What is the function of knock sensor

[Zrun]

The knock sensor error is always on.. It costs a bomb to replace it, about 1000 USD..

 

My low rpm torque is bad, and engine's rough at low rpm, seemed like everything is out of synced.

 

My questions...

 

What is "timing" of engine? I only know faster is better, and if tuned too fast, the engine knocks... What's the mechanical/logical description of it?

 

How does the knock sensor affects timing?

I heard multiple answers, some say that knock sensor will sense bad fuel that knocks and automatically slow down timing.. Another say that the (modern)engine will constantly increase the timing until it knocks, retard, and advance again, maintaining highest performance.

[Wymun]

Good article about iginition timing here:

http://members.aol.com/dvandrews/timing.htm

http://www.howstuffworks.com/ignition-system1.htm

 

On Knock Sensors:

Many late model engines have a "knock sensor" on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to momentarily retard ignition timing until the detonation stops.

If the "check engine" light is on, check the vehicle's onboard computer system using the prescribed procedure for a "trouble code" that would correspond to a bad knock sensor (code 42 or 43 for GM, code 25 for Ford, or code 17 for Chrysler).

 

A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself!) and watching for the timing change while the engine is idling. If the timing fails to retard, the sensor may be defective -- or the problem may be within the electronic spark timing control circuitry of the computer itself. To determine the cause, you'll have to refer to the appropriate diagnostic chart in a service manual and follow the step-by-step test procedures to isolate the cause.

 

Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.

Edited November 14, 2002 by Wymun