Engine Speed Check
The ECM monitors engine speed through the tach lead from terminal "R" of the alternator. With key "ON," CEL will remain "ON" until engine speed exceeds 400 RPM. If tach signal is lost, CEL will remain "ON" after engine exceeds 400 RPM.
- This step checks for a 5 volt reference. Normal reading should be about 5 volts. If voltage reference is lost, CEL will remain "ON" at any RPM.
- This step checks for a grounded wire on the ECM side of the CEL bulb circuit. A grounded wire will keep CEL "ON" with engine running.
- This step checks for tach signal output for alternator. This signal should be more than 7 volts A.C. at idle and increase slightly with increased RPM. If voltage is more than 7 volts A.C., fault is not in alternator or wiring up to point of measurement.