Knock Sensor: Description and Operation

PURPOSE
Varying octane levels in todays gasoline can cause detonation (also known as spark knock) in an engine. The Knock Sensor (KS) system has 2 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
The knock sensor system components are:
^ 2 Knock sensors.
^ Powertrain Control Module (PCM).

A 5 volt reference is applied to the knock sensors which have an internal resistance of about 8200 ohms each. This resistance combined 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 PCM riding on top of the already existing 2.5 volts. An AC voltage monitor inside the PCM will detect the knock and trigger the PCM to start retarding the spark incrementally.