Electronic Spark Timing

The Electronic Spark Timing (EST) system uses an HEI distributor with a 7-terminal HEI module. The distributor has no provision for vacuum or centrifugal advance. The HEI distributor communicates a reference pulse (indicating engine RPM) to the ECM via a 4-terminal connector. The ECM determines the proper spark advance for engine operating conditions, then sends an "EST" pulse to the distributor.

The ECM controls spark advance under normal operating conditions. Under certain operating conditions such as cranking, or when setting base timing, the distributor can operate without ECM control. This operating condition is called the by-pass mode. It is determined by the by-pass lead from the ECM to the distributor. When the by-pass lead is 5 volts, the ECM will control the spark. When the by-pass lead is grounded or open-circuited, the HEI module will control the spark.

Disconnecting the 4-terminal EST connector causes the engine to operate in the by-pass mode. It will also cause trouble code 41 to be set in the memory. Use the by-pass mode when setting base timing. After setting base timing, clear trouble code 41 from the memory.