Combination Vehicle Inspection and Coupling/Uncoupling
What this domain covers
Combination-vehicle inspection questions test whether you can verify a tractor-trailer as a system: fifth wheel, kingpin, air lines, landing gear, and the order of coupling steps. Expect sequence questions ("what comes next?") and defect questions.
Pre-coupling inspection
- Fifth wheel: jaws fully open, mounting bolts tight, no cracks, plate lubricated. Check trailer apron height so the kingpin will not strike the plate.
- Trailer: kingpin not bent or worn, landing gear secure, air lines and electrical cord undamaged.
- Chock the trailer wheels before any hookup.
Coupling sequence
- Inspect and align the tractor squarely with the trailer.
- Back slowly until the fifth-wheel jaws close on the kingpin.
- Tug test: pull forward gently in low gear with the trailer brakes set.
- Get out and look: jaws closed around the kingpin, release handle in.
- Connect the service (blue) and emergency (red) lines and the electrical cord.
- Raise the landing gear fully, then supply air and release the trailer spring brakes.
Uncoupling
Park on level ground, chock the trailer wheels, lower the landing gear to firm ground, disconnect the lines, pull the fifth-wheel release, and pull forward slowly.
Worked example
After coupling, the trailer will not roll and the dash valve is in "supply." Reasoning: the emergency (red) line is still disconnected or the trailer parking valve is closed, so the spring brakes stay applied. Reconnect the red line and charge the trailer; the brakes release.
Traps
Skipping the tug test; raising the landing gear before the air lines are connected; trusting the dash valve instead of checking the trailer; failing to check jaw position visually.