Ignition System: Description and Operation

Timing Control
All spark timing changes are controlled by the ECM--distributor contains no centrifugal or vacuum advance--which monitors information from the various engine sensors, computes the correct spark timing, and adjusts the timing accordingly.

Module
EST distributor uses a seven terminal High Energy Ignition (HEI) module

Distributor Connectors
A four-terminal connector is used for EST use, in addition to the connectors normally found on HEI distributors.

Bypass Mode
The distributor operates in bypass mode--without ECM control--during cranking or when setting basic ignition timing.

Bypass mode is controlled through the bypass lead from the ECM to the distributor.
- When the bypass lead voltage is approximately 5 volts the ECM performs the spark timing function.
- When the bypass lead is grounded or open circuited the HEI module assumes the spark control function.
- Disconnecting the four terminal EST connector causes the engine to automatically operate in the bypass mode.

NOTE: Before setting basic ignition timing always disconnect the EST connector.