Knock Sensor: Description and Operation

PURPOSE
Varying octane levels in today's gasoline can cause detonation in high performance engines. Detonation is sometimes called spark knock. The Knock Sensor (KS) is used to detect abnormal vibration (spark knocking) in the engine. The Powertrain Control Module (PCM) will retard the ignition control timing up to 10° to reduce spark knock in the engine based on the signal being received. This allows the engine to use maximum spark advance to improve driveability and fuel economy.
The KS system has two major components:
^ The knock sensor.
^ The KS module (located in the PCM).

OPERATION
The circuitry within the knock sensor causes the PCM's 5 volts to be pulled down so that under a no knock condition circuit 496 would measure about 2.5 volts. The KS produces an AC output voltage which increases with the severity of the knock signal and rides on the 2.5 volts DC voltage. The amplitude and frequency are dependent upon the knock level. The PCM used with this engine contains a KS "module" which contains the spark control logic. Spark timing is retarded by this circuit which reduces engine detonation. The KS module receives the signal on circuit 496, then commands the PCM to retard spark timing.