DELCO-REMY Electronic Spark Timing Ignition System: Operation

Part of the ECM, Programmed Read Only Memory (PROM), carries basic engine timing based on engine speed only. The ECM uses this basic timing (from PROM) plus information from other engine sensors to advance or retard engine timing to suit driving conditions.

Fig 1: General Motors Electronic Spark Timing Wiring Schematic

Courtesy of GENERAL MOTORS COMPANY