Diagnostic Aids

1989 GMC Rally G3500, 7.4 N, 400/M40SECTION Diagnostic Aids

SCAN tester reading should closely match speedometer reading with drive wheels turning. See PARK/NEUTRAL SWITCH DIAGNOSIS chart if vehicle is equipped with automatic transmission. If Park/Neutral switch is okay, see INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

NOTE: Disregard CODE 24 if set while drive wheels are not turning.
RENDER: 1.0x

NO RELATED

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • β€’ You smell fuel, burning insulation, or see smoke.
  • β€’ Brakes feel soft, pull hard to one side, or make grinding noises.
  • β€’ The engine overheats, stalls repeatedly, or misfires under load.
  • β€’ You are missing required tools, torque specs, or safe lifting equipment.
  • β€’ You are not confident in the next step or safety outcome.