E023

TEST CONDITIONS
Code EO23 is tested under two separate conditions:
A.
1. Bypass line (CKT 424) low (0 volts);
2. Reference pulses being received by the ECM.
B.
1. Bypass line (CKT 424) high (5 volts);
2. Reference pulses being received by the ECM.

FAILURE CONDITIONS
A. EST pulses detected on CKT 423.
B. No EST pulses detected on CKT 423.

ACTION TAKEN
1. ECM turns on "Engine Control System Fault" telltale;
2. ECM commands BCM to display "Service Soon" message on Driver Information Center (DIC);
3. AIR is diverted to the air cleaner;
4. ECM-controlled spark is disabled for ignition cycle.

DESCRIPTION
Ignition spark is controlled by either the ECM or the HEI module. The bypass line (CKT 424) is used as the switch to determine which one will control timing. When the Bypass line is low, the HEI module ignores the EST signal (grounds CKT 423) and controls spark by built in calibration (called backup spark). When the ECM commands the Bypass line high (5 volts), the HEI module accepts EST signals sent from the ECM. The ECM controls spark by modifying base timing pulses sent from the HEI module on the distributor reference line (CKT 430). One reference pulse is sent every time a cylinder reaches 10° BTDC (base timing). The ECM adds spark timing information to the reference and sends it out on the EST line (CKT 423).

Code EO23 will set under either of the two following conditions:
1. Reference pulses are being received by the ECM (engine is turning) and the bypass line is low (HEI module should be grounding the EST line) but the ECM detects pulses on the EST line (line not grounded). This could be caused by an open EST line (CKT 423) or a shorted to ground bypass line (CKT 424).
2. Reference pulses are being received by the ECM (engine is turning) and the bypass line is commanded high (ECM controlling spark) but the ECM detects a grounded EST line (CKT 423). This could be caused by an open or shorted to ground bypass line or a shorted to ground EST line.

Possible causes of Code EO23 are:
1. CKT 423 (EST) open/grounded;
2. CKT 424 (Bypass) open/grounded;
3. CKT 453 (ground) open at the HEI or ECM;
4. ECM poor connection to battery negative through CKT 1030;
5. ECM or HEI module faults.

Notes on Fault Tree
1. Four jumper wires must be obtained for use in the diagnostic procedure. The jumpers should be about 12 inches long with a male and female weatherpack connector on either end (P/N 12014836 and 12014837). With the distributor connector disconnected, use the jumpers to reconnect the terminals per instruction provided on the Code EO23 chart.
2. With only Distributor reference and Distributor reference ground jumpered to the ECM, the engine will run on backup spark. Checking for proper ground connection between ECM and the engine and the ECM and HEI.
3. Checking for ECM providing an EST output on CKT 423.
4. Checking for HEI module able to ground EST signal with open Bypass circuit.
5. Checking for HEI module able to recognize a voltage on the Bypass circuit and to stop grounding EST (ECM controlled timing).
6. Checking for Bypass signal to the module. If Bypass is being sent by the ECM to the HEI and if the module is interpreting Bypass voltage correctly, then the module will switch off the ground to the EST.
7. If the chart leads to "EST circuit is OK", then a fault may exist in the 6-way weatherpack connector. Check for proper connector mating and for pins backing out of the weatherpack. If no trouble is found with the connector, reconnect the 6-way connector, clear codes and retest.