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Entering Diagnostic Mode

  1. Turn ignition on (engine off). "CHECK ENGINE" lamp should glow. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness under instrument panel. Insert lug terminal across "TEST" terminal and "GROUND" terminal. See Fig 1.
    NOTE: Early model "J" cars are equipped with a 5-pin ALDL connector. Later model "J" cars and all other models are equipped with a 12-pin connector.
    Fig 1: Exploded View of ALDL Connectors
    G09340223Courtesy of GENERAL MOTORS COMPANY
  2. "CHECK ENGINE" lamp should flash code "12". Code "12" consists of "FLASH", pause, "FLASH", "FLASH" followed by a longer pause. Trouble code "12" will be repeated 2 more times, then if any trouble codes are stored in ECM memory, they will be displayed in same manner.
  3. Trouble codes will be displayed from lowest to highest numbered code (3 times each) and be repeated as long as the "TEST" terminal of ALDL connector is grounded.
    CAUTION: Inserting lug in terminals of ALDL connector grounds "TEST" terminal lead. Do not ground ALDL connector before ignition is turned on or engine is started.
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