Knock Sensor: Description and Operation

PURPOSE
Varying octane levels in today's gasoline may cause detonation in some engines. Detonation is caused by an uncontrolled explosion (burn) in the combustion chamber. This uncontrolled explosion could produce a flame front opposite that of the normal flame front produced by the spark plug.

The "rattling" sound normally associated with detonation is the result of two or more opposing pressures (flame fronts) colliding within the combustion chamber. Though "light" detonation is sometimes considered normal, "heavy" detonation could result in engine damage.

To control spark knock, a Knock Sensor (KS) system is used. This system is designed to retard spark timing up to 15° to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

OPERATION
The KS system has two major components:
^ PROM.
^ Knock sensor.

The knock sensor detects abnormal vibration (spark knocking) in the engine. The sensor is mounted in the engine block near the cylinders. The sensor produces an AC output voltage which increases with the severity of the knock. This signal voltage inputs to the Powertrain Control Module (PCM). The PCM then adjusts Ignition Control (IC) timing to reduce spark knock.