot freely on the plate

Explanation

The release arm must be fully in the locked position with its safety latch engaged before the combination moves. An arm that is partly out of position can let the jaws open in transit and separate the trailer from the tractor.

Question 4Vehicle Inspection, Coupling and Uncoupling

Before you back the tractor under a trailer, how should the fifth wheel be set?

  1. Tilted up with the jaws open
  2. Tilted up with the jaws closed
  3. Tilted down with the jaws open Correct answer.
  4. Flat and level with the jaws closed

Explanation

The fifth wheel should be tilted down with the jaws open so the kingpin can slide into the throat and trip the locking mechanism. A plate tilted up or jaws that are already closed block the kingpin and prevent a solid coupling.

Question 5Vehicle Inspection, Coupling and Uncoupling

When uncoupling, after the landing gear makes firm contact with the ground, what should you do with the crank?

  1. Turn the crank a few extra turns in low gear Correct answer.
  2. Crank until the handle will not turn any further
  3. Leave the crank in high gear so the gear holds
  4. Crank until the trailer tires clear the ground

Explanation

A few extra turns in low gear lift some of the trailer weight off the tractor and make the fifth wheel easier to release. Stop at firm contact plus those turns; raising the trailer until the tires leave the ground overloads the landing gear.

Question 6Vehicle Inspection, Coupling and Uncoupling

After the fifth wheel is locked and the tug test is passed, what is the correct next step?

  1. Pull the tractor forward to test the coupling before connecting anything
  2. Raise the landing gear first, then connect the air lines and cord
  3. Release the trailer parking brake, then connect the air lines
  4. Connect the air lines and electrical cord, then raise the landing gear Correct answer.

Explanation

The coupling sequence is to lock and check the fifth wheel, connect the air lines and electrical cord, and then raise the landing gear. Getting air to the trailer before the gear comes up keeps the trailer brakes charged and supported while you finish the hookup.

Question 7Vehicle Inspection, Coupling and Uncoupling

After connecting the air lines to a trailer, what must be done before releasing the parking brakes and moving?

  1. Charge the trailer tanks to full pressure and check for leaks Correct answer.
  2. Release the tractor parking brake immediately to test the connection
  3. Open the trailer air supply valve to drain the tanks
  4. Disconnect the service line and feel for escaping air

Explanation

The trailer air tanks must be charged to normal pressure before you release the brakes and move; otherwise the spring brakes may stay applied or drag. Watch the gauge after charging to confirm the coupling and lines are not leaking.

Question 8Vehicle Inspection, Coupling and Uncoupling

Where should you position a combination vehicle before uncoupling?

  1. On a slope, so the trailer rolls away from the tractor
  2. On any surface, as long as the tractor parking brake is set
  3. On firm, level ground with the trailer brakes set and wheels chocked Correct answer.
  4. On soft ground, so the landing gear sinks in and holds

Explanation

Uncouple only on firm, level ground, with the trailer parking brake set and the wheels chocked. A slope lets the trailer roll once the tractor is pulled away, and soft ground lets the landing gear sink and the trailer drop or tip.

Question 9Vehicle Inspection, Coupling and Uncoupling

You are backing under a trailer, but the trailer sits too high for the fifth wheel. What problem does this create?

  1. The fifth wheel will lift the trailer off the ground
  2. The trailer brakes will not release
  3. The kingpin may miss the jaws or fail to lock fully Correct answer.
  4. The landing gear will collapse under the load

Explanation

Trailer height must match the fifth wheel. If the trailer sits too high, the kingpin can ride over the jaws or never seat fully, so check the gap and adjust the landing gear before backing under. A trailer that is too low is the case where the tractor frame may strike the trailer.

Question 10Vehicle Inspection, Coupling and Uncoupling

How should the electrical cord be handled after it is plugged into the trailer?

  1. Coil it tightly around the air lines to keep it off the ground
  2. Leave it hanging free so it swings with the trailer
  3. Plug it in only after the trailer is rolling
  4. Secure it so it cannot drag or chafe against the coupling Correct answer.

