Combination Vehicle Inspection and Coupling/Uncoupling
This domain tests whether you can verify a tractor-trailer is mechanically sound and securely joined before you move it. Exam questions focus on the fifth wheel, kingpin, air and electrical connections, landing gear, and the correct order of coupling and uncoupling steps.
Core inspection and coupling rules
- Inspect the fifth wheel for secure mounting, damaged jaws, and proper lubrication; confirm the release handle is in the locked position and the jaws are open before coupling.
- Check the kingpin for wear, cracks, and secure mounting; check the apron for damage.
- Chock the trailer wheels, align the tractor squarely, and back slowly until the fifth wheel locks. Listen for the click, then perform a tug test in a low gear.
- After the tug test, visually confirm the jaws are closed around the kingpin and the release handle is secured.
- Connect the service (blue) and emergency (red) air lines to the correct glad hands, then the electrical cord. Ensure lines are not crossed, kinked, or dragging.
- Raise the landing gear fully, secure the crank, release the trailer parking brake, and check lights and air pressure before moving.
- Uncoupling: park on firm level ground, chock wheels, lower landing gear to the ground, disconnect lines, pull the release handle, and pull forward slowly.
Worked example
A driver backs under a trailer, hears a click, and immediately connects the lines without a tug test. On the road the trailer separates. The likely cause is a high hook: the tractor's fifth wheel rode over the kingpin instead of locking. The tug test would have revealed the failed lock.
Traps
Skipping the tug test; failing to confirm jaw closure visually; crossing service and emergency lines; forgetting to chock wheels or lower landing gear; leaving the release handle unsecured.