The Electronic Spark Control (ESC) system is a closed loop system that controls engine detonation by adjusting the spark timing. There are two basic components in this system, the controller and the sensor.
The controller processes the sensor signal and remodifies the EST signal to the distributor to adjust the spark timing. The process is continuous so that the presence of detonation is monitored and controlled. The controller is not capable of memory storage.
The sensor is a magnetorestrictive device, mounted in the engine block that detects the presence, or absence, and intensity of detonation according to the vibration characteristics of the engine. The output is an electrical signal which is sent to the controller.
Service on vehicles ESC system is complex and should be maintained by a qualified mechanic.