Chevrolet Full Size Cars 1979-1989

Electronic Spark Timing (EST)


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Fig. Fig. 1 Common electronic spark control module

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Fig. Fig. 2 Schematic of the early model EST circuitry

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Fig. Fig. 3 HEI system schematic with EST 1985 and later 262 V6 engine

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Fig. Fig. 4 Remote coil/EST schematic, late model 305 and 350 Chevrolet produced V8 engines

All 1980 models with the 231 V6 engine and most 1981-89 models, except some with the 229 V6 engine, are equipped with Electronic Spark Timing (EST). The EST distributor, as described in Sections 2 and 3 of this guide, 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. 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 the circuit is opened, 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.