Combination Vehicles Practice Test
The Combination Vehicles endorsement covers the knowledge needed to safely operate tractor-trailer combinations. Our free practice test covers coupling, uncoupling, air lines, and preventing jackknife situations.
What's on the Combination Vehicles Test
The combination vehicles knowledge test covers the safe operation of vehicles with a tractor and trailer. You'll be tested on coupling and uncoupling procedures, air supply systems, trailer brake operation, and how to prevent and recover from jackknife and trailer swing.
Who Needs a Combination Vehicles Endorsement
You need a Combination Vehicles endorsement if you plan to drive a Class A tractor-trailer. This test is required for all Class A CDL applicants and covers the specific knowledge needed to safely operate combination vehicles, which behave differently from single-unit trucks due to the articulation point between tractor and trailer.
Sample Practice Questions
1. What is the purpose of the tractor protection valve?
- A.To protect the tractor from theft
- B.To keep air in the tractor if the trailer breaks away
- C.To protect the trailer from over-pressurizing
- D.To shut off the engine
Answer: B. The tractor protection valve keeps air in the tractor brake system if the trailer develops a major leak or breaks away, allowing the tractor to stop safely.
2. What should you check before coupling a trailer?
- A.Only the lights
- B.That the fifth wheel is lubricated and the trailer height is correct
- C.Only the air lines
- D.Nothing, coupling is automatic
Answer: B. Before coupling, ensure the fifth wheel is properly lubricated and the trailer is at the correct height so the kingpin engages the fifth wheel jaw securely.
3. What causes a trailer to jackknife?
- A.Driving too slowly
- B.The trailer wheels lock up and the trailer swings outward
- C.The tractor wheels lock up
- D.Using the trailer hand valve too little
Answer: B. A jackknife happens when the trailer wheels lock and the trailer swings around, pushing the tractor out of alignment. It's often caused by over-braking on a slippery surface.
4. How do you test the trailer emergency brakes?
- A.Pull the trailer air supply valve and try to drive forward
- B.Honk the horn
- C.Step on the brake pedal three times
- D.Release the parking brake only
Answer: A. Pull out the trailer air supply valve to apply the trailer emergency brakes, then try to drive forward. If the vehicle doesn't move, the emergency brakes are working.
5. What are glad hands used for?
- A.To secure cargo
- B.To connect the air lines between the tractor and trailer
- C.To couple the fifth wheel
- D.To adjust the mirrors
Answer: B. Glad hands are the coupler fittings that connect the tractor's air supply line and service line to the trailer's air system.
Frequently Asked Questions
How many questions are on the Combination Vehicles test?
The combination vehicles knowledge test typically has 20 questions. You must answer at least 16 correctly (80%) to pass.
Do I need this endorsement for a Class A CDL?
Yes. The combination vehicles test is required for all Class A CDL applicants, since Class A vehicles are combination vehicles by definition.
What's the difference between this and the Air Brakes test?
The Air Brakes test covers brake system operation, while the Combination Vehicles test covers coupling, uncoupling, and the unique handling characteristics of tractor-trailers.
Can I take this test before my skills test?
Yes. You can take the knowledge test at any time after getting your CLP. Many drivers take it along with the General Knowledge and Air Brakes tests.
Ready to start studying? Create a free account or view our pricing plans.