Piezo Electric

Piezo electricity is generated by pressure on certain crystals, such as quartz, which will develop a potential difference, or voltage, on the crystal face. When the crystal flexes or vibrates, an AC voltage is produced.

Knock sensors, which are becoming more common, take advantage of this phenomenon by sending the ECU a signal that engine knock is occurring. The ECU in turn retards the ignition timing to stop the knocking. Knock sensors contain a piezo electric element which, when deformed by cylinder block vibration caused by knocking, generates a voltage.

There are two styles of knock sensors used. The mass type produces a voltage output over wide range, but the signal is greatest at a vibration of approximately 7 kHz. The other style is the resonance type which only produces a significant voltage signal when exposed to a vibration of approximately 7 kHz. Since the voltage output from either knock sensor varies continually, the system is highly susceptible to electromagnetic and radio interference. The computer can be fooled by these stray electrical signals if they get mixed with the knock sensor signal. For this reason the signal wire running from the sensor to the ECU is a special ground-shielded type. The shield surrounds the signal wire and is connected to ground so any electrical interference is taken to ground. If this shield is damaged or not grounded, the electrical interference can reach the ECU and cause it to retard the timing unnecessarily.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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