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HOMESERVICE MANUALSCHEVROLET1987HI-CUBE 6.2 J, 400/M40REPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 24 (ENGINE CONTROLS - TESTS W/CODES - 6.5L DIESEL)DIAGNOSTIC TROUBLE CODE CHARTSDIAGNOSTIC SYSTEM CHECK
1987 Chevrolet Hi-Cube 6.2 J, 400/M40

Diagnostic System Check

1987 Chevrolet Hi-Cube 6.2 J, 400/M40SECTION Diagnostic System Check
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1994 GMC Yukon, 1994 GMC Suburban, 1994 GMC Pickup, 1994 GMC Cab & Chassis, and 1994 GMC C3500 HD. However, it is still accessible from the selected car via links, so may be relevant.

The diagnostic system check is an organized approach to identifying a problem created by a control module system malfunction. This is the starting point for any driveability complaint diagnosis, as it directs technician to the next logical step in diagnosis, helps reduce diagnostic time and prevents the unnecessary replacement of good parts.

NOTE: Test numbers refer to numbers on diagnostic chart.
  1. This tests MIL operation. With ignition on and engine off, light should be on.
  2. This step will isolate of customer complaint is a MIL or driveability problem.
  3. Although the control module is powered up, a symptom could exist because of a system fault.
  4. Use Tech-1 to aid in diagnosis (to check if serial data is available). If a PROM error is present, the PCM may have been able to flash DTC 12 or 51, but not enable serial data.
  5. Although the control module is powered up, a "CRANKS BUT WILL NOT RUN" symptom could exist because of a PCM or system fault.
  6. This step will isolate if customer complaint is a MIL or driveability problem with no MIL displayed. See DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION  under TROUBLE CODE IDENTIFICATION, to determine if code is valid. An invalid DTC may be the result of a faulty scan tester, PROM or PCM.
  7. Comparison of actual control system data with typical scan tester data values is a quick check to determine if any parameter is not within limits. A base engine problem (i.e. advanced cam timing) may substantially alter sensor values. See TYPICAL TECH-1 DATA VALUES  in introduction.
Fig 1: Diagnostic System Check Schematic ("C" & "K" Series)
G94F65602
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When to See a Mechanic

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

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