Ignition System - HEI-Est (CALIF): Operation
During cranking or EST failure, a by-pass signal from ECM terminal 11 to HEI module terminal "B" is either absent or low. See Fig 1. This notifies the HEI module to ignore EST information coming from the ECM. During this period, poor engine performance may result but, the diagnostic "CHECK ENGINE" light will not come on.
The HEI module transmit RPM signals through the by-pass switch and terminal "C" directly to the negative "TACH" terminal of the ignition coil. These signals cause electrical pulses in the ignition coil primary circuit creating high voltage surge by the secondary windings to fire the spark plugs.
The Programmed Read Only Memory (PROM) portion of the ECM carries the basic spark advance curve based on engine RPM. Spark timing is calculated by the ECM whenever an ignition pulse is present. Spark advance information is sent to the distributor when the engine is running (not during cranking). Engine sensor values are used by the ECM to modify the PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. The coolant temperature switch informs the system when engine is cold so that ignition timing can be advanced. Advance will be reduced as engine reaches normal operating temperature. As the engine reaches high temperature, timing is retarded to prevent detonation.
The ECM is continually computing sensor information to maintain efficient engine performance with low emission levels. The system must function under constantly changing engine conditions.