Knock Sensor: Description and Operation

Knock Sensor (KS) Sensor:

Knock Sensor Circuit:

Purpose:
The Knock Sensors send an AC voltage signal to the ECM when a knock occurs in the engine. The ECM responds to this signal by retarding ignition timing. When detonation stops, timing is advanced in small increments back to a pre-programmed value for the engine speed, load, and temperature conditions present. This allows the ECM to maintain maximum timing advance under various engine load conditions.

Operation:
When abnormal engine vibrations occur (spark knock), slight distortions of an element in the sensors generate an AC voltage signal. The AC voltage produced depends on the severity of the knock. These signals are monitored by the ECM. The Electronic Spark Control (ESC) system, built into the ECM, varies ignition timing to maximize performance and minimize spark knock.

A resistor to ground within each knock sensor causes the ECM's 5 volt "bias" to drop, so that under a no knock condition CKT 496 should measure about 1.5 volts. Since the sensors are connected in parallel, if one or the other sensor or circuits is open or disconnected, the voltage in circuit 496 will be greater than 1.5 volts but less than 5.0 volts, under a no-knock condition.

Location:
The knock sensors are located in the lower sides of the engine block.