Louisiana Commercial Driver License
Combination Vehicles Exam Practice
Questions And Correct Answers (Verified
1. During a pre-trip inspection of a tractor-trailer with air brakes, you notice that the
air pressure gauge shows 100 psi, but the low air pressure warning buzzer does not
sound when you turn the ignition on. What is the most likely cause and appropriate
action?
A. The warning buzzer is faulty; proceed with the trip but monitor pressure closely.
B. The system is functioning correctly; the buzzer only activates below 60 psi.
C. The warning buzzer circuit is defective; the vehicle must be placed out of service until
repaired.
D. The gauge is inaccurate; use a handheld tester to verify pressure before departure.
Answer: C. The warning buzzer circuit is defective; the vehicle must be placed out of
service until repaired.
Rationale: Federal regulations require a low air pressure warning device that activates
before pressure drops below 60 psi; a missing warning at 100 psi indicates a defect. The
vehicle is unsafe and must be repaired before operation. Options A, B, and D incorrectly
suggest proceeding or misdiagnose the issue.
2. When coupling a tractor to a semi-trailer, which sequence ensures proper
engagement and safety?
A. Back under trailer, secure kingpin, connect air lines, then test tug.
B. Connect air lines, back under trailer, secure kingpin, then test tug.
C. Back under trailer, test tug, secure kingpin, then connect air lines.
D. Secure kingpin, back under trailer, connect air lines, then test tug.
Answer: A. Back under trailer, secure kingpin, connect air lines, then test tug.
Rationale: The correct coupling procedure is to back under the trailer, ensure the kingpin
is locked (visually check and tug test), then connect air lines and electrical cord. Testing
tug before securing kingpin risks separation. Options B, C, and D have incorrect order that
could lead to dropped trailers or air line damage.
Page 1 of 85
,3. A tractor-trailer is loaded with a high center of gravity cargo. Which driving
technique best reduces the risk of rollover during a sharp curve?
A. Increase speed slightly to maintain momentum and reduce time in curve.
B. Brake firmly while turning to shift weight to the outside wheels.
C. Reduce speed before the curve and steer smoothly without abrupt movements.
D. Accelerate through the curve to keep the trailer in line.
Answer: C. Reduce speed before the curve and steer smoothly without abrupt
movements.
Rationale: High center of gravity increases rollover risk; reducing speed before entering
the curve and avoiding sudden steering or braking minimizes lateral forces. Braking or
accelerating in the curve can upset balance. Options A, B, and D are unsafe practices that
exacerbate rollover potential.
4. What is the primary purpose of the trailer hand valve (trolley valve) in a
combination vehicle?
A. To apply the trailer brakes independently for testing and emergency use.
B. To release the tractor protection valve during normal braking.
C. To modulate air pressure to the trailer spring brakes.
D. To activate the trailer anti-lock braking system.
Answer: A. To apply the trailer brakes independently for testing and emergency use.
Rationale: The hand valve allows the driver to apply only the trailer brakes, useful for
testing brake connections and in some emergency situations. It does not release the tractor
protection valve or modulate spring brakes. Options B, C, and D misstate its function.
Page 2 of 85
,5. When backing a tractor-trailer, which mirror observation technique helps prevent
jackknifing?
A. Use only the driver-side mirror to track the trailer's rear.
B. Alternate between both mirrors to keep the trailer visible and correct early.
C. Rely on the rearview camera exclusively for precise alignment.
D. Focus on the tractor's position relative to the dock, ignoring trailer angle.
Answer: B. Alternate between both mirrors to keep the trailer visible and correct
early.
Rationale: Alternating between mirrors allows early detection of trailer drift and prevents
oversteering that leads to jackknife. Using one mirror or ignoring trailer angle increases
risk. Rearview cameras may not show clearance on both sides. Options A, C, and D are
inadequate techniques.
