Commercial Vehicle Systems & Controls practice test: free CDL General Knowledge questions

Sample Practice Questions for Commercial Vehicle Systems & Controls

1. Question 1Ref: AAMVA CDL Manual — Chapter 6: Air Brakes

   What does the air compressor do in a CMV air brake system?

   ASupplies air to the brake chambersBBuilds and maintains air pressure in the systemCConverts air pressure to mechanical forceDFilters moisture from the air lines
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   B. Builds and maintains air pressure in the system

   The air compressor is the heart of the air brake system. It pumps air into the system's storage tanks, building pressure to the normal operating range of 100 to 125 psi. Without a functioning compressor, the system cannot maintain the pressure needed for braking.

2. Question 2Ref: AAMVA CDL Manual — Chapter 6: Air Brake System Components

   What color is the air line connected to the emergency (spring) brake relay valve?

   ABlueBGreenCYellowDRed
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   D. Red

   The emergency (supply) line is color-coded red. If air pressure is lost in this line, the spring brakes on both the tractor and trailer will apply automatically. The service line, which is controlled by the brake pedal, is color-coded blue. Never mix up these connections.

3. Question 3Ref: 49 CFR 393.95 — Spring Brake Operation

   What happens to the spring brakes when air pressure in the system drops below 60 psi?

   AThey disengage to allow normal drivingBNothing, until the engine is shut offCThey lock in the released positionDThey begin to apply gradually
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   D. They begin to apply gradually

   As system air pressure falls below 60 psi, the spring brake parking brake valves begin to apply. At approximately 20 to 45 psi (depending on the system), the spring brakes will fully engage. Never drive with low air pressure warnings active.

4. Question 4Ref: AAMVA CDL Manual — Chapter 6: Tractor Protection System

   What is the purpose of the tractor protection system on a tractor-trailer?

   ATo protect the engine from overheatingBTo disconnect the trailer during a jackknifeCTo limit the speed of the trailer on downhill gradesDTo automatically apply trailer brakes if air supply is lost
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   D. To automatically apply trailer brakes if air supply is lost

   The tractor protection system is a safety device that closes the trailer air supply valve when air pressure drops too low, applying the trailer spring brakes. This ensures the trailer brakes even if the tractor-trailer air connection fails completely. Without it, a runaway trailer could result.