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CDL · Inspection

During the pre-trip inspection of a doubles combination, what must be verified about the dolly's connection to the second semitrailer?

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

The dolly's fifth wheel must be properly coupled to the second trailer's kingpin (using the same verification as a primary fifth wheel coupling: tug test, jaw engagement, no high hookup); safety chains crossed; air lines connected; lights functioning

  1. A The dolly's fifth wheel must be properly coupled to the second trailer's kingpin (using the same verification as a primary fifth wheel coupling: tug test, jaw engagement, no high hookup); safety chains crossed; air lines connected; lights functioning
  2. B The second connection requires no inspection
  3. C Only that the dolly is visible
  4. D Only verify the dolly's tires

Why this is the answer

The dolly-to-second-trailer coupling is a full fifth wheel coupling that requires the identical inspection as the primary tractor-to-trailer coupling: back the dolly under the second trailer's kingpin at the correct height; confirm jaw engagement; perform the tug test with the dolly's brakes set; verify no high hookup; connect air lines and electrical; verify safety chains are crossed with correct slack. A failed dolly coupling at highway speed separates the rear half of the combination.
Source: FMCSA CDL Manual, Chapter 7, Doubles and Triples, Dolly-to-Trailer Coupling Inspection

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About this topic: Inspection

Air brake inspection is its own structured procedure: build-up, leakage rate, low-air warning, and spring-brake activation must all check out before you drive. This is one of 15 Doubles and Triples questions on inspection, each with a full explanation on a single study page.

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