Coupling and Uncoupling a Double/Triple Combination
This domain tests the fixed order for joining a tractor to a trailer and for adding a converter dolly and further trailers, plus the reverse. Examiners look for sequence and for the checks that prove the coupling actually locked.
Backing under a trailer
Inspect the fifth wheel before you move: jaws open, mounting bolts tight, no cracks, plate greased. Center the trailer in the mirrors, back in a straight line, and stop just short of the kingpin so you can judge trailer height. Back under too low and you strike the landing gear; too high and the jaws ride over the kingpin instead of locking around it.
The two proof checks
When the jaws close and the release handle flips, do both checks:
- Visual: jaws closed around the kingpin, safety latch engaged.
- Tug test: set the trailer spring brakes, pull gently forward, and confirm the trailer holds the tractor.
Adding trailers
Chock wheels, couple the converter dolly to the first trailer's rear, then the next trailer's kingpin to the dolly's fifth wheel. Each coupling gets its own visual check and tug test. Connect air lines and the electrical cord last, open the shut-off valves, and confirm nothing drags.
Uncoupling
Park on firm, level ground, set the parking brake, chock the wheels, and lower the landing gear until it firmly contacts the ground. Disconnect the lines and cord, pull the release handle, and pull forward slowly.
Worked example
You hear the click and see the handle flipped, so you skip the tug test. Why is that a failure? The click only proves the jaws touched the kingpin. If you came in high, the jaws can close above the kingpin and release the moment you pull. The tug test, with trailer brakes locked, forces the jaws to carry the load; if the tractor moves and the trailer does not, the coupling is bad.
Traps that cost marks
Leaving shut-off valves closed; forgetting the safety latch check; not chocking before uncoupling; using the hand valve instead of the trailer parking brake to hold the trailer; skipping the dolly's own coupling checks.