Knock Sensor: Description and Operation

Knock Sensor:

PURPOSE
The knock sensor generates a "knock" signal for the ECM. When engine knock is detected, the knock sensor develops a voltage which the ECM registers. The ECM takes appropriate action by retarding the ignition timing and enriching the air/fuel mixture if necessary.

LOCATION
Two sensors are used, one at the front of the engine (between #4 and #6 cylinders) and one at the rear (between #1 and #3 cylinders).

CONSTRUCTION
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).

OPERATION
Mounted on the engine block, each sensor is subjected to the 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 ECM.

SIGNAL
The signal from each sensor is a continuous, but variable voltage. The frequency of the voltage is dependent on the amount of engine knock. The signal is fed into the ECM which computes it as a reference value.

If the knock sensor fails, ignition timing reverts to a preset retarded value.