6. A combination vehicle has a total weight of 80,000 lbs with a trailer weight of 45,000
lbs. The tractor has a wheelbase of 200 inches and the trailer has a wheelbase of 300
inches. What is the approximate off-tracking (in inches) when making a 90-degree
turn?
A. 25 inches
B. 50 inches
C. 75 inches
D. 100 inches
Answer: B. 50 inches
Rationale: Off-tracking is roughly proportional to the square of the trailer wheelbase
divided by the turning radius, but a common rule of thumb for a 90-degree turn is about 1/4
of the trailer wheelbase. Here, 300/4 = 75 inches, but considering the tractor wheelbase
and typical geometry, 50 inches is a more realistic estimate for this configuration. Options
A, C, and D are either too low or too high based on standard calculations.
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, 7. Which of the following best describes the correct action when the tractor protection
valve pops out during a brake check?
A. It indicates normal operation; continue driving.
B. It signals a major air leak; shut down and inspect the system.
C. It means the trailer brakes are fully applied; release the parking brake.
D. It shows the compressor is overworking; reduce engine RPM.
Answer: B. It signals a major air leak; shut down and inspect the system.
Rationale: The tractor protection valve closes to prevent total air loss when pressure drops
too low, indicating a serious leak or malfunction. Continuing to drive is unsafe. Options A,
C, and D misinterpret the valve's purpose and required response.
8. When securing cargo on a flatbed trailer, which tie-down arrangement is most
effective for preventing forward movement?
A. Use only vertical tie-downs over the cargo.
B. Use diagonal tie-downs angled toward the rear.
C. Use diagonal tie-downs angled toward the front.
D. Use a combination of vertical and diagonal tie-downs with at least one angled
forward.
Answer: D. Use a combination of vertical and diagonal tie-downs with at least one
angled forward.
Rationale: To prevent forward movement, tie-downs should be angled toward the front
(opposing forward motion) and combined with vertical restraints for stability. Solely
vertical or rear-angled tie-downs are less effective. Option D provides the most
comprehensive restraint.
Page 4 of 85
Combination Vehicles Exam Practice
Questions And Correct Answers (Verified
1. During a pre-trip inspection of a tractor-trailer with air brakes, you notice that the
air pressure gauge shows 100 psi, but the low air pressure warning buzzer does not
sound when you turn the ignition on. What is the most likely cause and appropriate
action?
A. The warning buzzer is faulty; proceed with the trip but monitor pressure closely.
B. The system is functioning correctly; the buzzer only activates below 60 psi.
C. The warning buzzer circuit is defective; the vehicle must be placed out of service until
repaired.
D. The gauge is inaccurate; use a handheld tester to verify pressure before departure.
Answer: C. The warning buzzer circuit is defective; the vehicle must be placed out of
service until repaired.
Rationale: Federal regulations require a low air pressure warning device that activates
before pressure drops below 60 psi; a missing warning at 100 psi indicates a defect. The
vehicle is unsafe and must be repaired before operation. Options A, B, and D incorrectly
suggest proceeding or misdiagnose the issue.
2. When coupling a tractor to a semi-trailer, which sequence ensures proper
engagement and safety?
A. Back under trailer, secure kingpin, connect air lines, then test tug.
B. Connect air lines, back under trailer, secure kingpin, then test tug.
C. Back under trailer, test tug, secure kingpin, then connect air lines.
D. Secure kingpin, back under trailer, connect air lines, then test tug.
Answer: A. Back under trailer, secure kingpin, connect air lines, then test tug.
Rationale: The correct coupling procedure is to back under the trailer, ensure the kingpin
is locked (visually check and tug test), then connect air lines and electrical cord. Testing
tug before securing kingpin risks separation. Options B, C, and D have incorrect order that
could lead to dropped trailers or air line damage.
Page 1 of 85
,3. A tractor-trailer is loaded with a high center of gravity cargo. Which driving
technique best reduces the risk of rollover during a sharp curve?
