Coupling and Uncoupling Procedures
This domain covers the sequence for joining a tractor to a trailer and separating them safely - the order of steps, the tug test, and the mistakes that drop a trailer.
Before you back under
Inspect the fifth wheel and kingpin. Chock the trailer wheels. Set the trailer at the right height - low enough that the tractor lifts it slightly as you back under, high enough that the kingpin clears the plate. Jaws open, plate tilted down at the rear.
Coupling and the tug test
Back under slowly and in line. Listen and feel for the jaws closing. Then do the tug test: with the trailer parking brake set, pull forward gently in a low gear. If the coupling is not locked, the tractor slides out from under the trailer - you will see and feel it. Get out and look at the jaws before you trust a sound.
Finishing the connection
Connect the service (blue) and emergency (red) lines and the electrical cord. Charge the trailer and listen for leaks. Raise the landing gear fully and secure the crank. Release the trailer spring brake and remove the chocks.
Uncoupling
Park on firm, level ground. Set the tractor parking brake and chock the trailer wheels. Lower the landing gear until it touches the ground and carries the trailer's weight before you pull out. Disconnect the lines and cord. Pull out slowly, straight ahead; the emergency line separates last and sets the trailer brakes through the tractor protection valve.
Worked example
You back under, hear the click, connect the lines, and start raising the gear. The trailer hisses and settles. The red line was never fully seated, so the emergency line lost pressure and the trailer's spring brakes set. Reseat the glad hand, recharge the trailer, and recheck before raising the gear.
Traps
- High hooking: the tractor is too high, jaws close, the kingpin is not seated - the tug test catches it.
- Driving away with the trailer spring brake still set.
- Pulling out at an angle and bending the landing gear.
- Skipping chocks on even a slight slope.