CDL Combination Vehicles TestCoupling and UncouplingMedium
During the uncoupling process, after the landing gear is fully extended and supporting the trailer, what is the next critical step before pulling the tractor away?
- AEngage the trailer's parking brake.
- BDisconnect the electrical cable and air lines.
- CRelease the tractor's parking brake.
- DLower the fifth wheel to its lowest position.
Show answer & explanationAnswer & explanation
Correct answer: B. Disconnect the electrical cable and air lines.
Once the landing gear is supporting the trailer, the air lines and electrical cable are the next items to be disconnected to fully separate the tractor from the trailer's systems.
Why the other options are wrong
- A. The trailer's parking brake (spring brakes) should already be engaged by the loss of air pressure from the disconnected lines, or manually set before uncoupling.
- C. Releasing the tractor's parking brake is done much later, as the tractor needs to be held in place for disconnection.
- D. Lowering the fifth wheel is typically done after the tractor has been pulled clear of the trailer, not before disconnecting lines.
Uncoupling Sequence (Lines)
After the trailer is supported by its landing gear, the air lines and electrical cable are disconnected to sever the operational link between the tractor and trailer.
- Ensures trailer stability before disconnection.
- Prevents damage to lines if tractor moves prematurely.
- Part of a systematic uncoupling procedure.
Memory trick: Park, Prop, Pull the Plugs, then Part ways.