Ignition System - HEI-Est: Operation: Sensors
The coolant temperature sensor advances timing on a cold engine and retards timing as the engine reaches normal operating temperature. If the engine overheats, 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 engine RPM, and Manifold Absolute Pressure (MAP) sensors. When MAP output voltage is low, the ECM gives more advance. Less spark advance 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, increased spark advance is given. When engine is under heavy load based on RPM and high air-flow, less spark advance is given.