Knowledge area

CDL Combination Vehicles

18 practice questions cite 6 distinct 49 CFR sections and 1 whole part. Drivers preparing for combination vehicles are tested on coupling, uncoupling and combination vehicle inspection knowledge in 49 CFR 383.111(c).

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Source: PracticeDMV CDL question bank, citations checked against eCFR 2026-09-26

A truck tractor unit parked in a depot yard

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From this area's questions

What this test covers, by regulation

Knowledge standard cited in this bank: 49 CFR 383.111(c).

49 CFR sections and parts cited by the Combination Vehicles question bank
Section	Topic in this bank	Questions in our bank
49 CFR 383.111	Combination vehicle knowledge, paragraph (c)	6
49 CFR 383.113	Vehicle inspection and control skills	4
49 CFR 393.43	Trailer air supply protection	2
49 CFR 393.45	Trailer air line connections	2
49 CFR 393.70	Fifth wheels and coupling devices	2
49 CFR 392.7	Equipment checks before driving	1
49 CFR Part 393	Vehicle equipment and connections	1

Source: PracticeDMV CDL question bank, citations checked against eCFR 2026-09-26. Subsections of the same numbered section are counted together.

Three questions worked through

These questions explain rules from the combination vehicles bank.

Why does connecting the tractor and trailer air lines to the wrong glad hands (crossing service and emergency lines) create a hazard?

It only affects the trailer's turn signals
It prevents the landing gear from retracting
It has no effect as long as the lines are physically connected
It can disable the trailer's normal and emergency braking response when the driver needs it (correct)

The service and emergency lines each do a different job in controlling trailer air brakes, so swapping them can leave the trailer brakes unresponsive or behaving unpredictably. This is exactly why a pre trip inspection includes confirming the glad hands are matched correctly, not merely that air is flowing.

Source: 49 CFR 393.45.

What are glad hands used for on a combination vehicle?

Locking the fifth wheel onto the kingpin
Adjusting trailer suspension height
Connecting the tractor's air lines to the trailer's air brake system (correct)
Connecting the tractor and trailer electrical wiring

Glad hands are the coupling devices that join the service and emergency air lines between tractor and trailer, letting compressed air control and supply the trailer brakes. A driver should always check that both glad hands seal properly and that the lines are not crossed.

Source: 49 CFR 393.45.

During a pre trip coupling check, why should a driver pull forward gently against the coupled fifth wheel before getting under the trailer to inspect it?

To warm up the tractor engine
To confirm the kingpin is locked into the fifth wheel jaws and the coupling will hold (correct)
To test the trailer's turn signals
To release the trailer parking brakes

Pulling gently against the coupling puts tension on the kingpin and jaws, so if the connection is not actually locked the tractor will pull partly out from under the trailer instead of driving off with a trailer that later separates. Federal skills standards for combination vehicles require drivers to be able to inspect and confirm a coupling before relying on it.

Source: 49 CFR 383.113.

Knowledge area

Take the combination vehicles test

18questions

15correct to pass

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