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 resultant voltage "spike" from the sensor oscillates about this 2.5 volt "bias". This voltage "spike" rises above the thresh-hold voltage (about 3 volts) indicating an engine knock.

The Mem-Cal used with this engine, contains the functions which were part of remotely mounted ESC modules used on other GM vehicles. The ESC portion of the Mem-Cal signals other parts of the ECM to retard the spark timing. 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.