Explanation

The cord must be plugged in fully and supported so it does not drag on the road or get pinched between the tractor and trailer. A worn or pinched cord causes lighting and ABS failures that put the combination out of service.

Question 11Vehicle Inspection, Coupling and Uncoupling

Before connecting the trailer air lines to the tractor, what should you check on each glad hand?

  1. That the rubber seal is present and not cracked or missing. Correct answer.
  2. That the shut-off valve is closed and locked with a pin.
  3. That the glad hand is filled with grease to ease connection.
  4. That the coupler is wrapped with tape to prevent leaks.

Explanation

Glad hands seal against each other with rubber gaskets. A missing, cracked, or hardened seal will leak air and can keep the trailer brakes from releasing or charging properly. Check the seal before every connection and replace damaged seals.

Question 12Vehicle Inspection, Coupling and Uncoupling

While inspecting the fifth wheel before coupling, what condition should the top surface be in?

  1. Coated with a light film of grease. Correct answer.
  2. Covered with a non-slip rubber mat.
  3. Painted with a fresh coat of enamel.
  4. Clean and dry with no lubricant.

Explanation

The fifth wheel must be lubricated so the trailer can pivot without binding or damaging the kingpin. A light film of grease on the fifth wheel plate is normal. Do not paint the plate or cover it with a mat, because that prevents proper sliding and inspection.

Question 13Vehicle Inspection, Coupling and Uncoupling

During the walk-around inspection, what should you check on the tractor-to-trailer electrical cord?

  1. That the cord is disconnected before the engine is started.
  2. That the cord is long enough to reach from the tractor to the trailer.
  3. That the cord is tied securely to the air lines with wire.
  4. That the plug pins are straight and the casing is not worn. Correct answer.

Explanation

The electrical cord and plug carry the trailer light and ABS signals. Bent pins, cracked plugs, or worn casing can cause lights to fail or create a short. Inspect the cord during the walk-around and secure it so it does not drag.

Question 14Vehicle Inspection, Coupling and Uncoupling

When inspecting the trailer kingpin, which condition requires the trailer to be taken out of service?

  1. The kingpin is painted the same color as the trailer frame.
  2. The kingpin is stamped with a manufacturer's number.
  3. The kingpin has a light film of grease on it.
  4. The kingpin is bent or has a cracked weld at the apron. Correct answer.

Explanation

A bent kingpin or a cracked weld at the trailer apron can prevent the fifth wheel from locking securely. Either defect can let the trailer separate from the tractor. The trailer must be repaired before it is moved.

Question 15Vehicle Inspection, Coupling and Uncoupling

After connecting the air lines and opening the shut-off valves, what does pushing in the trailer air supply knob do?

  1. It drains moisture from the trailer air tanks.
  2. It disconnects the tractor protection valve from the circuit.
  3. It applies only the trailer emergency brakes for a leak test.
  4. It charges the trailer air tanks and releases the trailer parking brakes. Correct answer.

Explanation

Pushing in the trailer air supply knob sends tractor air to the trailer and releases the trailer spring brakes. Pulling the knob out applies the trailer emergency brakes and cuts off the air supply. This is why the knob must be pushed in before you release the tractor parking brake and move.

Question 16Vehicle Inspection, Coupling and Uncoupling

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

  1. It increases engine rpm when the trailer is loaded.
  2. It keeps air in the tractor if the trailer breaks away. Correct answer.
  3. It automatically locks the fifth wheel when the kingpin enters the jaws.
  4. It prevents the trailer lights from flashing when the turn signal is on.

Explanation

The tractor protection valve senses a major air loss, such as a separated trailer line. It then closes to keep the tractor's air supply from escaping and applies the trailer emergency brakes. This gives the driver a chance to stop safely.

Question 17Vehicle Inspection, Coupling and Uncoupling

During uncoupling, after you lower the landing gear and disconnect the air lines and electrical cord, what should you do next?

  1. Raise the landing gear until the trailer is lifted off the fifth wheel.
  2. Connect the trailer hand valve and apply the trailer brakes.
  3. Back the tractor farther under the trailer to relieve pressure.
  4. Pull the release handle and move the tractor slowly forward. Correct answer.

