Knock Sensor: Description and Operation

Knock Sensor Circuit:

PURPOSE
To reduce spark knock (detonation) by retarding timing advance.

OPERATION
The Knock Sensor (KS) system is one of the inputs to the Powertrain Control Module and is used to determine control of the Ignition Control (IC) circuit. The KS system is comprised of a knock sensor and KS module (part of PROM). When the knock sensor detects detonation, the PCM retards timing in small increments (up to 10°) to reduce detonation. This allows the engine to use maximum spark advance to improve driveability and fuel economy. Retarded timing can also be a result of excessive valve lifter, pushrod or other mechanical engine or transmission noise.

CIRCUIT DESCRIPTION
The knock sensor is used to detect engine detonation and the PCM will retard the electronic spark timing based on the signal being received, The circuitry within the knock sensor causes the PCM's supplied 5 volt signal to be pulled down so that under a no knock condition, CKT 496 would measure about 2.5 volts.

The knock sensor produces an Alternating Current (AC) signal which rides on the 2.5 volts DC voltage. The amplitude and frequency are dependent upon the intensity of the knock.

The PROM used with this vehicle contains the functions which were part of the remotely mounted KS modules used on other GM vehicles. The KS portion of the PROM then sends a signal to other parts of the PCM which adjusts the spark timing to reduce detonation.