Suburban 3/4 Ton 4WD V8-305 5.0L (1986)
Engine Control Module: Description and Operation
Electronic Spark Timing (EST)
Electronic Spark Timing is used on all Calif. vehicles. The EST distributor contains no centrifugal or vacuum advance and uses a 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 5
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.
