Ignition System - HEI-Est: Operation: Sensors

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 provides more spark advance. Less spark advance is given when MAP output voltage is high (low vacuum), helping to reduce the possibility of engine detonation.

On engines not equipped with MAP sensors, ECM will calculate spark advance using information from Mass Airflow (MAF) sensor, coolant temperature sensor and engine RPM. When engine is under minimum load, based on low RPM and low mass airflow signals, more spark advance is given. When engine is under heavy load based on high RPM and high mass airflow signals, less spark advance is given to eliminate possible detonation.