Knock Sensor: Description and Operation

PURPOSE
Varying octane levels in today's gasoline can cause detonation in an engine. This detonation is sometimes called a spark knock.

The Knock Sensor (KS) system is used to detect engine detonation. This allows the engine to have maximum spark advance for improved driveability and fuel economy.


OPERATION
A Vehicle Control Module (VCM) is used in conjunction with one knock sensor in order to control detonation. On a VCM application no KS module will be found as it is internal to the control module.

A 5 volt reference is applied to the knock sensor which has an internal resistance of about 100,000 ohms. This resistance will lower the applied voltage to about half or 2.5 volts. When a knock is present, a small AC voltage is produced by the knock sensor and transmitted to the control module riding on top of the already existing 2.5 volts. An AC voltage monitor inside the control module will detect the knock and trigger the control module to start retarding the spark incrementally.

The VCM will retard the spark timing based on the signals from the KS module. The Knock Sensor(s) produce an AC voltage that is sent to the KS module. The amount of AC voltage produced is proportional to the amount of knock. An operating engine produces a normal amount of engine mechanical vibration (Noise). The knock sensor(s) will produce an AC voltage signal from this Noise. When an engine is operating, the VCM will learn the minimum and maximum frequency of the noise the engine produces. When the VCM determines that this frequency is less than or greater than the expected amount, a knock sensor DTC will set.