A. Increase speed slightly to maintain momentum and reduce time in curve.
B. Brake firmly while turning to shift weight to the outside wheels.
C. Reduce speed before the curve and steer smoothly without abrupt movements.
D. Accelerate through the curve to keep the trailer in line.
Answer: C. Reduce speed before the curve and steer smoothly without abrupt
movements.
Rationale: High center of gravity increases rollover risk; reducing speed before entering
the curve and avoiding sudden steering or braking minimizes lateral forces. Braking or
accelerating in the curve can upset balance. Options A, B, and D are unsafe practices that
exacerbate rollover potential.
4. What is the primary purpose of the trailer hand valve (trolley valve) in a
combination vehicle?
A. To apply the trailer brakes independently for testing and emergency use.
B. To release the tractor protection valve during normal braking.
C. To modulate air pressure to the trailer spring brakes.
D. To activate the trailer anti-lock braking system.
Answer: A. To apply the trailer brakes independently for testing and emergency use.
Rationale: The hand valve allows the driver to apply only the trailer brakes, useful for
testing brake connections and in some emergency situations. It does not release the tractor
protection valve or modulate spring brakes. Options B, C, and D misstate its function.
Page 2 of 85
,5. When backing a tractor-trailer, which mirror observation technique helps prevent
jackknifing?
A. Use only the driver-side mirror to track the trailer's rear.
B. Alternate between both mirrors to keep the trailer visible and correct early.
C. Rely on the rearview camera exclusively for precise alignment.
D. Focus on the tractor's position relative to the dock, ignoring trailer angle.
Answer: B. Alternate between both mirrors to keep the trailer visible and correct
early.
Rationale: Alternating between mirrors allows early detection of trailer drift and prevents
oversteering that leads to jackknife. Using one mirror or ignoring trailer angle increases
risk. Rearview cameras may not show clearance on both sides. Options A, C, and D are
inadequate techniques.
6. A combination vehicle has a total weight of 80,000 lbs with a trailer weight of 45,000
lbs. The tractor has a wheelbase of 200 inches and the trailer has a wheelbase of 300
inches. What is the approximate off-tracking (in inches) when making a 90-degree
turn?
A. 25 inches
B. 50 inches
C. 75 inches
D. 100 inches
Answer: B. 50 inches
Rationale: Off-tracking is roughly proportional to the square of the trailer wheelbase
divided by the turning radius, but a common rule of thumb for a 90-degree turn is about 1/4
of the trailer wheelbase. Here, 300/4 = 75 inches, but considering the tractor wheelbase
and typical geometry, 50 inches is a more realistic estimate for this configuration. Options
A, C, and D are either too low or too high based on standard calculations.
Page 3 of 85
, 7. Which of the following best describes the correct action when the tractor protection
valve pops out during a brake check?
A. It indicates normal operation; continue driving.
B. It signals a major air leak; shut down and inspect the system.
C. It means the trailer brakes are fully applied; release the parking brake.
D. It shows the compressor is overworking; reduce engine RPM.
Answer: B. It signals a major air leak; shut down and inspect the system.
Rationale: The tractor protection valve closes to prevent total air loss when pressure drops
too low, indicating a serious leak or malfunction. Continuing to drive is unsafe. Options A,
C, and D misinterpret the valve's purpose and required response.
8. When securing cargo on a flatbed trailer, which tie-down arrangement is most
effective for preventing forward movement?
A. Use only vertical tie-downs over the cargo.
B. Use diagonal tie-downs angled toward the rear.
C. Use diagonal tie-downs angled toward the front.
D. Use a combination of vertical and diagonal tie-downs with at least one angled
forward.
Answer: D. Use a combination of vertical and diagonal tie-downs with at least one
angled forward.
Rationale: To prevent forward movement, tie-downs should be angled toward the front
(opposing forward motion) and combined with vertical restraints for stability. Solely
vertical or rear-angled tie-downs are less effective. Option D provides the most
comprehensive restraint.
Page 4 of 85
