Hard Trouble Code E23

NOTE: If this code is not displayed during second pass of diagnostic codes, it is an intermittent failure and cannot be diagnosed using this procedure.

When engine is cranking, ECM pulls HEI by-pass line (circuit No. 424) down to 0 volts and the 0 volt signal commands HEI module to use base timing. During this by-pass mode, HEI module grounds spark timing pulses that are supplied by ECM on EST line (circuit No. 423). When engine starts, ECM pulls HEI by-pass line up to about 5 volts and the 5 volt signal commands HEI module to use spark timing pulses that are supplied by ECM and EST line.

The ECM monitors EST line to determine if HEI module is operating properly. Trouble code indicates that ECM has failed to see proper EST signal on circuit No. 423. Possible causes are: Defective HEI module, defective wiring, terminals, etc., or ECM not processing EST signal properly or not pulling by-pass line up to 5 volts.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

Use the jumpers described previously in this article. Disconnect 4-way connector at distributor. Install jumper wires as explained.
With terminals B and D reconnected, EST signal can be measured on terminal A after the vehicle is started. If voltage is not between 2.0 and 2.8 volts, an open or short must exist either in circuit No. 423 or ECM itself.
With circuit No. 423 between 2.0 and 2.8 volts, next step is to reconnect terminal A and determine if HEI module is grounding EST signal properly in by-pass mode. Make sure ignition is left off for at least 10 seconds to allow ECM to "power down". If voltage is greater than 0.5 volts with engine idling, then circuit No. 423 is open either in distributor harness or HEI module itself.
With circuit No. 423 below 0.5 volts, 12 volts should then be applied to distributor harness terminal C through resistance of test light. This will force HEI module out of by-pass mode. If voltage on circuit No. 423 is not 0.5-2.0 volts, or if engine stalls, fault is in distributor harness wiring or HEI module itself.
If voltage on circuit No. 423 is now between 0.5 and 2.0 volts, turn ignition off for at least 10 seconds to allow ECM to "power down". With terminal C reconnected and engine started, circuit No. 462 should go to greater than 2.0 volts. If voltage is greater than 2.0 volts, system is working properly at this time. If voltage remains below 2.0 volts, fault is either in circuit No. 424 to ECM or in ECM itself.

Fig 1: Hard Trouble Code E23

Fig 2: Hard Trouble Code E23