Coupling and Uncoupling
What this domain tests
The exam tests the safe sequence for connecting and separating a tractor and trailer, including fifth wheel and kingpin inspection, the tug test, air line connection, landing gear, and trailer brake checks.
Core rules
- Before coupling, inspect the fifth wheel, jaws, and kingpin for damage, wear, and secure mounting.
- Back under the trailer squarely, engage the jaws, and perform a tug test with the trailer brakes set.
- Connect the emergency and service air lines in the correct order, then the electrical cord. Check for chafing and proper seating.
- Raise the landing gear fully, but not so high that the trailer is lifted off the fifth wheel.
- Before uncoupling, park on firm ground, set the trailer brakes, chock wheels if needed, lower the landing gear, and disconnect lines.
Worked example
After backing under a trailer, you hear the jaws close. You set the trailer brakes, put the tractor in a low gear, and pull gently against the trailer. The tractor moves forward without pulling the trailer. The tug test has failed, so the jaws are not locked. The correct response is to stop, inspect the fifth wheel and kingpin, and recouple. The reasoning: the sound of the jaws is not proof of a locked coupling; only a tug test confirms it.
Traps that cost marks
- Skipping the tug test.
- High hooking, where the tractor is not fully under the trailer.
- Forgetting to check the gap between the fifth wheel and trailer apron.
- Connecting air lines but not testing trailer brakes.