DTC 23

Fig. 11 High energy ignition wiring circuit:

During engine cranking, the ECM pulls HEI bypass line (circuit 462) down to 0 volts. This 0 volt signal commands the HEI module to use base timing. During this ``bypass'' mode, the HEI module grounds spark timing pulses supplied by the ECM on EST line (circuit 423). When the engine starts, the ECM pulls the HEI bypass line up to approximately 5 volts. This 5 volt signal commands the HEI module to use spark timing pulses supplied by the ECM on the EST line. Code 23 indicates that ECM has failed to see correct EST signal on circuit 423. Possible causes of this condition are:
a. Defective HEI module.
b. Open set timing connector.
c. Defective wiring or electrical connectors.
d. ECM failing to process signal properly or pull bypass line up to 5 volts.
1. Ensure set timing connector is not open.
2. Disconnect 4-way connector from distributor.
3. Connect jumper wire between distributor terminal B and harness terminal B. Connect a second jumper wire between distributor terminal D and harness terminal D.
4. Start engine and run at idle until normal operating temperature is reached.
5. Measure voltage on ECM side of terminal A. If voltage is between 1.0 and 2.8 volts, proceed to step 6. If voltage is not between 1.0 and 2.8 volts, check circuit 423 for an open or short to ground. If circuit is satisfactory, check for faulty ECM or ECM connector and repair or replace as necessary.
6. Turn ignition off for a minimum of 10 seconds, then connect a third jumper wire between distributor terminal A and harness terminal A.
7. Run engine at idle and measure voltage at terminal A. If voltage is less than .5 volt, proceed to step 8. If voltage is .5 volt or more, check circuit 423 between connector and HEI module for an open. If circuit is satisfactory, check for faulty module or HEI module connector and repair or replace as necessary.
8. Connect a test lamp between distributor terminal C and 12 volt power source and recheck voltage at terminal A with engine idling. If terminal A voltage is now .5 to 2.0 volts, proceed to step 9. If voltage is not .5 to 2.0 volts, check for an open or short to ground in circuit 462 between connector and HEI module. If engine is stalled, check circuit 423 for a short to ground between connector and HEI module. If circuits are satisfactory, check for a faulty HEI module or module connector and repair or replace as necessary.
9. Turn ignition off for a minimum of 10 seconds, then connect a fourth jumper wire between distributor terminal C and harness terminal C.
10. Run engine at idle and measure voltage at terminal C. If voltage is greater than 2.0 volts, there is no problem in the system at this time. If voltage is 2.0 volts or less, check circuit 462 for an open or short to ground between connector and ECM. If circuit is satisfactory, check for faulty ECM or ECM connector and repair or replace as necessary.