Ignition System - HEI/Est/ESC: Operation: Sensors

On EST systems, the coolant temperature sensor signals ECM to advance timing on a cold engine and return timing to programmed advance curve as engine reaches normal operating temperature. If engine overheats, spark is retarded to prevent detonation. During light throttle operation, throttle position sensor input to ECM allows for additional advance.

Spark advance is also governed by input from engine RPM and Manifold Absolute Pressure (MAP) sensor. When MAP output voltage is low (high vacuum), ECM gives less spark advance. More spark advance is given when MAP output voltage is high (low vacuum).