Knock Sensor: Description and Operation

DESCRIPTION

Two knock sensors are used to signal the Engine Control Module (ECM) when a sudden severe detonation (knock) occurs in the engine. One sensor is fastened to the engine block, in the valley beneath the intake manifold. The other is bolted to the left side of the engine block, beneath the cylinder head. When a knock is sensed, the ECM retards the ignition timing, in steps, until the knock ceases.

Ignition Maps And Knock Sensor Control:

The ECM has two ignition "maps" programmed into its memory. One for premium fuel and one for lower octane fuels. The premium fuel ignition map is normally the one referenced by the ECM. If knocking cannot be eliminated by retarding the ignition timing the maximum number of degrees from the mapped point on the premium ignition map (12 degrees), the ECM switches to the other ignition map. If the knocking stops, the ECM advances the timing, in steps, back to the mapped point for that engine speed and load.

Knock Sensor:

The knock sensor contains a piezo-ceramic crystal element. When mechanical forces act upon a crystal, causing distortion of the crystal, small electrical charges are generated on the crystals surface. When a knock occurs in the engine, the frequency and magnitude of the sound wave that is generated causes small distortions in the knock sensor crystal, generating a voltage on the surface of the crystal which the ECM reads.

SELF DIAGNOSIS

The ECM will recognize the absence of a signal, indicating an open or shorted sensor or sensor circuit. There is no substitute signal for this input.

Diagnostic Trouble Code # 00524 is displayed on the VAG 1551 scan tool for Knock sensor I.

Blink Code # 2142 is displayed using blink code extraction for Knock sensor I.

Diagnostic Trouble Code # 00540 is displayed on the VAG 1551 scan tool for Knock sensor II.

Blink Code # 2144 is displayed using blink code extraction for Knock sensor II.