CDL General Knowledge TestCombination Vehicles (if applicable)Medium
A driver is preparing to uncouple a combination vehicle. After lowering the landing gear until it makes firm contact with the ground, what is the next critical step?
- ARelease the fifth wheel locking jaws.
- BDisconnect the glad hands and electrical cable.
- CBack the tractor slightly to relieve pressure on the kingpin.
- DCrank the landing gear a few extra turns to lift the trailer slightly.
Show answer & explanationAnswer & explanation
Correct answer: D. Crank the landing gear a few extra turns to lift the trailer slightly.
After the landing gear makes firm contact, cranking it a few extra turns lifts the trailer's weight slightly off the fifth wheel. This takes pressure off the kingpin and allows the fifth wheel jaws to release more easily.
Why the other options are wrong
- A. Releasing the jaws without relieving kingpin pressure can damage the mechanism or make it difficult to operate.
- B. These connections are disconnected later in the process, after the fifth wheel is ready to release.
- C. Backing the tractor slightly is done before lowering the landing gear to ensure the kingpin is not binding.
Uncoupling: Lifting Trailer
Before releasing the fifth wheel jaws during uncoupling, crank the landing gear a few extra turns to lift the trailer slightly off the fifth wheel, relieving kingpin pressure.
- Done after landing gear touches ground.
- Relieves pressure on kingpin.
- Facilitates fifth wheel release.
- Prevents damage to coupling components.
Memory trick: Landing gear down, then a little lift to ease the shift.