Knock Sensor: Description and Operation

The engine has two identical knock sensors, one for the front and one for the rear bank of cylinders. Each knock sensor contains a piezoelectric element which produces an electric current when subjected to pressure.

When knocking occurs in any of the cylinders it gives rise to severe vibration in the engine block, causing the sensor to generate alternating current. In the case of extremely severe knocking, the current thus generated may amount to about 500 mV.

The knock sensors are mounted on the cylinder banks by means of a through bolt. Tightening torque is extremely important for proper operation of the sensors and should therefore be strictly observed.

Tightening torque: 22 Nm (16 ft lb).

The front knock sensor is connected to pin 29 and the rear sensor to pin 11 of the electronic control module. The sensors are screened and grounded via pin 30 of the control module.

The control module reads the voltage of the front sensor during 45° after cylinder 2, 4 or 6 has fired and the voltage of the rear sensor during 45° after cylinder 1, 3 or 5 has fired.





If knocking occurs in any of the cylinders, the ignition will be retarded by 3° for that particular cylinder. If the knocking then stops, the ignition will be restored to what it was before in steps of 0.75°.

On the other hand, should knocking continue in the same cylinder in spite of retarding the ignition, the control module will retard the ignition an additional 3° and so on in steps of 3° up to a maximum of 13.5°.

At high intake air temperatures (above 50°C [122°F]), the ignition will also be retarded in order to counteract knocking. When the temperature drops the ignition will be restored in steps of 0.75° to what it was before.

When the throttle angle exceeds a certain figure, the control module interprets this as full load. The ignition timing angle is then obtained from another map or matrix with lower ignition timing values.

If either of the sensors fails to work or in the event of a break (no continuity) in the circuit, the ignition will be retarded 12° in the event of a load on the engine which would entail a risk of knocking.

The MIL (CHECK ENGINE) lamp will light up.