Explanation

Once the landing gear supports the trailer and the lines are disconnected, release the fifth wheel and move the tractor forward slowly. Pulling away slowly lets you watch the trailer for stability and keeps the landing gear from being damaged. Do not raise the landing gear while the trailer is still on the fifth wheel.

Question 18Vehicle Inspection, Coupling and Uncoupling

After coupling, connecting the air lines, and plugging in the electrical cord, how should you check the trailer lights?

  1. Watch the trailer lights reflect in the mirrors while you operate the switches alone.
  2. Turn on the four-way flashers and check only the front tractor lights.
  3. Have a helper behind the trailer call out each light as you test it. Correct answer.
  4. Assume the lights work if the cord is plugged in fully.

Explanation

Trailer lights must be checked from outside the vehicle because the driver cannot see all of them from the cab. A helper can confirm the clearance lights, turn signals, brake lights, and four-way flashers as you operate each switch. Never assume the lights work just because the cord is connected.

Question 19Vehicle Inspection, Coupling and Uncoupling

After backing under a trailer, the fifth wheel release arm moves freely and will not stay in the locked position. What should you do?

  1. Do not move until the locking mechanism is repaired and the arm stays locked. Correct answer.
  2. Tie the release arm down with wire and continue.
  3. Drive slowly and check the arm again after a few miles.
  4. Pull the tractor forward until the trailer drops onto the landing gear.

Explanation

The release arm must stay in the locked position with the safety latch engaged. If it will not lock, the fifth wheel may not be fully engaged, and the trailer can separate. Do not drive until the cause is found and corrected.

Question 20Vehicle Inspection, Coupling and Uncoupling

During inspection you find the fifth wheel mounting bolts are loose. What is the correct action?

  1. Replace the bolts with shorter ones so they do not protrude.
  2. Tighten them after the trip if the trailer tows normally and the bolts do not back out.
  3. Ignore them if the trailer is empty and the route is short.
  4. Do not use the vehicle until the bolts are tightened to specification. Correct answer.

Explanation

Loose fifth wheel mounting bolts can let the fifth wheel tilt or separate from the tractor frame. The vehicle must be taken out of service until the bolts are tightened to the manufacturer's specification. Do not wait until after a trip or ignore the condition because the trailer is empty.

Question 21Vehicle Inspection, Coupling and Uncoupling

When uncoupling, what must you do with the trailer landing gear before pulling the tractor away?

  1. Lower it until it firmly contacts the ground. Correct answer.
  2. Leave it retracted so the trailer can settle.
  3. Raise it fully before the tractor is moved.
  4. Remove the crank handle and leave the gear up.

Explanation

The landing gear must support the front of the trailer once the tractor is removed. Lower it until it makes firm contact with the ground, then make sure the trailer is stable before disconnecting anything else. If the gear is raised or left retracted, the trailer can drop when the tractor pulls out.

Question 22Vehicle Inspection, Coupling and Uncoupling

After connecting the trailer air lines, what position should the shut-off valves be in before you drive?

  1. Open only on the service line.
  2. Closed on both lines.
  3. Open only on the emergency line.
  4. Open fully on both lines. Correct answer.

Explanation

Both shut-off valves must be open fully so air can flow from the tractor to the trailer. If either valve is closed, the trailer may not charge properly, the spring brakes may not release, or the trailer brakes may not work. Check the valves as part of the coupling inspection.

Question 23Vehicle Inspection, Coupling and Uncoupling

When uncoupling a trailer on a slope, what should you do before lowering the landing gear?

  1. Disconnect the electrical cord first.
  2. Set the tractor parking brake only.
  3. Raise the landing gear fully.
  4. Place chocks under the trailer wheels. Correct answer.

Explanation

A trailer on a slope can roll once the tractor is removed, so the trailer wheels must be chocked. Setting the tractor parking brake alone does not protect the trailer after uncoupling. Wheel chocks are the extra safeguard required when level ground is not available.

Question 24Vehicle Inspection, Coupling and Uncoupling

After the fifth wheel locks and you complete the tug test, how can you visually confirm the coupling is secure?

