E023 Part 1 of 2

Diagnostic Chart.:

Wiring Diagram.:

DESCRIPTION:

The EST distributor produces one 5 volt Distributor Reference Pulse each time that a cylinder reaches 10° BTDC (base timing). The reference pulses are sent from the distributor to the ECM on circuit 430 (the distributor reference signal). The ECM adds spark timing information to the reference pulses received from the HEI and sends out 5 volt Electronic Spark Timing pulses (EST) on circuit 423.

The ECM can choose to control spark timing. The HEI bypass circuit 424 is turned on by the ECM (approximately 5 volts) when the ECM wishes to control timing. When the ECM wants the HEI module to control timing, the HEI bypass circuit is turned off and the HEI module grounds circuit 423 (EST). The ECM sends EST pulses to the HEI; the HEI module shorts the EST pulses to ground when the bypass signal from the ECM to the module is low (less than .5 volts).

TEST CONDITION #1 CRANK:

1. Bypass line low (0 volts).
2. References pulses being sent by HEI module to the PCM/ECM.

TEST CONDITION #2 RUN:

1. Bypass line high (5 volts).
2. References pulses being received by the PCM/ECM.

FAILURE CONDITION #1:

EST pulses are detected on CKT 423 in bypass.

FAILURE CONDITION #2:

No EST pulses are detected on CKT 423.

ACTION TAKEN:

1. PCM/ECM turns on the "SERVICE ENGINE SOON" telltale.
2. The PCM/ECM controlled spark is disabled for ignition cycle.

NOTES ON FAULT TREE:

1. When circuit 423 is shorted to ground, the engine will have a long crank and hard start. When the ECM detects the grounded circuit, the bypass line will be shifted low.

2. With only distributor reference and distributor reference ground jumpered, engine will run at 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 circuit 423.

4. Checking for HEI module able to ground the EST signal with an open bypass circuit.

NOTE ON INTERMITTENTS:

Possible causes of code E23 are:

1. Reference pulses are being received by the PCM/ECM and the bypass line is low, but the PCM/ECM detects pulses on the EST line. This could be caused by the bypass line shorted to voltage at the distributor.

2. Reference pulses are being received by the PCM/ECM and the bypass line is high, and the PCM/ECM does not detect a signal on the EST line. This could be caused by an open or shorted to ground on the EST line or the bypass line open or a shorted to ground.