Combination Vehicles · Section 6.4

Coupling and uncoupling: CDL practice questions

Section 6.4 covers how to couple a tractor to a semitrailer, how to check the connection, and how to uncouple safely. It is the largest chapter of the Combination Vehicles test.

Examlane has 43 questions on this chapter. Ten are below; open one to see the answer and why. The rest are in the practice tests.

Practice this chapter

1. Why is backing a tractor under a trailer at an angle a mistake?
1. The glad hands will lock together
2. The fifth wheel jaws will close too early and miss the kingpin
3. The kingpin will drop into the wrong slot on the fifth wheel plate
4. The trailer could slide sideways and break the landing gear
Answer: D. The trailer could slide sideways and break the landing gear
Backing in at an angle can shove the trailer sideways and break its landing gear, so line the tractor up straight in front of the trailer.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
2. What can happen if the trailer is too low when you back the tractor under it?
1. The trailer air lines may reverse and cross each other
2. The tractor may hit and damage the trailer's nose
3. The landing gear may fold up on its own as you back
4. The fifth wheel jaws may stay open
Answer: B. The tractor may hit and damage the trailer's nose
A trailer that is too low can be hit by the tractor, damaging the trailer nose. Raise or lower it so the tractor lifts it slightly.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
3. While testing the trailer brakes by applying and releasing them, why do you shut the engine off?
1. So the air pressure drops to zero
2. So the trailer rolls freely
3. So you can hear the brakes
4. So the tractor protection valve resets
Answer: C. So you can hear the brakes
With the engine off, you can hear the trailer brakes apply and release, which confirms they work.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
4. From the cab, which action supplies air to the trailer brake system during coupling?
1. Pushing in the air supply knob
2. Pulling out the air supply knob
3. Setting the tractor parking brake
4. Raising the landing gear
Answer: A. Pushing in the air supply knob
The trailer brake system gets air when the tractor protection valve control is moved to normal or the air supply knob is pushed in.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
5. After backing under the trailer, how can you confirm it has locked onto the tractor?
1. Pull gently forward with the trailer brakes released
2. Back up hard against the kingpin
3. Pull forward quickly in a high gear
4. Pull gently forward with the trailer brakes still locked
Answer: D. Pull gently forward with the trailer brakes still locked
A gentle pull forward against locked trailer brakes shows whether the trailer is really locked onto the tractor.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
6. After you couple, you see a gap separating the upper fifth wheel from the lower one. What may be wrong?
1. The trailer brakes are locked
2. The kingpin may be resting on the closed jaws
3. The landing gear is fully raised off the ground
4. The electrical cord is plugged in and fastened
Answer: B. The kingpin may be resting on the closed jaws
A gap means something is wrong: the kingpin might be sitting on the closed jaws, so the trailer could come loose very easily.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
7. Why do you take the key with you when you secure the vehicle and go under the trailer?
1. To keep the engine from overheating while it sits idle
2. To keep the parking brakes from setting while you work
3. So no one can start the truck while you are under it
4. To keep the air pressure from dropping
Answer: C. So no one can start the truck while you are under it
Taking the key keeps anyone else from starting and moving the truck while you are under it.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
8. Why must the landing gear be raised all the way up before you drive?
1. It may snag on railroad tracks or other objects
2. It may drain air from the trailer brakes
3. It may overheat the tractor tires
4. It may lock the fifth wheel open
Answer: A. It may snag on railroad tracks or other objects
Landing gear left part way up can catch on railroad tracks or other objects, so raise it all the way.
Based on: CDL manual, Section 6.4.1 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
9. Before uncoupling, what must be true of the surface where you park the trailer?
1. It must be sloped so the trailer rolls clear
2. It must be able to support the weight of the trailer
3. It must be soft so the landing gear digs in
4. It must be marked with painted lines
Answer: B. It must be able to support the weight of the trailer
The parking surface has to hold up the trailer's weight once the tractor is no longer under it.
Based on: CDL manual, Section 6.4.2 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.
10. What can happen if you pull the tractor out from under a trailer at an angle?
1. The fifth wheel jaws can lock shut
2. The air lines can tangle
3. The trailer brakes can release
4. The landing gear can be damaged
Answer: D. The landing gear can be damaged
If the tractor leaves at a slant, it can harm the landing gear, so keep it lined up straight with the trailer as it pulls out.
Based on: CDL manual, Section 6.4.2 (Georgia edition of the national model manual, read 2026-10-24). Question in our own words.

Read the chapter itself in your state's CDL manual, Section 6.4. Your state's manual is free: find it in the state table.