  1. Check that the trailer apron is touching the fifth wheel.
  2. Verify the release arm is in the locked position only.
  3. Look under the trailer to see the jaws closed around the kingpin. Correct answer.
  4. Confirm the electrical cord is plugged in.

Explanation

A visual check under the trailer confirms that the fifth wheel jaws have closed around the kingpin. The release arm position and the tug test are important, but they do not by themselves prove the jaws are wrapped around the kingpin. If the jaws are not closed, do not drive.

Question 25Vehicle Inspection, Coupling and Uncoupling

After the trailer is coupled and the air lines are connected, what must you do with the landing gear before driving?

  1. Leave it on the ground for stability.
  2. Remove the crank and leave the gear down.
  3. Lower it until it touches the ground.
  4. Raise it fully before moving. Correct answer.

Explanation

Once the trailer is supported by the fifth wheel and the air lines are connected, the landing gear must be raised completely. Driving with the landing gear down can damage it, tear up pavement, or cause the trailer to hang up. Confirm it is fully retracted before you move the combination vehicle.

Question 26Vehicle Inspection, Coupling and Uncoupling

During the coupling inspection, what should you check about the trailer apron?

  1. It is lower than the fifth wheel.
  2. It is coated with grease.
  3. It is straight, undamaged, and at the proper height. Correct answer.
  4. It is resting on the landing gear.

Explanation

The trailer apron is the front lower surface that meets the fifth wheel during coupling. It should be straight, free of cracks or bends, and at the correct height so the kingpin can enter the fifth wheel properly. A damaged or misaligned apron can prevent a secure coupling.

Question 27Vehicle Inspection, Coupling and Uncoupling

After coupling and charging the trailer air system, how should you test the trailer brakes?

  1. Push in the trailer air supply knob and listen for a buzzer.
  2. Check that the glad hands are disconnected.
  3. Release the parking brake and accelerate to 30 mph.
  4. Pull forward slowly and apply the trailer hand valve. Correct answer.

Explanation

Test the trailer brakes by moving slowly and applying the trailer hand valve or brake control. The trailer brakes should engage and hold the vehicle. This confirms that air is reaching the trailer and that the brake system is working before you drive on the road.

Question 28Vehicle Inspection, Coupling and Uncoupling

You are backing under a trailer, but the trailer sits too low for the fifth wheel. What is the main risk if you continue?

  1. The electrical cord will disconnect.
  2. The landing gear will retract automatically.
  3. The kingpin may strike the fifth wheel instead of entering the jaws. Correct answer.
  4. The trailer lights will not work.

Explanation

If the trailer is too low, the kingpin or trailer apron can hit the fifth wheel before the kingpin enters the jaws. That can damage the fifth wheel and prevent a secure lock. Raise the trailer to the proper height before backing under it.

Question 29Vehicle Inspection, Coupling and Uncoupling

The fifth wheel release arm is in the locked position, but the safety latch is broken and missing. What should you do?

  1. Remove the release arm entirely.
  2. Do not drive until the safety latch is repaired or replaced. Correct answer.
  3. Wire the arm in place and continue.
  4. Drive normally because the arm is already locked.

Explanation

The safety latch helps keep the release arm from vibrating out of the locked position. Without it, the fifth wheel could unlock while the vehicle is moving, which can separate the tractor from the trailer. The defect must be repaired before the combination vehicle is driven.

Question 30Vehicle Inspection, Coupling and Uncoupling

After backing under a trailer, the fifth wheel jaws do not close around the kingpin. What is the correct response?

  1. Lower the landing gear and disconnect the air lines.
  2. Force the release arm into the locked position with a tool.
  3. Pull forward, recheck height and alignment, and back under again. Correct answer.
  4. Drive slowly to see if the jaws close on their own.

Explanation

If the jaws do not close, the coupling is not secure and the vehicle must not be moved. Pull the tractor forward, check the trailer height and alignment, and try again. Forcing the mechanism or driving with open jaws can damage the fifth wheel or allow the trailer to separate.

