Knock Sensor: Description and Operation

PURPOSE
The knock sensor is used to detect engine detonation. The control unit uses this signal to retard ignition timing when a knock occurs. This helps to maintain optimum ignition timing advance while avoiding damaging (and annoying) engine "PING" or "KNOCK" due to detonation in the combustion chambers.

OPERATION
The circuitry, within the knock sensor, causes the ECM's 5.0 volts to be pulled down so that under a no knock condition, CKT 496 would measure about 2.5 volts. The knock sensor produces an AC signal, which rides on the 2.5 volts DC voltage. When a knock occurs, the sensor produces a voltage "spike" that rides on the 2.5 volt bias. The resultant voltage "spike" rises above the thresh-hold voltage (about 3 volts) indicating an engine knock.

When this signal is detected by the PCM, ignition timing is retarded. When the knocking stops, the control module advances the timing in small increments back to its preset value. This allows the PCM to maintain optimum ignition timing advance for better fuel economy and performance under all operating conditions.