Knock Sensors

The knock sensors are piezo-ceramic sensors that allow the ECM to employ active knock control and prevent engine damage from pre-ignition or detonation. Knocking or 'pinking' describes the combustion process in which flame speeds in the region of the speed of sound occur. This can happen towards the end of the combustion process if, after the normal combustion process has started, unburnt air/fuel mixture on the combustion chamber walls self-ignites under increasing pressure. These pressure peaks can damage the pistons, cylinder head gasket and the cylinder head.

Two knock sensors are installed on the left side of each cylinder block, below the intake manifold, one mid-way between cylinders 1 and 2, and one mid-way between cylinders 3 and 4. Each knock sensor is secured with a single screw. On each knock sensor, a two pin electrical connector provides the interface with the engine harness.

The ECM compares the signals from the knock sensors with mapped values stored in memory to determine when detonation occurs on individual cylinders. When detonation is detected, the ECM retards the ignition timing on that cylinder for a number of engine cycles, then gradually returns it to the original setting.

The ECM cancels closed loop control of the ignition system if the signal received from a knock sensor becomes implausible. In these circumstances the ECM defaults to base mapping for the ignition timing. This ensures the engine will not become damaged if low quality fuel is used. The MIL will not illuminate, although the driver may notice that the engine 'pinks' in some driving conditions and displays a drop in performance and smoothness. The ECM calculates the default value if one sensor fails on each bank of cylinders.