Combination Vehicles
Combination Vehicles is the Pennsylvania CDL domain that tests how safely you connect, inspect, drive, and disconnect a tractor-trailer. Expect questions on coupling sequence, fifth-wheel and kingpin checks, air-line placement, brake tests, turning geometry, and skids and jackknifes.
Coupling and uncoupling in order
Before coupling, check that the trailer kingpin and fifth-wheel jaws are undamaged, the fifth wheel is tilted and greased, and the trailer brakes are set. Back slowly so the tractor frame aligns with the trailer; after the jaws close, perform a tug test, then inspect the coupling under. Connect the air lines-blue for service and red for emergency-then the electrical cord. Raise the landing gear fully and secure the crank handle.
Air and brake checks
With the engine running and trailer brakes released, charge the trailer air tanks, then shut off the engine. A tractor protection valve should keep air from escaping when the emergency line is disconnected; the trailer spring brakes should set. Test service brakes and trailer hand valve response at low speed. Confirm the emergency line separates and the trailer brakes apply automatically.
Turning and following
Tractors turn around a rear axle pivot point, so the trailer cuts the corner. Swing wide before a right turn without entering the opposing lane, and watch the trailer's outside wheels in the mirror. At highway speed, the trailer amplifies lateral movement; steer smoothly and avoid sudden lane changes.
Worked example
A driver backs under a trailer, hears the jaws lock, and drives away. The missing step is the tug test: pull gently against the trailer brakes while still coupling, then get out and verify the jaws are closed around the kingpin. A click is not proof; a high hitch can leave the trailer resting on the jaws.
Traps
Do not connect the electrical cord before the air lines; do not leave the landing gear touching the ground after coupling; do not rely on the trailer hand valve for parking. When uncoupling, set the trailer brakes, lower the landing gear, disconnect the electrical cord and air lines, then pull forward to clear the kingpin.