Question 31Vehicle Inspection, Coupling and Uncoupling

Before you back the tractor under a parked trailer to couple, what should you do with the trailer wheels?

  1. Place wheel chocks in front of and behind the trailer wheels. Correct answer.
  2. Inflate the trailer tires to the maximum sidewall pressure.
  3. Remove any chocks so the trailer can roll into alignment.
  4. Turn the trailer wheels so they point toward the tractor.

Explanation

Wheel chocks keep the trailer from rolling while you back under it. If the trailer moves during coupling, the kingpin and fifth wheel can misalign, and the landing gear or trailer can be damaged.

Question 32Vehicle Inspection, Coupling and Uncoupling

On a combination vehicle with color-coded glad hands, which trailer air line must be connected to the tractor's red coupler?

  1. Either line, as long as both are connected.
  2. The blue service line.
  3. The electrical cord.
  4. The red emergency line. Correct answer.

Explanation

The red glad hand is the emergency line and must be connected red to red. The blue line is the service line and connects blue to blue; crossing them can prevent the trailer brakes from working correctly.

Question 33Vehicle Inspection, Coupling and Uncoupling

During the walk-around inspection of a trailer you are about to couple, what should you check about the landing gear?

  1. That the crank handle has been removed and stored in the cab.
  2. That the legs, foot pads, and bracing are free of cracks, bends, and missing parts. Correct answer.
  3. That the landing gear is greased with the trailer brakes released.
  4. That both legs are fully retracted for travel.

Explanation

Landing gear supports the trailer when it is not connected to a tractor, so cracked legs, bent foot pads, or missing bracing can cause it to collapse. The gear is inspected in its supporting position before coupling, not fully retracted.

Question 34Vehicle Inspection, Coupling and Uncoupling

Before coupling to a trailer equipped with a sliding tandem, what should you confirm about the slider locking pins?

  1. The pins are lightly greased and left partly engaged.
  2. The locking handles are removed so the tandem can center itself.
  3. Only one pin on each side is engaged for slight movement.
  4. All pins are fully engaged and the handles are seated. Correct answer.

Explanation

The slider locking pins hold the trailer tandem in position on the frame. If any pin is not fully engaged, the tandem can slide while the vehicle is moving, which can cause loss of control or damage.

Question 35Vehicle Inspection, Coupling and Uncoupling

After coupling and charging the trailer air system, you hear a steady hiss at a glad hand connection. What should you do?

  1. Close the shut-off valve on the tractor and continue the trip.
  2. Drive to the nearest shop if the trailer brakes still release.
  3. Increase engine speed until the hiss stops.
  4. Shut down and correct the leak before driving. Correct answer.

Explanation

A steady hiss at a glad hand means air is escaping, and a serious leak can keep the trailer brakes from releasing or cause them to apply unexpectedly. The connection must be corrected before the vehicle is driven.

Question 36Vehicle Inspection, Coupling and Uncoupling

During the pre-trip inspection of a coupled combination vehicle, what should you check about the tractor-to-trailer air lines?

  1. That they are wrapped tightly around the fifth wheel release arm.
  2. That they are not tangled, dragging, or rubbing against sharp surfaces. Correct answer.
  3. That they are disconnected so the trailer brakes stay applied.
  4. That they are crossed to keep them away from the exhaust.

Explanation

Air lines must be supported and kept clear of sharp edges, exhaust components, and moving parts. Tangled or dragging lines can rub through, leak, or disconnect while driving.

Question 37Vehicle Inspection, Coupling and Uncoupling

While inspecting the fifth wheel before coupling, you notice deep cracks or excessive wear on the locking jaws. What is the correct action?

  1. Tap the jaws with a hammer to see whether the cracks close.
  2. Lubricate the jaws and continue if they still close.
  3. Remove the vehicle from service until the jaws are repaired or replaced. Correct answer.
  4. Couple the trailer and check the jaws again after the tug test.

Explanation

The locking jaws must hold the kingpin securely, and cracks or heavy wear can cause the jaws to open or slip. The vehicle should be taken out of service until the fifth wheel is repaired or replaced.

