Ignition System - HEI-Est: Operation: Sensors
The coolant temperature sensor advances spark on a cold engine and retards advance as the engine reaches normal operating temperature. If the engine is too hot, spark is retarded to prevent detonation. During light throttle operation, the throttle switch allows for additional advance.
Spark advance is also governed by input from coolant temperature, engine RPM, and Manifold Absolute Pressure (MAP) sensors. When MAP output voltage is low, ECM gives more advance. Less spark is given when MAP output voltage is high.
On engines with port fuel injection, the ECM will calculate spark advance using information from the Mass Air Flow (MAF) sensor, coolant temperature, and engine RPM. When the engine is under minimum load based on RPM and low air flow, more spark advance is given. When engine is under heavy load based on RPM and high air flow, less spark advance is given.