944 Turbo

The knock sensor, which is bolted on the engine block next to the cylinder head, monitors ignition knocking. The knock sensor consists of 2 seismic materials, between which a "piezo" crystal is located. Oscillation, especially engine knocking, produces forces on the crystal by way of the seismic materials and the crystal reacts to these forces with changes in the electric voltage. These changes in voltage are sent to the knock and charging air pressure control unit as electrical signals

With the trigger signal for each individual cylinder, the DME ignition signal (TD) and the signal from the knock sensor can recognize in which cylinder there has been uncontrolled combustion (knocking).

With the next ignition pulse from the DME control unit for this cylinder the "TD" signal will be delayed in the knock control unit and fed back as a "TD" signal, so that the ignition for the pertinent cylinder is retarded 3°. If knocking occurs again, the ignition of the cylinder will be retarded again by 3°. If this then stops the engine knocking, the ignition will be advanced to the correct value in steps of .3° per 100 ignition pulses.

If knock pulses continue, the charging air pressure will then be taken back in steps of (.03-.05°). Taking back the charging air pressure is accomplished by changing the rectangular signals to the cycling valve and therefore increasing the pressure on the diaphragm of the charging air pressure control valve.