968 Cabriolet L4-2990cc 3.0L DOHC (1993)
Knock Sensor: Description and Operation
Knock Sensor Construction
PURPOSE AND LOCATION
Two knock sensors are used to detect engine knocking and are mounted on the left side of the engine block, just below the cylinder head, between
cylinders 1 and 2, and 3 and 4.
CONSTRUCTION
Each sensor is comprised of a piezo-ceramic crystal sandwiched between two electrical contacts with a fixed weight on top.
OPERATION
When a knock occurs in the engine, the resulting vibrations cause the crystal to be compressed. When compressed, the internal structure of the crystal
is stressed. This stress on the crystal produces a low voltage signal which is read by the Digital Motor Electronics (DME) control unit. The knock
control function is cylinder-selective and adaptive, retarding the ignition timing by a maximum of 9° in 3° increments.