Vehicle combination knowledge: tractor, trailer, coupling and braking
Domain focus
This section tests the tractor unit, semitrailer, fifth-wheel coupling, air-line connection and braking system of a CE combination. The exam expects a safe coupling sequence and an understanding of what each system does.
Core rules
- The fifth-wheel plate on the tractor locks onto the trailer kingpin. Before reversing, align the tractor and trailer, secure the trailer with its parking brake and check that the kingpin and jaws are clean and undamaged.
- After reversing, confirm the jaws are locked and do a tug test. Connect the air lines in the correct order: red supply/emergency line and yellow service-brake line, then the electrical susa. Check for leaks and correct trailer brake operation.
- Raise the landing legs only after coupling is confirmed. Release the trailer parking brake only when the tractor-trailer combination is secure.
- The braking system is dual-circuit and includes ABS/EBS, a spring parking brake and, on many tractors, an endurance brake or retarder. The trailer must brake with the tractor; a disconnected or leaking line is a serious defect.
Worked example
After coupling, the driver notices the trailer parking brake will not release and the trailer wheels stay locked. The likely cause is an air-supply problem: the red line may be disconnected, leaking or not charged. The driver should not force the trailer to move. The correct response is to shut down, check the red line connection, charge the system and test the brakes before moving.
Traps
Driving off without a tug test; confusing the red supply line with the yellow service line; raising the landing legs before the fifth wheel is locked; and treating an ABS warning as harmless on a laden combination.