Question 38Vehicle Inspection, Coupling and Uncoupling

After uncoupling a trailer and disconnecting the electrical cord, what should you do with the cord?

  1. Plug it back into the trailer to keep the socket covered.
  2. Wrap it around the trailer air lines for storage.
  3. Leave it on the ground so the next driver can find it.
  4. Secure it on the tractor so it cannot drag or be damaged. Correct answer.

Explanation

The electrical cord should be secured on the tractor so it does not drag, get crushed, or contact moisture and dirt. A damaged cord can cause lights and signals to fail.

Question 39Vehicle Inspection, Coupling and Uncoupling

After coupling, connecting the air lines, and charging the system, you release the tractor parking brake but the trailer brakes remain locked. Which condition is the most likely cause?

  1. The trailer landing gear is still on the ground.
  2. The trailer service (blue) line is connected to the blue coupler.
  3. The electrical cord is not plugged in.
  4. The trailer emergency (red) line is not connected or its shut-off valve is closed. Correct answer.

Explanation

The red emergency line supplies air that releases the trailer spring brakes. If it is disconnected or its shut-off valve is closed, the trailer brakes will stay applied even after the tractor parking brake is released.

Question 40Vehicle Inspection, Coupling and Uncoupling

During uncoupling, you release the fifth wheel before confirming the landing gear is firmly on the ground. What is the main risk?

  1. The tractor protection valve will close and set the trailer brakes.
  2. The fifth wheel jaws will re-lock around the kingpin.
  3. The trailer air lines will automatically disconnect.
  4. The trailer can drop or tip, damaging the landing gear and kingpin. Correct answer.

Explanation

The landing gear must be firmly on the ground and supporting the trailer before the fifth wheel is released. If it is not, the trailer can fall, bend the landing gear, or damage the kingpin and tractor.

Question 41Vehicle Inspection, Coupling and Uncoupling

Before backing the tractor under a parked trailer to couple, what should be the condition of the fifth wheel?

  1. Jaws open, plate tilted down toward the rear, and the top lightly greased. Correct answer.
  2. Jaws closed, plate tilted up toward the front, and the top left completely dry.
  3. Jaws closed, plate level, and the safety latch wired open.
  4. Jaws open, plate tilted up toward the front, and the top heavily greased.

Explanation

The fifth wheel must be ready to accept the kingpin: jaws open, the plate tilted down at the rear so the trailer apron can ride up onto it, and a light film of grease so the surfaces slide. Closed jaws or a plate tilted the wrong way will stop the kingpin from entering the throat and locking.

Question 42Vehicle Inspection, Coupling and Uncoupling

When connecting a trailer's air lines to the tractor's glad hands, which line should be connected first?

  1. The electrical cord first, then both air lines at the same time.
  2. The emergency line (red), then the service line (blue). Correct answer.
  3. Either line, as long as both are connected before charging the system.
  4. The service line (blue), then the emergency line (red).

Explanation

Connect the emergency (red) line first so supply air reaches the trailer, and then connect the service (blue) line. That order lets supply pressure release the trailer spring brakes and lets you confirm the supply circuit is seated before you fully charge the system.

Question 43Vehicle Inspection, Coupling and Uncoupling

During the walk-around inspection of a coupled combination vehicle, what should you check about the fifth wheel mounting?

  1. That the release arm has been tied in the open position with wire.
  2. That the jaws are open and lightly greased.
  3. That the mounting bolts are tight and the frame is not cracked. Correct answer.
  4. That the plate is tilted rearward as it is before coupling.

Explanation

The fifth wheel transfers the trailer load to the tractor frame through its mounting, so loose bolts, cracks in the mounting brackets, or a cracked frame can let the fifth wheel shift or break away. Check bolt tightness and look for cracks around the mounting during the inspection.

Question 44Vehicle Inspection, Coupling and Uncoupling

When a trailer is uncoupled and the tractor's air lines are not in use, where should the glad hands be kept?

  1. Stored on the dummy couplers to keep out dirt and moisture. Correct answer.
  2. Hanging from the tractor frame so any water inside can drain out.
  3. Wrapped around the electrical cord and hung on the mirror.
  4. Plugged into each other and left lying on the ground.

