CDL General Knowledge — All Questions

You arrive for your shift and the tractor was driven by another driver yesterday. Before you move the vehicle, what does the federal rule require you to do first?

• a.Road test the vehicle for at least two miles before loading
• b.Review the last driver vehicle inspection report and be satisfied the vehicle is in safe operating condition✓
• c.Weigh the loaded vehicle at the nearest certified scale
• d.Drain the air tanks and rebuild pressure from zero

A driver must review the previous driver's inspection report and sign it if defects were noted and certified as repaired, then be satisfied the vehicle is safe to operate. Draining tanks completely and rebuilding pressure is not a required daily step, weighing is a cargo issue rather than a pre-trip requirement, and no minimum road-test distance is required before loading.49 CFR §396.13

What is the minimum legal tread depth in a major groove of a front steering-axle tire on a commercial motor vehicle?

• a.6/32 of an inch
• b.2/32 of an inch
• c.1/32 of an inch
• d.4/32 of an inch✓

Steer tires must have at least 4/32 inch of tread in every major groove because they carry the steering and a large share of the load. The 2/32 figure is the minimum for all other tires on the vehicle, and 6/32 and 1/32 are not federal minimums for any position.49 CFR §393.75

During your walkaround you measure 3/32 inch of tread on a drive-axle tire and 5/32 inch on a steer tire. What is the correct conclusion?

• a.The steer tire is illegal but the drive tire is legal
• b.Both tires meet the federal minimum tread requirements✓
• c.Both tires are illegal but the vehicle may be driven to the nearest shop
• d.Both tires are illegal and must be replaced

Non-steering tires need at least 2/32 inch, so 3/32 is legal, and steer tires need at least 4/32 inch, so 5/32 is legal. Neither tire is below the minimum, so no replacement is required and no exception for driving to a shop is needed.49 CFR §393.75

At what air pressure must the low-air-pressure warning device on an air-brake vehicle come on?

• a.At or below 20 psi
• b.At or below 60 psi✓
• c.At or below 85 psi
• d.At or below 45 psi

The low-pressure warning light and buzzer must activate at or before pressure drops to 60 psi, giving the driver time to stop while service brakes still work. Around 85 psi is a typical governor cut-in point, and 20 to 45 psi is the range in which spring brakes apply automatically, which is far too late for a warning.49 CFR §393.51

You are running the air-brake check with a fully charged system, engine off, and the service brakes released. What is the maximum allowable air pressure loss per minute for a combination vehicle?

• a.6 psi per minute
• b.2 psi per minute
• c.4 psi per minute
• d.3 psi per minute✓

With brakes released, a combination vehicle may lose no more than 3 psi per minute; a single vehicle is limited to 2 psi per minute. With the brakes applied the limits rise to 3 psi for a single vehicle and 4 psi for a combination, and 6 psi per minute always indicates an unsafe leak.

With the engine off and the air system fully charged, you begin fanning off air pressure with the brake pedal. At roughly what pressure should the tractor protection valve and parking brake valve pop out?

• a.Between 80 and 100 psi
• b.Between 10 and 15 psi
• c.Between 20 and 45 psi✓
• d.Between 60 and 80 psi

Spring-brake control valves must pop out somewhere between 20 and 45 psi, applying the parking brakes automatically before the system loses the ability to hold the vehicle. Pressures of 60 to 80 psi or higher would mean the valve trips too early, and 10 to 15 psi would leave the vehicle without brakes at a dangerously low pressure.

Which sequence correctly describes the seven-step inspection method?

• a.Start engine, road test, walkaround, engine compartment, cab check, lights, paperwork
• b.Vehicle overview, engine compartment, start engine and check the cab, turn off engine and check lights, walkaround, check signal lights, start engine and check the brake system✓
• c.Check lights, check tires, check brakes, check cargo, check mirrors, check horn, sign the report
• d.Walkaround, engine compartment, brake check, cab check, lights, signals, road test

The standard method starts with a general overview, moves to the engine compartment, then inside the cab, then lights, then the full walkaround, then signal lights, and finishes with the air-brake system check. The other sequences scramble the order or leave out required steps such as the engine compartment and the final brake check.

You finish a run in a property-carrying truck and discover the right rear turn signal stopped working during the trip. What must you do at the end of the day?

• a.Prepare a written driver vehicle inspection report listing the defect and give it to the motor carrier✓
• b.Wait until the defect happens a second time before reporting it
• c.Nothing, because lights are not covered by the inspection report rule
• d.Note it verbally to the next driver only

A driver must prepare a written report whenever a defect or deficiency that could affect safety is discovered, and lighting devices are specifically listed. Verbal notice does not satisfy the recordkeeping requirement, and there is no rule allowing a driver to ignore a defect until it recurs.49 CFR §396.11

Which set of emergency equipment must be on board a commercial motor vehicle?

