Action Taken
- ECM turns on SERVICE SOON light.
- If ECM sees 5-volt EST pulses with by-pass voltage at zero volts (caused by EST circuit open or by-pass circuit shorted to voltage), ECM will not enable spark. The engine will start and run on base timing.
- If EST circuit is shorted to ground, vehicle will start and stall when engine reaches 648 RPM (when EST is enabled). When restarted, ECM will not try to turn by-pass on. Code E23 will set, and vehicle will start and run on base timing.
The EST distributor produces one 5-volt distributor reference pulse each time a cylinder reached 10 degrees BTDC (base timing). The reference pulses are sent from distributor to ECM on circuit No. 430 (distributor reference signal).
The ECM adds spark timing information to reference pulses received from HEI and sends out 5-volt EST on circuit No. 423.
The ECM can choose to control spark timing. The HEI by-pass circuit No. 424 is turned on by ECM (about 5 volts) when ECM wishes to control timing. When ECM wants HEI module to control timing, HEI by-pass circuit is turned off and HEI module grounds circuit No. 423 (EST). The ECM always sends EST pulses to HEI. The HEI module shorts EST pulses to ground, when by-pass signal from ECM to module is low (less than .5 volt).
Test Number 1) Four jumper wires must be obtained for use in diagnostic procedure. The jumper wires should be about 12" long with Weatherpack Connector (12014836 and 12014837) on either end. With distributor connector disconnected, use jumpers to reconnect terminals per instructions provided on Code E23 chart.
Test Number 2) With only distributor reference and distributor reference ground jumpered, engine will run at back-up spark. Check for proper ground connection between ECM and engine, and ECM and HEI.
Test Number 3) Check for ECM providing an EST output on circuit No. 423.
Test Number 4) Check for HEI module ability to ground EST signal with open by-pass circuit.