semplr.

Exams · Certifications · Licensing

CDL Combination Vehicles Test

Inspecting combinations: practice questions

Our CDL Combination bank has 21 questions on this topic. Here are 5 of them with answers; the full mock exam draws from every topic, like the real one.

Start the free mock exam

Question 1

What should you check on the upper fifth wheel?

  1. Jaws open and handle pulled out
  2. Landing gear lowered
  3. Air lines disconnected
  4. Glide plate securely mounted and kingpin not damaged
Show the answer

D. Glide plate securely mounted and kingpin not damaged — Upper fifth wheel: glide plate securely mounted to trailer frame, kingpin not damaged (CDL Manual 6.5.1)

Question 2

Before testing that air flows to all trailers, how do you hold the vehicle in place?

  1. Use the tractor parking brake and/or chock the wheels
  2. Leave it in gear with the engine off and the trailer brakes released
  3. Apply the trailer hand valve only
  4. Lower the landing gear
Show the answer

A. Use the tractor parking brake and/or chock the wheels — Use the tractor parking brake and/or chock the wheels to hold the vehicle, wait for normal pressure, then push in the red knob (CDL Manual 6.5.2)

Question 3

How do you check that air flows to all trailers?

  1. Watch the trailer ABS lamp come on
  2. Drive forward and brake with the hand valve
  3. Feel the brake drums for warmth
  4. Open the emergency line shut-off at the rear of the last trailer and listen for air
Show the answer

D. Open the emergency line shut-off at the rear of the last trailer and listen for air — Open the emergency line shut-off valve at the rear of the last trailer; you should hear air escaping, showing the system is charged (CDL Manual 6.5.2)

Question 4

What could happen if the tractor protection valve doesn't work right?

  1. The trailer brakes would never release, even with full air pressure
  2. A trailer leak could drain all the air from the tractor
  3. The trailer lights would fail
  4. The fifth wheel would unlock
Show the answer

B. A trailer leak could drain all the air from the tractor — If it doesn't work right, an air hose or trailer brake leak could drain all the air from the tractor, with possible loss of control (CDL Manual 6.5.2)

Question 5

How do you test the tractor protection valve?

  1. Pull the knob out while driving at low speed
  2. Disconnect the service line and drive
  3. Charge the trailer system, shut off the engine and fan the brakes until the knob pops out
  4. Rev the engine to cut-out and listen
Show the answer

C. Charge the trailer system, shut off the engine and fan the brakes until the knob pops out — Charge the trailer air system, shut off the engine, step on and off the brake to reduce pressure; the trailer air supply control should pop out (CDL Manual 6.5.2)

More CDL Combination topics