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CDL Practice Test: General Knowledge, Air Brakes & Combination

The knowledge tests every commercial driver takes: general knowledge, air brakes, combination vehicles and the pre-trip inspection.

170 questions · 4 topics · free, no sign-up

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Mixed practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

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Sample questions

At about what tank pressure does the governor stop the compressor from pumping air (cut-out)?

  1. 85 psi
  2. 100 psi
  3. 125 psi
  4. 150 psi
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C. 125 psi

The governor cuts the compressor out at around 125 psi. About 100 psi is the cut-in level where pumping starts again, and 150 psi is where the safety valve opens.

Reference: CDL Manual Section 5.1, The Parts of an Air Brake System

Compared with an empty rig, how much more likely is a fully loaded rig to roll over in a crash?

  1. 2 times
  2. 5 times
  3. 10 times
  4. 50 times
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C. 10 times

A fully loaded rig is about ten times more likely to roll over in a crash than an empty one, because the load raises the center of gravity. That is why cargo is kept low and turns are taken slowly.

Reference: CDL Manual Section 6.1, Driving Combination Vehicles Safely

A truck whose electrical circuits are protected by fuses must carry which set of emergency equipment?

  1. Spare fuses, a first-aid kit and one reflective warning triangle
  2. A fire extinguisher, a spare tire and a set of jumper cables
  3. Three reflective triangles, tire chains and a tow strap or chain
  4. Spare fuses, three reflective triangles and a fire extinguisher
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D. Spare fuses, three reflective triangles and a fire extinguisher

The required items are spare electrical fuses (unless the vehicle uses circuit breakers), warning devices for a parked vehicle such as three reflective triangles, and a properly charged and rated fire extinguisher. A first-aid kit, spare tire or chains may be useful but are not the required set.

Reference: CDL Manual Section 2.1, Vehicle Inspection; 49 CFR 393.95

How do you show the examiner that the engine oil level is acceptable?

  1. Read the dipstick with the engine off
  2. Watch the oil gauge with the engine idling
  3. Look for oil stains on the engine block
  4. Open the filler cap and look inside
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A. Read the dipstick with the engine off

You point out the dipstick and, with the engine off, show that the level is within the safe operating range, above the refill mark. The oil pressure gauge shows pressure while running, not how much oil is in the pan.

Reference: CDL Manual Section 11.1, All Vehicles (Internal Inspection)

At about what tank pressure does the governor let the compressor start pumping again (cut-in)?

  1. 20 psi
  2. 60 psi
  3. 100 psi
  4. 125 psi
Show the answer

C. 100 psi

When tank pressure falls to about 100 psi the governor cuts the compressor back in. The 125 psi level is the cut-out point where it stops pumping.

Reference: CDL Manual Section 5.1, The Parts of an Air Brake System

Exams this helps with

ExamWho runs itWhere
CDL air brakesState DMVsUnited States
CDL combination vehicles (Class A)State DMVsUnited States
CDL general knowledgeState DMVs (FMCSA standards)United States
Pre-trip vehicle inspectionState DMVsUnited States

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