Knock Sensor (KS) System Description: Operation

The KS detects when the engine is experiencing detonation. The KS (1) has a Piezo ceramic element (3) that generates a signal at the same resonance point as the expected knocking frequency of the engine. When engine detonation is detected, the sensor signals the PCM to reduce the spark advance until detonation is no longer detected.

In response to the KS signal the PCM retards the spark advance in order to reduce the detonation. The amount of timing retard that the PCM applies is based on the engine speed and the length of time that the engine detonation is detected. Once the spark timing is retarded, the KS circuitry in the PCM performs calculations in order to determine how much spark advance should be re-introduced. Normally the ignition timing advance is increased until zero retard, or normal ignition timing, is re-established. If detonation occurs again, the whole cycle will repeat. The alteration of the ignition timing by the KS often occurs continuously while the engine is running, even though no detonation is heard by the vehicle's operator.

Fig 1: Identifying Knock Sensor Components

Courtesy of GENERAL MOTORS CORP.