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CDL · Coupling and Uncoupling

When completing a coupling, you pull forward gently against the locked kingpin to test the connection. The trailer does NOT move but you hear a loud 'clunk'. What does this indicate?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

A high hookup — the kingpin may be resting on the fifth wheel lip rather than locked in the jaws; visually inspect from the side before moving

  1. A The landing gear needs adjustment
  2. B Successful coupling — the clunk is the fifth wheel locking
  3. C A broken kingpin
  4. D A high hookup — the kingpin may be resting on the fifth wheel lip rather than locked in the jaws; visually inspect from the side before moving

Why this is the answer

A 'clunk' during the tug test without trailer movement can indicate the fifth wheel jaws are resting ON the kingpin rather than locked AROUND it — a 'false coupling.' This can hold briefly but release at speed or on uneven road, dropping the trailer. Always visually confirm from the side: you should not be able to see daylight between the upper and lower fifth wheel plates, and the locking jaw should be fully closed around the kingpin shank, not the head.
Source: FMCSA CDL Manual Chapter 6, Coupling Verification

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About this topic: Coupling and Uncoupling

A bad coupling can drop the trailer or separate it from the tractor at speed. The tug test, visual inspection of the kingpin, and proper line-up are the defenses. This is one of 19 Combination Vehicles questions on coupling and uncoupling, each with a full explanation on a single study page.

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