Knock Sensor: Description and Operation

Knock Sensor Location:

Knock Sensor - Cross Sectional View:

Knock Sensor Circuit Diagram:

Purpose:
The Knock Sensor sends an AC voltage signal to the ECM when a knock occurs in the engine. The ECM responds to this signal by retarding ignition timing. When detonation stops, timing is advanced in small increments back to a pre-programmed value for the engine speed, load, and temperature conditions present. This allows the ECM to maintain maximum ignition timing advance under various operating conditions, for improved driveability and fuel economy.

Operation:
When abnormal engine vibrations occur (spark knock), slight distortions of an element in the sensors generate an AC voltage signal that is detected by the ECM. The voltage potential produced depends on the severity of the knock. When a knock signal is detected, the Electronic Spark Control (ESC) system (built into the ECM) varies the timing signal to the ignition module to maximize performance and minimize spark knock.

The ECM applies a 5 volt reference voltage to the knock sensor circuit through a resistor in the ECM, and then monitors the voltage in the circuit. A resistor to ground within the knock sensor causes the ECM's 5 volt reference to drop, so that under a no knock condition, the signal circuit should measure about 2.5 volts. This is the "bias" voltage that the knock sensor signal is carried on, to ensure that even a slight knock will produce a voltage "spike" that rises above the threshold voltage of about 3 volts to indicate a knock.

Location:
The knock sensor is located in the lower side of the engine block.