LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> GMC >> 1987 >> Forward Control 5.7 M, Standard >> Repair and Diagnosis >> Engine Performance >> System >> CCC TBI Tests W/Codes >> Testing & Diagnosis >> Basic Diagnostic Procedures
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Basic Diagnostic Procedures

NOTE: Most computerized engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoes. Before considering the computer system as a possible cause of problems ignition high tension wires, fuel supply, electrical connections and vacuum hoses should be checked. Failure to do so may result in lost diagnostic time.

Diagnosis of the CCC system should be performed in the following order:

  1. Make sure that all engine systems not related to the CCC system are operating properly. Do not proceed with testing unless all other problems have been repaired.
  2. Go the SYSTEM CHECK chart and follow all instructions given there to verify proper operation of ECM self-diagnostics and to obtain any stored trouble codes.
  3. If trouble codes were displayed, determine whether the codes are "intermittent" or "hard". Go to numbered CODE CHARTS for further diagnosis of stored trouble codes.
  4. If no trouble codes were displayed, proceed to FIELD SERVICE MODE check. This will determine if CCC system can enter "Closed Loop" mode of operation.
  5. If no trouble is indicated by any of these charts, use the TROUBLE SHOOTING material in this article. The comments there will send you to the proper component charts or tell you what to fix.
  6. After any repairs have been made, always perform SYSTEM CHECK. Clear any trouble codes.
NOTE: Each of the steps listed here are described later in this section. If you are unsure of the proper way to test, read through the following material.
scientia non olet · About LEMON Manuals