• a.A first aid kit, a flashlight, and a tire gauge
• b.A tow strap, road flares only, and a hard hat
• c.A properly rated and charged fire extinguisher, spare electrical fuses if the vehicle uses them, and warning devices for stopped vehicles✓
• d.A fire blanket, a spill kit, and two wheel chocks

Federal rules require a fire extinguisher, spare fuses for vehicles that use fuses, and warning devices such as three reflective triangles. First aid kits, tow straps, spill kits, and chocks may be useful or required by a company policy, but they are not the federal emergency-equipment list.49 CFR §393.95

While checking the tractor's fifth wheel during your walkaround, what condition tells you the coupling is unsafe?

• a.There is a light coat of grease on the fifth wheel plate
• b.The release handle is in the locked position
• c.There is visible space between the upper and lower fifth wheel plates✓
• d.The locking jaws are closed around the shank of the kingpin

Any gap between the upper and lower fifth wheel plates means the trailer is not fully seated and the coupling can fail. Jaws closed around the kingpin shank, grease on the plate, and a locked release handle are all signs of a proper coupling.

You are checking steering-system play on a vehicle with a 20-inch steering wheel. How much free play is generally the maximum before the steering is considered defective?

• a.About 4 inches
• b.About 6 inches
• c.Any play at all is a defect
• d.About 2 inches✓

Roughly 10 degrees of free play, which is about 2 inches on a 20-inch wheel, is the practical limit before steering becomes hard to control. Four to six inches is excessive and would be an out-of-service condition, while a small amount of play is normal in a mechanical steering system.

You are inspecting the suspension on a trailer axle and find the main leaf of a leaf spring is broken. What is the correct action?

• a.Continue the trip but keep the load light
• b.Reduce tire pressure on that axle to compensate
• c.Continue and note the defect on tomorrow's report
• d.Place the vehicle out of service until the spring is repaired✓

A broken main leaf is an out-of-service condition because the axle can shift and the load is no longer properly supported. Lightening the load, delaying the report, or changing tire pressure does nothing to restore the suspension's ability to hold the axle in place.

Why is a leaking exhaust system a serious defect that must be corrected before driving?

• a.It prevents the transmission from shifting into high gear
• b.It automatically triggers the low-air-pressure warning
• c.It always causes the engine to stall at highway speed
• d.Exhaust gas including carbon monoxide can enter the cab and poison the driver✓

Leaking exhaust can allow carbon monoxide and heat into the cab and can ignite nearby fuel or wiring, which is why it must be repaired before operating. Exhaust leaks do not stall the engine on their own, have nothing to do with air-pressure warnings, and do not lock out the transmission.

During the applied-pressure portion of your air-brake test, what does the low-pressure warning coming on at 55 psi tell you?

• a.The system is fine because the warning came on below 60 psi
• b.The governor is set too high
• c.The spring brakes are already applied
• d.The warning device is defective because it must activate at or before 60 psi✓

The warning must activate at or before 60 psi, so a device that waits until 55 psi is out of adjustment and gives the driver too little time to react. It is not evidence of a governor problem, and spring brakes do not apply until roughly 20 to 45 psi.

During the static leak test you find air pressure dropping steadily and you can hear a hiss near the trailer air lines. What should you do?

• a.Take the vehicle out of service until the leak is repaired✓
• b.Add air with a shop line before each leg of the trip
• c.Apply the trailer hand valve to seal the leak
• d.Drive slowly to the delivery point and report it there

An audible, ongoing air leak means the system may not hold enough pressure to operate the brakes, so the vehicle must not be driven until it is repaired. Driving to a delivery point, topping off pressure, or applying a hand valve masks the problem without fixing the loss of braking capability.

You start the engine with the air tanks nearly empty and watch the gauge. On a dual air system, roughly how quickly should pressure build from 85 to 100 psi at operating rpm?

• a.Within 15 seconds
• b.Within about 45 seconds✓
• c.Within 3 minutes
• d.Within 5 seconds

A properly working dual air system should build from 85 to 100 psi in about 45 seconds at operating rpm. Five or fifteen seconds is unrealistically fast for a truck compressor, and three minutes indicates a weak compressor or a leak that must be repaired.

At what pressure should the air compressor governor normally cut out and stop pumping air?

• a.Between 120 and 140 psi✓
• b.Between 85 and 100 psi
• c.Between 150 and 170 psi
• d.Between 60 and 80 psi

The governor typically cuts out somewhere between 120 and 140 psi and cuts back in around 85 to 100 psi. Cutting out at 60 to 80 psi would leave too little reserve, and 150 to 170 psi would over-pressurize the system.

You are performing the tug test after coupling. What are you actually checking?

• a.That the landing gear is strong enough to hold the trailer
• b.That the trailer brakes release quickly enough
• c.That the electrical cord carries enough current
• d.That the fifth wheel locking jaws have fully closed around the kingpin and the trailer will not pull free✓

With the trailer brakes locked, gently pulling forward proves the coupling is holding the kingpin. The tug test says nothing about brake release timing, landing gear strength, or the electrical circuit, each of which is checked separately.

Which wheel and rim condition requires the vehicle to be taken out of service?

