Knock Sensor: Description and Operation

PURPOSE
The Knock Sensor (KS) system enables the Powertrain Control Module (PCM) to control the ignition timing advance for the best possible performance while protecting the engine from potentially damaging levels of detonation. The sensors in the KS system are used by the PCM as microphones to listen for abnormal engine noise that may indicate pre-ignition/detonation.

SENSOR DESCRIPTION
There are 2 types of KS currently being used:
^ The broadband single wire sensor
^ The flat response two wire sensor

Both sensors use piezo-electric crystal technology to produce and send signals to the PCM. The amplitude and frequency of this signal will vary constantly depending on the vibration level within the engine. Flat response and broadband KS signals are processed differently by the PCM. The major differences are outlined below:
^ All broadband sensors use a single wire circuit. Some types of controllers will output a bias voltage on the KS signal wire. The bias voltage creates a voltage drop the PCM monitors and uses to help diagnose KS faults. The KS noise signal rides along this bias voltage, and due to the constantly fluctuating frequency and amplitude of the signal, will always be outside the bias voltage parameters. Another way to use the KS signals is for the PCM to learn the average normal noise output from the KS. The PCM uses this noise channel, and KS signal that rides along the noise channel, in much the same way as the bias voltage type does. Both systems will constantly monitor the KS system for a signal that is not present or falls within the noise channel.
^ The flat response KS uses a two-wire circuit. The KS signal rides within a noise channel which is learned and output by the PCM. This noise channel is based upon the normal noise input from the KS and is known as background noise As engine speed and load change, the noise channel upper and lower parameters will change to accommodate the KS signal, keeping the signal within the channel. If there is knock, the signal will range outside the noise channel and the PCM will reduce spark advance until the knock is reduced. These sensors are monitored in much the same way as the broadband sensors, except that an abnormal signal will stay outside of the noise channel or will not be present.

KS diagnostics can be calibrated to detect faults with the KS diagnostic inside the PCM, the KS wiring, the sensor output, or constant knocking from an outside influence such as a loose or damaged component. In order to determine which cylinders are knocking, the PCM uses KS signal information when the cylinders are near Top Dead Center (TDC) of the firing stroke.