Combination Vehicle Inspection and Coupling/Uncoupling
This domain tests the pre-trip inspection of a tractor-trailer and the correct sequence for coupling and uncoupling. The exam expects you to know what proves a fifth wheel is locked, not just that it sounded locked.
Core inspection and coupling rules
- Inspect the fifth wheel mounting bolts, jaws, locking lever, safety latch, and kingpin apron. The jaws must close around the kingpin, and the locking lever must be seated with its safety latch engaged.
- Chock the trailer wheels. Check trailer height: the trailer apron should contact the fifth wheel so the tractor lifts it slightly as you back under. Too low can damage the kingpin; too high prevents coupling.
- Back under slowly and squarely. After the click, perform a tug test with the trailer brakes locked, then get out and visually check the jaws, gap, and latch.
- Connect the service and emergency air lines and the electrical cord; make sure they are not crossed, kinked, or dragging. Raise the landing gear fully and secure the crank.
- To uncouple: chock wheels, lower landing gear to the ground, disconnect lines and cord, release the fifth-wheel locking lever, then pull forward slowly.
Worked example
A driver hears the fifth-wheel click, connects the lines, raises the gear, and drives off. A later walk-around shows a visible gap between the trailer apron and fifth wheel and the locking lever not fully seated. The click only means the jaws contacted the kingpin; the gap and lever show the coupling is incomplete. Correct action: stop, lower the trailer, pull forward, re-couple squarely, complete the tug test and visual check, then reconnect.
Traps that cost marks
Treating the click as proof of coupling; skipping the tug test; leaving the landing gear partly down; forgetting to chock before uncoupling; and checking only one side of the fifth wheel.