Knock Sensor: Description and Operation

Knock Sensors:

PURPOSE
Varying octane levels in today's gasoline can cause detonation (also known as spark knock) in an engine.

The Knock Sensor (KS) system has two knock sensors that are used on 4.3L engines. The KS system reduces spark knock (detonation) in the engine. This allows the engine to have maximum spark advance for improved driveability and fuel economy.

OPERATION
A control module Powertrain Control Module (PCM), Vehicle Control Module (VCM) or Vehicle Control Module (VCM-A) is used in conjunction with two knock sensors to control detonation. No KS module will he found on these applications as it is internal to the control module.

A 5 volt reference is applied to the knock sensors which has an internal resistance of about 8200 ohms. This resistance will lower the applied voltage to about half or 2.5 volts. When a knock is present, a small AC voltage is produced by the knock sensors and transmitted to the control module riding on top of the already existing 2.5 volts. An AC voltage monitor inside the control module will detect the knock and trigger the control module to start retarding the spark incrementally.