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6.4 – Coupling and Uncoupling

Knowing how to couple and uncouple correctly is basic to the safe operation of combination vehicles. Wrong coupling and uncoupling can be dangerous. General coupling and uncoupling steps are listed below. There are differences between different rigs, so learn the details of coupling and uncoupling the truck(s) you will operate.

6.4.1 – Coupling Tractor-Semitrailers

Step 1: Inspect Fifth Wheel

  • Check for damaged/missing parts.
  • Check to see that mounting to tractor is secure, no cracks in frame, etc.
  • Be sure that the fifth-wheel plate is greased as required. Failure to keep the fifth-wheel plate lubricated could cause steering problems because of friction between the tractor and trailer.
  • Check if fifth wheel is in proper position for coupling.
    • Fifth wheel tilted down toward rear of tractor.
    • Jaws open.
    • Safety unlocking handle in the automatic lock position.
  • If you have a sliding fifth wheel, make sure it is locked.
  • Make sure the trailer kingpin is not bent or broken.

Step 2: Inspect Area and Chock Wheels

  • Make sure area around the vehicle is clear.
  • Be sure trailer wheels are chocked or spring brakes are on.
  • Check that cargo (if any) is secured against movement due to tractor being coupled to the trailer.

Step 3: Position Tractor

  • Put the tractor directly in front of the trailer. (Never back under the trailer at an angle because you might push the trailer sideways and break the landing gear.)
  • Check position, using outside mirrors, by looking down both sides of the trailer.

Step 4: Back Slowly

  • Back until fifth wheel just touches the trailer. Do not hit the trailer.

Step 5: Secure Tractor

  • Put on the parking brake.
  • Put transmission in neutral.

Step 6: Check Trailer Height

  • The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed. (If the trailer is too low, the tractor may strike and damage the trailer nose; if the trailer is too high, it may not couple correctly.)
  • Check that the kingpin and fifth wheel are aligned.

Step 7: Connect Air Lines to Trailer

  • Check glad hand seals and connect tractor emergency air line to trailer emergency glad hand.
  • Check glad hand seals and connect tractor service air line to trailer service glad hand.
  • Make sure air lines are safely supported where they will not be crushed or caught while tractor is backing under the trailer.

Step 8: Supply Air to Trailer

  • From cab, push in “air supply” knob or move tractor parking valve control from the “emergency” to the “normal” position to supply air to the trailer brake system.
  • Wait until the air pressure is normal.
  • Check brake system for crossed air lines.
    • Shut engine off so you can hear the brakes.
    • Apply and release trailer brakes and listen for sound of trailer brakes being applied and released. You should hear the brakes move when applied and air escape when the brakes are released.
    • Check air brake system pressure gauge for signs of major air loss.
  • When you are sure trailer brakes are working, start engine.
  • Make sure air pressure is up to normal.

Multiple-Choice Questions:

Question #343 (1 of 1)

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When you're hooking up the tractor to the trailer, how high should the trailer be relative to the tractor?

  • None of these answers are correct
  • The trailer should be high enough that there is an inch of clearance between the 5th wheel and the trailer
  • The trailer should be high enough that the tractor does not touch the trailer as you back under it
  • The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it.
The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it.
This question will not be on the test, but it's very important in real life. You don't want the trailer too high or too low when attempting to back under it. If the tractor lifts the trailer just slightly as you back under it, you're at the perfect height.
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