Seville V8-273 4.5L (1989)
Ignition Control Module: Description and Operation
Ignition Module
The EST distributor contains no centrifugal or vacuum advance, but instead uses a modified seven terminal High Energy Ignition (HEI) module. A
four-terminal connector is used for EST use, in addition to the connectors normally found on HEI distributors. All spark timing changes are controlled by
the ECM, which monitors information from the various engine sensors, computes the correct spark timing, then signals the distributor to adjust the
timing accordingly. The distributor operates without ECM control during cranking or when setting basic ignition timing. This is known as the "Bypass"
mode and it is determined by the bypass lead from the ECM to the distributor. When the bypass lead voltage is approximately 5.0 volts, the ECM
performs the spark timing function. However, 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. The EST connector should be
disconnected whenever setting basic ignition timing.
