Knock Sensor

A knock sensor uses a "Piezooelectric Crystal" to turn vibrations into a voltage. If a knock is sensed on the engine, the crystal creates an A/C signal down the signal wires, which the ECU picks up. If knock is sensed, the ECU will place the engine on knock map, or "safe mode". This runs the engine rich to attempt to remove any predetonation in the combustion chamber, and remove any "hot spots". If your engine is on knock map, you will notice a reduction in power, and a rise in fuel usage.

To test the knock sensor, i had to hook the two wires up to my oscilliscope, and simulate an engine knock by gently tapping the sensor against a hard surface. What i got was a small analog signal on the oscilliscope.