CDL Combination Vehicles TestCoupling and UncouplingHard
After coupling, a driver observes that the kingpin is visible through the inspection hole in the fifth wheel's throat, but the release arm is not fully seated. What does this indicate?
- AThe kingpin may not be fully engaged, and the fifth wheel jaws are not securely locked.
- BThe fifth wheel is correctly locked, but the release arm spring is weak.
- CThe trailer is too heavy for the tractor, causing stress on the kingpin.
- DThe fifth wheel lubrication is insufficient, preventing the arm from seating.
Show answer & explanationAnswer & explanation
Correct answer: A. The kingpin may not be fully engaged, and the fifth wheel jaws are not securely locked.
If the kingpin is visible through the inspection hole but the release arm is not fully seated, it indicates the fifth wheel jaws may not have closed completely around the kingpin, meaning an insecure connection.
Why the other options are wrong
- B. A visible kingpin with an unseated arm means an insecure lock, regardless of spring strength.
- C. Trailer weight doesn't directly prevent the release arm from seating after coupling.
- D. Insufficient lubrication might make the arm stiff, but an unseated arm with a visible kingpin points to an incomplete lock.
Incomplete Kingpin Lock
A condition where the kingpin is partially engaged in the fifth wheel jaws, but the jaws are not fully closed and locked, often indicated by an unseated release arm.
- Extremely dangerous, can lead to trailer separation.
- Requires immediate correction before driving.
- Often detected by visual inspection of release arm and kingpin.
Memory trick: Visible kingpin, unseated arm, means alarm!