Coupling System and Converter Dolly
What the exam checks
This domain tests your knowledge of the fifth wheel, kingpin, converter dolly, pintle hook, safety chains, and locking devices that hold a double or triple combination together. You must know what a correct coupling looks like and what to check after backing under a trailer.
Core rules
The fifth wheel jaws must close completely around the kingpin. The release handle must be in, the safety latch engaged, and no gap should show between the fifth wheel and trailer apron. A converter dolly has a drawbar or pintle eye that engages the pintle hook on the trailer ahead; the hook latch must close and the safety chains must be attached with enough slack to turn but not drag. The dolly's fifth wheel then carries the front of the following trailer. Check that the dolly is rated for the load and that its locking pins, pivot, and axle are secure. Never use a dolly with a bent drawbar, cracked casting, or missing latch.
Worked example
After backing under a second trailer, a driver sees the release handle out and the safety latch open. The jaws may look closed, but the handle position shows the lock is not fully seated. The driver should pull forward, re-couple, and perform a tug test with the trailer brakes set. Only after the handle is in and the latch is closed is the coupling acceptable.
Traps that cost marks
A visual "jaw looks shut" is not enough; use the handle, latch, and tug test. Do not confuse the dolly pintle hook with the tractor fifth wheel. Do not rely on safety chains to carry the trailer; they are a backup.