Carbureted Engines

With selector lever in "P" (auto. trans.) or Neutral (man. trans.), run engine at fast idle. Note timing change as "TEST" terminal is grounded. See Fig 1. If timing changes, system is operating properly.

Fig 1: Test Terminal Location Timing should change when test terminal is grounded.

CAUTION: Prior to placing automatic transmission in "D" position in following step, ensure emergency brake is FIRMLY applied and wheels are securely blocked.
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 selector lever in "D" and note timing change as test terminal is grounded. If timing changes, system is operating properly.
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).
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 wire from terminal "H" of ECM to park/neutral switch. If wire is not grounded, replace ECM.