• a.Light surface rust on the rim
• b.A cracked rim or a wheel with missing, loose, or broken lugs✓
• c.A missing hubcap
• d.Mud packed in the wheel well

Cracked rims and missing or loose lug nuts can cause a wheel to separate from the vehicle and are out-of-service conditions. Surface rust, a missing hubcap, and packed mud are housekeeping issues rather than defects that make the wheel unsafe.

You notice oil or grease on the outside of a brake drum during your walkaround. Why does this matter?

• a.It prevents the ABS light from working
• b.It causes the drum to rust faster
• c.Contamination on the friction surfaces greatly reduces braking power✓
• d.It makes the wheel bearings spin too freely

Oil or grease on brake linings and drums destroys the friction the brake depends on, so stopping distance increases sharply and braking can become uneven. Rust, bearing speed, and the ABS lamp are unrelated to contaminated friction surfaces.

Which item is checked as part of the in-cab portion of the pre-trip inspection?

• a.Kingpin wear on the trailer
• b.Gauges, warning lights, wipers, horn, mirrors, and emergency equipment✓
• c.Slack adjuster travel at each wheel
• d.Tread depth on the drive tires

The in-cab step covers instruments, warning lamps and buzzers, wipers, horn, mirror adjustment, seat belt, and the presence of emergency equipment. Slack adjusters, the kingpin, and tire tread are all checked outside during the walkaround.

The previous driver's inspection report lists a defect that the shop has certified as repaired. What must you do before driving?

• a.Contact the previous driver to confirm
• b.Prepare a brand new report describing the old defect
• c.Review the report and sign it to acknowledge the repair or the certification that no repair was needed✓
• d.Nothing, since a mechanic already signed off

The next driver must review the prior report and sign it, confirming that the listed defect has been repaired or certified as not needing repair. Doing nothing skips a required signature, writing a new report about an old defect is not the rule, and contacting the previous driver is unnecessary.49 CFR §396.11

You are checking slack adjusters on an S-cam brake by pulling the pushrod by hand with the brakes released. What indicates the brakes are out of adjustment?

• a.The pushrod does not move at all
• b.The pushrod moves more than about one inch where it leaves the chamber✓
• c.The brake chamber is mounted upside down relative to the axle
• d.The slack adjuster is warm to the touch

More than roughly one inch of hand movement at the point the rod leaves the chamber signals brakes that need adjusting and will not develop full force. No movement is normal for a properly adjusted brake, warmth is expected after driving, and chamber orientation varies by design.

After a trip you find a small crack in the windshield directly in the driver's line of sight and a wiper blade that is torn. What is the correct assessment?

• a.Neither condition affects safety
• b.Only the wiper matters because glass damage is cosmetic
• c.Only the windshield matters because wipers are optional equipment
• d.Both are defects that affect the driver's ability to see and must be reported and repaired✓

Damaged glass in the driver's field of view and a torn wiper blade both degrade visibility, so both must be reported and corrected. Wipers are required equipment, and glass damage in the sightline is a safety defect rather than cosmetic.

Which statement best describes the driver's legal responsibility before operating a commercial vehicle?

• a.The driver must be satisfied that service brakes, parking brake, steering, lights, tires, horn, wipers, mirrors, coupling devices, wheels, and emergency equipment are in good working order✓
• b.Only the motor carrier is responsible for vehicle condition
• c.Responsibility begins only after the first 50 miles of the trip
• d.The mechanic's signature transfers responsibility away from the driver

The regulation places a personal duty on the driver to be satisfied that a specific list of components is in good working order before driving. A mechanic's signature and the carrier's own duties do not remove the driver's responsibility, and it applies from the first mile.49 CFR §392.7

You are inspecting the air lines between tractor and trailer. Which condition is acceptable?

• a.A line temporarily patched with electrical tape
• b.A hose rubbing against the frame with the outer cover worn through
• c.Glad hands sealed with rubber grommets in good condition and lines secured so they cannot chafe✓
• d.Air lines dragging on the ground behind the tractor

Air lines must be secured, free of chafing, and sealed at the glad hands with sound grommets. Worn-through covers, dragging lines, and tape patches all invite a sudden loss of air pressure while driving.

Why must you drain moisture from the air tanks or verify that the automatic drain valves are working?

• a.Water and oil in the tanks can freeze in cold weather and damage the system, reducing braking ability✓
• b.Draining is only needed on trailers, not tractors
• c.It keeps the compressor from over-charging the batteries
• d.Water increases the governor cut-out pressure

Compressed air carries moisture and oil that collect in the tanks, where they can freeze and block valves or corrode components until braking is compromised. Draining does not affect governor settings or the charging system, and it applies to tanks on every unit.

You are stopped at a roadside inspection and the officer places the vehicle out of service. What does that mean?

• a.The vehicle may not be driven until the defects that caused the order are corrected✓
• b.The driver may continue if the load is time sensitive
• c.The order applies only to the trailer, never the power unit
• d.The vehicle may be driven to the nearest terminal at reduced speed

An out-of-service order prohibits operating the vehicle until the specific defects are repaired and the order is cleared. There is no reduced-speed exception, the order can apply to any unit in the combination, and a time-sensitive load never justifies moving an unsafe vehicle.