Description And Operation: 2.9L
DESCRIPTION
The knock sensor consists of piezoelectric material. Depending on the vibration frequency of the cylinder walls caused by ignition, the sensor generates voltage proportional to it. The Powertrain Control Module (PCM) detects the voltage value generated by the sensor its rate of change, and as a function of the latter, it can understand whether there is knocking in the cylinder. To reduce the "pinging" phenomenon caused by knocking, the PCM starts reducing the ignition advance in order to limit the maximum pressure that can be reached in the combustion chamber (the higher the pressure, the greater the risk of knocking).
OPERATION
The knock sensor:
- detects a knock in one of the cylinders on the corresponding bank
- sends an input signal to the PCM
In response, the PCM retards ignition timing for all cylinders by a scheduled amount.
Knock sensors contain a piezoelectric crystal which constantly vibrates and sends an input voltage (signal) to the PCM while the engine operates. As the intensity of the crystal's vibration increases, the knock sensor output voltage also increases.
The voltage signal produced by the knock sensor increases with the amplitude of vibration. The PCM receives the knock sensor voltage signal as an input. If the signal rises above a predetermined level, the PCM will store that value in memory and retard ignition timing to reduce engine knock. If the knock sensor voltage exceeds a preset value, the PCM retards ignition timing for all cylinders. It is not a selective cylinder retard.
The PCM ignores knock sensor input during engine idle conditions. Once the engine speed exceeds a specified value, knock retard is allowed.
Knock retard uses its own short term and long term memory program.
Long term memory stores previous detonation information in its battery-backed RAM. The maximum authority that long term memory has over timing retard can be calibrated.
Short term memory is allowed to retard timing up to a preset amount under all operating conditions (as long as RPM is above the minimum RPM) except at Wide Open Throttle (WOT). The PCM, using short term memory, can respond quickly to retard timing when engine knock is detected. Short term memory is lost any time the ignition key is turned off.
Over or under tightening the sensor mounting bolts will affect knock sensor performance, possibly causing improper spark control. Always use the specified torque when installing the knock sensors.