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CDL Practice Test: Combination Vehicles

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CDL Practice Test: Combination Vehicles

Combination Vehicles Questions

Click On The Picture To Begin

Good Luck!

What does the Service Air Line do?
  • Supplies air to the trailer air tanks
  • Carries air controlled by the foot brake or the trailer hand brake
  • Controls the parking brakes
  • Controls the emergency brakes
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

The service line (also called the "control line" or "signal line") carries air controlled by the foot brake or the trailer hand brake. Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change. The service line is connected to relay valves. These valves allow the trailer brakes to be applied more quickly than would otherwise be possible.

Next
What might happen if the trailer is too high when you try to couple?
  • The fifth wheel plate will automatically adjust to the trailer height
  • It may not couple correctly
  • The trailer will be forced downward into the fifth wheel plate, causing damage to the ground or landing gear
  • Modern trucks are installed with a "trailer height icon" which will illuminate on your dashboard
This is a question from page 51 - click here to look up the answer

Quote From Page 75 Of The Illinois CDL Manual:

Check Trailer Height

  • The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed. (If the trailer is too low, the tractor may strike and damage the nose of the trailer; if the trailer is too high, it may not couple correctly.)
  • Check that the kingpin and fifth wheel are aligned.

TruckingTruth's Advice:

In addition to incorrect coupling, if a trailer is too high off the ground and you try to back into it, you may cause damage as the rear of your truck cab could strike the front of the trailer. Further, the trailer king pin may become stuck in front of the fifth wheel plate which can be a costly and time wasting error. Before backing under any trailer, it's very important to visually check the trailer height in relation to your tractors fifth wheel.

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Next
Before disconnecting from a trailer, you should:
  • Set the trailer spring brakes but wait to set the tractor parking brakes until the landing gear has been lowered and the air lines have been disconnected
  • Apply the trailer spring brakes, then back up gently towards the trailer to ease tension off the fifth wheel locking lever
  • Apply the trailer spring brakes, then gently pull forward against the trailer to apply tension onto the fifth wheel locking lever
  • Set the tractor parking brakes but wait to set the trailer spring brakes until the landing gear has been lowered and the air lines have been disconnected
This is a question from page 53 - click here to look up the answer

Quote From Page 77 Of The Illinois CDL Manual:

Ease Pressure on Locking Jaws

  • Shut off trailer air supply to lock trailer brakes.
  • Ease pressure on fifth wheel locking jaws by backing up gently. (This will help you release the fifth wheel locking lever.)
  • Put on parking brakes while tractor is pushing against the kingpin. (This will hold rig with pressure off the locking jaws.
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On newer vehicles, what controls the trailer air supply?
  • A red 8-sided knob used to control the tractor protection valve
  • A lever on the side of the steering wheel used to control the trailer hand valve
  • A yellow diamond shaped knob used to control the tractor parking brake
  • There is no control for the trailer air supply on newer vehicles as it's all done automatically
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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How should you position your tractor before backing under and connecting to a trailer?
  • Back the tractor directly in front of the trailer
  • Back the tractor to the trailer on the left side
  • Back the tractor to the trailer on the right side
  • Back the tractor to the trailer at a 45 degree angle on either the left or right side
This is a question from page 51 - click here to look up the answer

Quote From Page 75 Of The Illinois CDL Manual:

  • Put the tractor directly in front of the trailer. (Never back under the trailer at an angle because you might push the trailer sideways and break the landing gear.)
  • Check position, using outside mirrors, by looking down both sides of the trailer.

TruckingTruth's Advice:

It will take some practice, but always back under a trailer as straight as possible to avoid turning the trailer over.

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What does the term "Bobtail" refer to?
  • A parked trailer with no truck attached
  • A fully loaded truck and trailer
  • A tractor without a trailer attached
  • A truck and trailer that is not loaded
This is a question from page 48 - click here to look up the answer

Quote From Page 71 Of The Illinois CDL Manual:

You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests show that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.

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Which of the following can help reduce the chance of a truck rollover?
  • Slow down to a safe speed before going into a turn
  • All of these can help reduce the chance of a truck rollover
  • At night, drive slow enough to see obstacles with your headlights before it is \ too late to change lanes or stop gently
  • Look far enough down the road to avoid being surprised and having to make a sudden lane change
This is a question from page 48 - click here to look up the answer

Quote From Page 70 Of The Illinois CDL Manual:

Steer gently and smoothly when pulling trailers. If you make a sudden movement with your steering wheel, you could tip over. Follow far enough behind other vehicles (at least 1 second for each 10 feet of your vehicle length, plus another second if going over 40 mph). Look far enough down the road to avoid being surprised and having

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How can you test the Tractor Protection Valve?
  • With the engine on, engage and release the parking brake several times until the trailer air supply control pops out
  • With the engine off, step on and off the brake pedal several times until the trailer air supply control pops out
  • With the engine off, engage and release the parking brake several times until the trailer air supply control pops out
  • With the engine on, step on and off the brake pedal several times until the trailer air supply control pops out
This is a question from page 54 - click here to look up the answer

Quote From Page 78 Of The Illinois CDL Manual:

Test tractor protection valve: Charge the trailer air brake system. (i.e., build up normal air pressure and push the "air supply" knob in.) Shut off the engine. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

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