Knock Sensor (KS)
- Discovery & Range Rover
KS are located on each side of engine block between cylinders No. 2 and No. 4, and between cylinders No. 3 and No. 5. KS contains a piezoelectric ceramic element which produces a voltage proportional to engine vibration. ECM uses KS signals and signals from CMP sensor and CKP sensor to verify engine knock based on positions of camshaft and crankshaft.ECM will calculate which cylinder is knocking and retard ignition timing until knock disappears. Ignition timing will then be advanced until optimum ignition timing for suspect cylinder is achieved. For this reason it is possible for all 8 cylinders to have different ignition timing simultaneously.
If KS fails, the ECM will use a "safe ignition map" to prevent engine damage, engine may run rough, and there may be a noticeable reduction in engine performance. A fault is indicated by illumination of the MIL.
- Freelander
KS allow the engine to operate at the limits of ignition advance, for optimum efficiency, without combustion knock. The ECM uses two KS, one for each cylinder bank, located between cylinder banks on cylinders No. 3 and 4. The knock sensors consist of piezo ceramic crystals that oscillate creating a voltage signal. During combustion knock, the crystal oscillation frequency increases, which alters the signal output to the ECM. The ECM compares the signal to known signal profiles in memory. If combustion knock is detected, the ECM retards ignition timing for a number of cycles. When combustion knock stops, ignition timing is gradually advanced to the original setting. To prevent incorrect installation, the knock sensor leads are different lengths.