ECM Code EO12 - No Distributor (Tach) Signal: Notes

TEST CONDITIONS:  Code EO12 is tested for during engine cranking operation (crank input to the ECM at system voltage).

FAILURE CONDITIONS:  If ECM does not see distributor reference pulses for 2.1 seconds with crank input to ECM at system voltage, code EO12 will be set.

ACTION:  ECM turns on "ENGINE CONTROL SYSTEM" light. ECM commands BCM to display "SERVICE NOW" message on CCDIC panel.

Possible causes of no distributor reference pulses are, open pick-up coil. HEI module unable to process pick-up coil signals. Open or shorts on circuit Nos. 430 and 453 from module 4-pin connector to ECM. Loss of 12 volts to "BAT" terminal of HEI.

1. This step checks for proper voltage of HEI system. If voltmeter shows .5-2.8 volts, HEI is producing reference pulses.
2. This step checks for proper ground connection between ECM and engine and also between ECM and HEI.
3. This step checks for proper voltage through circuit NO. 430BS from HEI to ECM. If ECM terminal "A7" sees .5-2.8 volts, then ECM is receiving reference pulses.
4. If HEI will produce spark, then fault is not within pick-up coil or module. Check circuit No. 430BS for opens or shorts to ground from module terminal "R" to pin "B" of 4-pin connector.