Coupling to a Trailer with a High Fifth Wheel
You are coupling a day cab to a loaded flatbed. The tractor's fifth wheel is set at the high mounting position, and the trailer's kingpin plate sits about 6 inches lower than the fifth wheel's throat when the tractor is backed straight in. You have already backed under the trailer, heard and felt a click, and done a visual check that the release handle is in. The trailer landing gear is still on the ground and the trailer brakes are set. The tractor has a manual transmission and you are on a slight grade with the trailer nose pointing downhill.
1. What must you verify before you pull forward, and how does the high fifth wheel change your procedure?
Read the worked answer
Do a pull test before anything else, and inspect the jaws directly. The click and the handle position are not proof of a complete couple. Get under the trailer with a light and confirm the jaws are closed around the kingpin, the kingpin is seated in the throat, and the safety latch is engaged. Then do the pull test: with the trailer brakes set, put the tractor in a low gear, apply light power, and feel for resistance. If the tractor moves forward without resistance, the kingpin is not captured and you must stop and recouple.
The high fifth wheel changes the geometry. Because the trailer plate sits lower than the throat, the trailer nose will ride up the ramps as you back in, and the kingpin may contact the top of the jaws rather than dropping into the throat. That can produce a click that sounds correct while the kingpin is actually riding on top of the locking mechanism. The 6-inch mismatch is large enough that you should confirm the fifth wheel height is correct for this trailer before backing in at all, and if it is not, adjust the fifth wheel or reject the trailer. Do not rely on sound.
2. After a successful couple, what checks confirm the connection is safe to move, and what would make you stop and uncouple?
Read the worked answer
With the trailer brakes still set, raise the landing gear fully and secure the crank handle. Connect the air lines, starting with the emergency (supply) line and then the service line, and confirm the trailer air pressure builds and the trailer brakes release when you charge the system. Connect the electrical cord and test the lights: running lights, left and right turn, brake, and four-way flashers, checked from the rear of the trailer. Confirm the trailer's spring brakes hold when the supply line is disconnected.
Stop and uncouple if any of the following appears: the pull test showed no resistance; the jaws are not visibly closed around the kingpin; the safety latch is missing or not engaged; the air lines leak or the trailer will not build pressure; the lights do not work; the landing gear will not retract fully; or the trailer frame sits at an angle that suggests the kingpin plate is not flat on the fifth wheel. On a downhill grade, any doubt about the couple is a reason to uncouple and start over rather than to test it on the road.