Explanation

Dummy couplers seal the glad hands when no trailer is connected, keeping out dirt and moisture that would damage the rubber seals and cause leaks later. Open glad hands hanging or lying on the ground collect contaminants that end up in the trailer air system.

Question 45Vehicle Inspection, Coupling and Uncoupling

While inspecting a glad hand before coupling, you find that the rubber seal is cracked and torn. What should you do?

  1. Coat the damaged seal with grease and connect the line.
  2. Connect it and check for leaks once the system is charged.
  3. Replace the seal before connecting the line. Correct answer.
  4. Connect it anyway, because the coupler will seal against bare metal.

Explanation

A cracked, torn, or missing glad hand seal leaks and can keep trailer supply pressure from building, which may leave the trailer spring brakes applied or trigger a low-air warning. Replace the seal before coupling rather than trying to grease it or relying on metal-to-metal contact.

Question 46Vehicle Inspection, Coupling and Uncoupling

Before coupling to a trailer, what should you check about the kingpin?

  1. That it is painted and slightly loose so the jaws can grab it.
  2. That it moves freely in the apron so the jaws can grab it.
  3. That it is straight, uncracked, and tight in the apron. Correct answer.
  4. That it is greased so it slides through the jaws without locking.

Explanation

The kingpin must be straight, free of cracks and heavy wear, and firmly attached to the trailer apron, because a bent or loose kingpin can let the jaws close without actually locking around it. If the kingpin is damaged, do not couple; report it and use a different trailer.

Question 47Vehicle Inspection, Coupling and Uncoupling

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

  1. It keeps the release arm from moving and uncoupling the trailer. Correct answer.
  2. It shows the driver whether the jaws are open or closed.
  3. It locks the kingpin into the jaws.
  4. It holds the trailer apron down against the fifth wheel plate at all times.

Explanation

The safety latch holds the release arm in the locked position so vibration or a bump in the road cannot move the arm and open the jaws. It is a backup to the locking mechanism, not an indicator of jaw position, so you still inspect the jaws and do a tug test after coupling.

Question 48Vehicle Inspection, Coupling and Uncoupling

During uncoupling, why do you pull out the red trailer air supply knob before disconnecting the air lines?

  1. To keep the trailer brakes from applying while you disconnect the lines.
  2. To release the trailer spring brakes so the wheels can roll.
  3. To drain moisture from the trailer air tanks.
  4. To shut off supply air so the trailer spring brakes set and hold the trailer. Correct answer.

Explanation

Pulling out the red trailer air supply knob shuts off supply air to the trailer, which causes the trailer spring brakes to apply and hold it in place. With the trailer brakes set, you can safely disconnect the lines and pull the tractor away without the trailer rolling.

Question 49Vehicle Inspection, Coupling and Uncoupling

You must uncouple a loaded trailer on soft ground. What is the best practice?

  1. Leave the landing gear retracted and let the tractor frame support the trailer.
  2. Lower the gear fully and rely on the wide foot pads to spread the load.
  3. Lower the gear only partway so the legs sink evenly into the ground.
  4. Place pads or blocks under the landing gear feet before lowering. Correct answer.

Explanation

Landing gear feet are small and will sink into soft ground, which can drop or tip the trailer once the tractor pulls away. Putting firm pads or blocks under the feet spreads the load and keeps the trailer stable while it is parked.

Question 50Vehicle Inspection, Coupling and Uncoupling

After coupling and connecting both air lines, you push in the red trailer air supply knob, but the trailer spring brakes stay applied. Which condition is the most likely cause?

  1. The landing gear is still touching the ground and bearing weight.
  2. The fifth wheel release arm is in the locked position.
  3. The electrical cord is plugged in upside down.
  4. The red glad hand is loose, so no supply air reaches the trailer. Correct answer.

Explanation

The spring brakes release only when supply air reaches the trailer through the emergency (red) line. A glad hand that is not fully seated, or a damaged seal on that coupling, blocks supply pressure and leaves the brakes applied even with the knob pushed in. Check both couplings and the seals before assuming a brake fault.