Coupling and Uncoupling
Coupling and uncoupling - what is tested
The examiner wants the coupling sequence in the right order, the three-point check after the trailer is hooked, and a clean uncoupling that leaves the trailer stable and the tractor clear.
Coupling
- Inspect the fifth wheel: jaws open, release handle out, mounting bolts tight, no cracks, plate greased.
- Chock the trailer wheels.
- Set trailer height: the trailer plate slightly lower than the fifth wheel so the tractor lifts it as it backs under. Too high and the tractor strikes the trailer; too low and the kingpin can hit the fifth wheel.
- Back slowly in a straight line, keeping tractor and trailer aligned.
- Tug test: pull gently forward in low gear. A properly coupled trailer resists; a bad hook pulls apart.
- Set the parking brake, get out, and check three things: jaws closed around the kingpin, no gap between the trailer apron and the fifth wheel, safety latch engaged.
- Connect the air lines by color (red emergency, blue service), then the electrical cord with slack to turn, secured so it cannot drag.
- Raise the landing gear fully and secure the crank handle.
- Charge the trailer, then test its brakes with the hand valve while pulling gently.
Uncoupling
- Park on level ground and chock the trailer wheels.
- Set the parking brake and the trailer spring brakes.
- Lower the landing gear to firm ground contact.
- Disconnect the cord and air lines, close the glad hands or use dummy couplers, and secure the lines.
- Release the tractor brake, pull forward slowly, stop and confirm the trailer is stable, then pull clear.
Worked example
A driver backs under a trailer left too high. The fifth wheel closes but never lifts the trailer, so the jaws do not lock around the kingpin, and the tug test pulls the tractor out from under it. Diagnosis: the trailer was set too high. Fix: lower the landing gear, reset the height, and re-couple.
Traps
- Skipping the tug test or the three-point visual check.
- Forgetting to chock wheels or leaving the landing gear partly down.
- Crossing the service and emergency lines.
- Driving off with the crank handle unsecured.