GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Electronic Spark Timing (EST)


See Figures 1 and 2

All 1980 models with the 231 V6 engine and all 1981 and later models, except those with the 229 V6 engine, are equipped with EST. The EST distributor, as described in Sections 2 and 3 of this information, contains no vacuum or centrifugal advance mechanism, but is instead equipped with a seven terminal HEI module. It has four wires going to a four terminal connector in addition to the connectors normally found on the HEI distributors. A reference pulse, indicating engine rpm is sent to the ECM; terminal R on the 7-terminal HEI provides this pulse on all models (except the 229 V6 engine). The ECM determines the proper spark advance for the engine operating conditions and then sends an EST pulse back to the distributor.

The distributor for the 1985 and later 262 V6 engine is equipped with a modified module which has eight terminals.

Under most normal operating conditions, the ECM will control spark advance. However, under certain operating conditions such as cranking or when setting base timing, the distributor is capable of operating without ECM control. This condition is called BYPASS and is determined by the BYPASS lead which runs from the ECM to the distributor. When the BYPASS lead is at the proper voltage, the ECM 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.

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Fig. Fig. 1: Schematic of the EST circuitry (except the 262 V6 engine)

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Fig. Fig. 2: HEI system schematic with EST-1985 and later 262 V6 engine