Knock Sensor: Description and Operation

Knock Sensor:

MODELS 300 CE, SL ONLY

The knock control system allows calibration of the ignition timing for normal operating conditions, while eliminating the danger of knock or detonation due to influences by heat, load, fuel quality or engine deposits.

Knock Sensor:

The knock sensor (C) consists of a housing (3) a piezoelectric crystal (4) mounted on a threaded sleeve (5) between two connector strips (6). One side of the assembly is fitted with a damping weight (7) and spring washer (8) secured by a nut (9).

The knock sensor which is mounted on the engine block is subjected to vibrations generated by knock. The vibrations cause a small distortion on the surface of the piezoelectric crystal, which consequently generates a small voltage. This voltage is read by the EZL/AKR control unit. Depending upon the intensity of the knock, the control unit will retard the ignition point for an individual cylinder between 2.5° and 5° of crankshaft angle per ignition cycle (Max. of 10°). The knock control system develops its maximum ignition retard of 10° crankshaft angle at a coolant temperature of 194°F. If the knock sensors, or the data evaluation circuit in the ignition control unit, or the camshaft position sensors malfunction, a safety-timing of 10° retard is imposed. If the engine coolant temperature sensor fails, a substitute temperature of 268°F is selected. Since this temperature lies within the range of overheating protection, the ignition timing will be retarded by an additional 3.5° for a total of 13.5°.