All models use EST. The EST distributor, as described in an earlier section, contains no vacuum or centrifugal advance mechanism and uses a seven terminal HEI module. It has four wires going to a four terminal connector in addition to the connectors normally found on HEI distributors. A reference pulse, indicating engine rpm is sent to the ECM/PCM. The ECM/PCM determines the proper spark advance for the engine operating conditions and then sends an EST pulse back to the distributor.
Under most normal operating conditions, the ECM/PCM will control the spark advance. However, under certain operating conditions such as cranking or when setting base timing, the distributor is capable of operating without ECM/PCM control. This condition is called BYPASS and is determined by the BYPASS lead which runs from the ECM/PCM to the distributor. When the BYPASS lead is at the proper voltage (5), the ECM/PCM will control the spark. If the lead is grounded or open circuited, the HEI module itself will control the spark. Disconnecting the 4-terminal EST connector will also cause the engine to operate in the BYPASS mode.