Knock Sensor: Description and Operation
The B258 engine has two knock sensors, one for each bank of cylinders. On the basis of information supplied by the camshaft position sensor, the control module knows which cylinder is under compression when ignition occurs.
If knocking is detected, the ignition will be retarded in the relevant cylinder in steps of 3° until the knocking ceases. It will then be advanced in steps of 0.75° until knocking recurs or until the original timing is restored.
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 generated in this way could 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 lbs).
The front knock sensor is connected to pin 40 and the rear sensor to pin 70 of the control module. The sensors are grounded via control module pin 71 and screened via pin 28.
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 the ignition being retarded, the control module will retard the ignition an additional step of 3°, and so on up to a maximum of 12°. The ignition will also be retarded at high ambient temperatures in order to counteract knocking tendencies provoked by high intake air temperatures.
When the throttle position exceeds a certain angle, the control module interprets this as full engine load or wide open throttle and obtains the ignition timing angle from another matrix with lower ignition timing values. Should either of the sensors fail to work or in the event of a break in the circuit (no continuity), the ignition will be retarded by 12° in the event of a load on the engine which would entail a risk of knocking.