ECM Code EO23 - Est Circuit Problem (1 Of 2): Action:

ECM turns on "ENGINE CONTROL SYSTEM" light. ECM commands BCM to display "SERVICE SOON" message on CCDIC. If ECM sees 5-volt EST pulses with by-pass voltage at zero, caused by EST circuit open or by-pass circuit shorted to voltage, the ECM will not enable spark timing. The engine will start and run on base timing. If EST circuit is shorted to ground, the vehicle will start and stall when engine reaches 300 RPM (as by-pass turns on). When restarted, the ECM will not try to turn by-pass on, code EO23 will set, and engine will start and run on base timing.

The EST distributor produces one 5-volt reference pulse each time that a cylinder reaches 10° BTDC (base timing). These reference pulses are sent from distributor to the ECM over circuit No. 430.

The ECM adds spark timing information to the reference pulses received from the HEI module and sends out a 5-volt electronic spark timing pulses over circuit No. 423.

The ECM can choose to control spark timing. The HEI by-pass circuit is turned on by the ECM when the ECM wishes to control timing. When timing control is desired, the HEI by-pass circuit is turned off and the HEI module ground circuit No, 423. The ECM always sends EST pulses to the HEI, the HEI module shorts EST pulses to ground when the by-pass signal from the ECM to module is low (less than .5 volts).

NOTE: Test numbers refer to test numbers on diagnostic chart.

Four 12" jumper wires must be obtained for use in diagnostic procedure. Jumper wires MUST have male and female Weatherpack Connectors (12014836 and 12014837) on either end. With distributor connector unplugged, use jumper wires to reconnect terminals. Use instructions provided in chart.
With only distributor reference and distributor reference ground jumpered, engine will run on back-up spark control. This step checks for proper ground connection between ECM and engine, and between ECM and HEI.
This step ensures that ECM is providing an EST output signal over circuit No. 423.
This step checks the ability of the HEI module to ground EST signal with an open by-pass circuit.

Fig 1: Schematic, Code EO23, EST Circuit Problem

Fig 2: Flow Chart, Code EO23, EST Circuit Problem (1 Of 2)

Fig 3: Flow Chart, Code EO23, EST Circuit Problem (1 Of 2)

Fig 4: Flow Chart, Code EO23, EST Circuit Problem (2 Of 2)