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Carbureted Engines

  1. With transmission in Park of Neutral, run engine at fast idle. Note timing change as the ALDL "test" terminal is grounded. See Figure. If timing changes, system is operating properly.
    CAUTION: Prior to placing automatic transmission in Drive in following step, ensure emergency brake is FIRMLY applied and DRIVE wheels are securely blocked.
  2. If timing does not change on vehicles with manual transmissions, go to step 3). If timing does not change on vehicles with automatic transmissions, let engine return to idle. Place transmission in Drive and note timing change as "test" terminal is grounded. If timing changes, system is operating properly.
  3. If no change in timing occurs, check voltage at output of MAP sensor or vacuum sensor as vacuum hose is removed and reinstalled. If no change occurs, problem is in the vacuum or MAP sensor circuit. If voltage output changes, proceed to step 4).
  4. On vehicles with automatic transmission, disconnect Park/Neutral switch and recheck for timing change. If timing changes, problem is either an improperly adjusted or faulty Park/Neutral switch. If timing does not change, check for grounded Park/Neutral signal wire between ECM and Park/Neutral switch. If wire is not grounded, replace ECM. Prior to replacement of ECM, see ECM REPLACEMENT in the appropriate THEORY & OPERATION article in the ENGINE PERFORMANCE section.
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