Knock sensor specs

[Barry Tisdale]

Since the KS thread began, been thinking how important it is.  Did some poking around, after putting together the 555/LED monitor (which didn't work).

DC on the KS is 2.36v; solid rapping on the block near the sensor puts an AC voltage of maybe 0.015v tops on the sensor line.  Scan tool reads knocks, but the value presented can be anything from 1 to 255; when it hears a knock, new (seemingly random) value is set & held till the next 'knock' comes along.  Knock retard degrees always reads zero.

Don't think 0.01VAC is enough fluctuation to trigger the 555 reliably; even using a comparitor, that's barely enough signal to pick out of the background noise - lots of ignition wires nearby. 

Are these readings within normal limits??  Helm manual just says "an AC voltage should be present" (when rapping) - no value given, but 0.01v seems real low.

Thanks for any feedback - Barry