Electronic Spark Control (ESC) System
The basic components of Electronic Spark Control (ESC) system are detonation (knock) sensor, HEI/EST distributor, controller (on some models) and ECM. On some fuel injected models, the function of the ESC controller is built into the Memory Calibration (MEM-CAL) unit of the ECM.
On controller-equipped systems, when detonation (knock) occurs, sensor sends an electrical signal to controller. Controller will then remove the signal voltage from the ECM-monitored knock signal wire. When the ECM senses a voltage drop (to less than one volt) on the knock sensor signal line, spark timing will be retarded. The ECM will retard spark timing until signals from detonation sensor cease.
On vehicles using ECMs containing MEM-CAL units, the ECM supplies and monitors a 5-volt reference signal on the knock sensor signal line. Internal circuitry of the knock sensor will pull this voltage down to about 2.5 volts. When knock occurs, the knock sensor produces an AC voltage signal which rides on the 2.5-volt DC signal. The voltage and frequency of this signal depend upon knock signals received by the sensor. The ECM will retard spark timing until signals from detonation sensor cease.