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CDL Practice Test: Combination Vehicles

These CDL practice questions are from our High Road Training Program, a CDL test preparation course designed to help you learn the CDL manual.

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CDL Practice Test: Combination Vehicles

Combination Vehicles Questions

Click On The Picture To Begin

Good Luck!

What are Glad Hands?
  • Coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer
  • Caps which cover the valve stems on tires
  • A name for the yellow and red air valves in the cab of a truck
  • Used to secure the trailer kingpin to the tractor 5th wheel
This is a question from page 50 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

"Glad hands" are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer.

Next
What does the Trailer Air Supply Control do?
  • Supplies the tractor with air from the trailer
  • Controls the parking brake on the tractor
  • Supplies the trailer with air from the tractor
  • Activates the air compressor located on the trailer
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

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Next
Which type of combination vehicle experiences the worst type of Rearward Amplification?
  • Trucks pulling 3 trailers
  • Trucks pulling 1 trailer
  • All trucks, regardless of how many trailers, experience the same amount of Rearward Amplification
  • Trucks pulling 2 trailers
This is a question from page 48 - click here to look up the answer

Quote From Page 70 Of The Illinois CDL Manual:

The rear trailer is twice as likely to turn over as the tractor. You can see that triples have a rearward amplification of 3.5. This means you can roll the last trailer of triples 3.5 times as easily as a 5-axle tractor semi.

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What happens if you cross the Emergency Air Line and the Service Air Line?
  • Air will continue filling the air holding tanks until the tanks rupture from excessive pressure
  • Supply air will be sent to the service line instead of going to charge the trailer air tanks which means the trailer spring brakes will not release
  • Newer vehicles automatically detect if the lines are crossed and will auto-correct itself
  • Supply air will cause all brakes to release and the truck will roll freely without any braking ability
This is a question from page 50 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes do not release when you push the trailer air supply control, check the air line connections.

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Next
Which statement about inspecting a combination vehicle is false?
  • Be sure the landing gear is fully raised, no missing parts, not bent or otherwise damaged
  • Check the the air lines and electrical line to be certain they are properly secured and connected
  • If you have a sliding fifth wheel, you must make sure the fifth wheel is securely locked in place
  • Make sure the fifth wheel locking jaws are around the head of the kingpin and not the shank of the kingpin
This is a question from page 54 - click here to look up the answer

Quote From Page 78 Of The Illinois CDL Manual:

Inspecting a Combination Vehicle

  • Locking jaws around the shank, not the head of kingpin.
  • Landing gear is fully raised, no missing parts, not bent or otherwise damaged.
  • Air lines properly connected to glad hands, no air leaks, properly secured with enough slack for turns.
  • All sliding fifth wheel locking pins present and locked in place
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Next
Which device inside the cab of a truck controls the Tractor Protection Valve
  • The brake pedal
  • The trailer hand valve
  • The red "trailer air supply" control valve
  • The yellow "parking brake" control valve
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The tractor protection valve keeps air in the tractor or truck should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the "trailer air supply" control valve in the cab. The control valve allows you to open and shut the tractor protection valve. The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on.

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Next
When pulling a single trailer with a Shut-Off Valve, the valves must be in the:
  • Open position
  • Off position
  • Closed position
  • Stopped position
This is a question from page 50 - click here to look up the answer

Quote From Page 74 Of The Illinois CDL Manual:

You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

TruckingTruth's Advice:

When only pulling one trailer, be sure the shut-off valves are all in the fully closed position.

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Next
When dropping an empty trailer, you should lower the landing gear until:
  • None of these answers are correct
  • You can't turn the landing gear handle any further
  • It makes firm contact with the ground
  • The landing gear is 1 to 3 inches above the ground
This is a question from page 53 - click here to look up the answer

Quote From Page 77 Of The Illinois CDL Manual:

Lower the Landing Gear

  • If trailer is empty, lower the landing gear until it makes firm contact with the ground.
  • If trailer is loaded, after the landing gear makes firm contact with the ground, turn crank in low gear a few extra turns. This will lift some weight off the tractor. (Do not lift trailer off the fifth wheel.) This will make it easier to unlatch the fifth wheel and easier to couple next time.
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