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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

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Good Luck!

How far should the trailer landing gear be raised?
  • All the way up
  • On light loads, only half way but on heavy loads, it should be raised all the way up
  • Half way up
  • 3/4 of the way up
This is a question from page 52 - click here to look up the answer

Quote From Page 77 Of The Illinois CDL Manual:

Raise The Front Trailer Supports (Landing Gear)

  • Use low gear range (if so equipped) to begin raising landing gear. Once free of weight, switch to the high gear range.
  • Raise landing gear all the way up. (Never drive with landing gear only part way up, as it may catch on railroad tracks or other things.)
  • After raising landing gear, secure crank handle safely.
  • When full weight of trailer is resting on tractor:
  • Check for enough clearance between rear of tractor frame and landing gear. (When the tractor turns sharply, it must not hit landing gear.)
  • Check that there is enough clearance between the top of the tractor tires and the nose of the trailer
Next
When you turn suddenly while pulling doubles, which trailer is most likely to turn over?
  • The tractor is more likely to turn over than either trailer
  • The rear trailer
  • Both trailers are equally as likely to turn over
  • The front trailer
This is a question from page 48 - click here to look up the answer

Quote From Page 70 Of The Illinois CDL Manual:

Trucks with trailers have a dangerous "crack-the-whip" effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.

"Rearward amplification" causes the crack-the-whip effect. The rear trailer is twice as likely to turn over as the tractor. You can roll the last trailer of triples 3.5 times as easily as a 5-axle tractor-semi.

TruckingTruth's Advice:

When pulling more than 1 trailer, the rear-most trailer is always at a higher risk of overturning than the front trailer.

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Next
The Tractor Protection Valve will close automatically in what air pressure range
  • 0 to 25 psi
  • 90 to 110 psi
  • 50 to 75 psi
  • 20 to 45 psi
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 will close automatically if air pressure is low (in the range of 20 to 45 psi).

TruckingTruth's Advice:

It is definitely worth memorizing the psi range the tractor protection valve will automatically close. Not only will you need to know this for the written and pre-trip exams, but you'll definitely want to know this info for real-world driving situations and when testing the air brake system.

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Next
When does a trailer jackknife most often occur?
  • When the trailer is empty
  • When the trailer is loaded to half of its maximum weight
  • All trailers, regardless of weight or cargo placement, have an equal chance of jackknifing
  • When the trailer is fully loaded to maximum weight
This is a question from page 48 - click here to look up the answer

Quote From Page 71 Of The Illinois CDL Manual:

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a "trailer jackknife"

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Next
What are shut-off valves
  • Designed to turn off the truck engine when a brake failure occurs
  • Placed at each trailer spring brake and are designed to engage the emergency brakes when air pressure becomes too high
  • The yellow and red air control knobs located in the cab of a truck
  • Used in the service and supply air lines at the back of trailers used to tow other trailers
This is a question from page 50 - click here to look up the answer

Quote From Page 74 Of The Illinois CDL Manual:

Shut-off valves (also called "cut-out cocks") are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. 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:

Only trailers designed to be pulled with another trailer behind it will have a shut off valve. If you are only pulling one trailer, make certain the valve is fully in the closed position or it will leak air out of the air system.

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Next
Why shouldn't you use the trailer hand valve while driving?
  • You may wear out the tractor brakes more quickly
  • There is an increased danger of making the trailer skid
  • It can throw your brakes out of adjustment
  • It only applies braking force to the tractor 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 trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s'). There is much less danger of causing a skid or jackknife when using just the foot brake.

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Next
What will happen if there is a major leak in the service air line?
  • Air pressure will quickly leak away, even if you are not using the service brakes
  • When you try to put the service brakes on, air will quickly leak and will lower the air tank pressure, possibly activating the trailer emergency brakes
  • The emergency brakes will come on automatically as soon as a major leak occurs
  • Nothing, but braking power will be reduced
This is a question from page 50 - click here to look up the answer

Quote From Page 74 Of The Illinois CDL Manual:

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

TruckingTruth's Advice:

Remember, the service line is not used unless you depress the brake pedal or use the trailer hand valve. Therefor, if there is a major leak in your service line, you may not know it until you try to use the service brake. A thorough pre-trip inspection should catch any major service line leaks before you begin your trip.

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Next
What is the usual color for the Emergency Line?
  • Red
  • Black
  • Green
  • Blue
This is a question from page 49 - click here to look up the answer

Quote From Page 73 Of The Illinois CDL Manual:

Emergency lines are often coded with the color red (red hose, red couplers or other parts) to keep from getting them mixed up with the blue service line.

TruckingTruth's Advice:

While the emergency line will almost always be red, the color is not a regulation so some trucks may use other colors.

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