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Bad bearings, engine noise, & knock sensors

In looking thru the archives, opinion seems to be [surprisingly ;-)] split wrt whether ornot piezo-crystal KSes can pick up engine noise (valves, chains, etc), accessory noise, and/or (poss inaudible/yet-to-be audible) bad bearing(s) noise (main bearings, conrod, wristpin)

One o-scope source (sorry - lost link - maybe Pico?) states that knocking occurs 15 KHz. I recall another source stating that KSes are "tuned" to normal cylinder occurences, generating AC when abnormal freqs appear. Maybe both are saying the same thing

At some point - late 90s?? - apparently the ECU/ECM would receive the raw signal from the KS, deciding what was knock or not (where earlier systems ECU just basically retarded timing as AC V increased?)?

So my question is, are knock vs other freqs (above) distinct?

Or has anyone scoped a (circa early 90s) piezo-crystal KS pattern - as spikey/erratic as they are - and correlated marked AC V output, with a bad bearing?

Thank you, Lance

Reply to
Lance Morgan
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The specific algorithm they use will depend on the engine, model, year, etc. but I know some of the newer GM setups have a "noise channel" that is learned at idle speeds, and from which it deduces the expected noise level at higher engine RPMs and load levels.

Reply to
Robert Hancock

My thanks to you and Robert. Interesting link above

Found the 15 KHz link

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Assume your 740T is a late 80's MY? it appears to use a Bosch PN 0 261

231 001 KS, replaced by PN 0 261 231 006 used on my 91 Saab 900S (non-T)

With a timing light on my Saab, as I slowly rev to ~3K RPM, the computer controlled timing (one coil, distrib, rotor, wires) retards! the timing instead of advancing it. If I snap throttle, there is a lag, but the timing will advance fairly decently. Above 3K the advance appears good

I believe the retarded timing is causing excessive secondary ignition probs/pressure, and seems to correlate well w/loss of low-end power, and the reproducible moderate-to-severe 2500-3100 RPM bucking at steady-state throttle, on the flats and going down gentle grades. Accel is fair but not good below 3K, but fairly strong above 3K

Seems like the KS is excited and retarding the timing. Trying to find the root cause. Nothing overtly audible from the engine (yet...)

Other tests done here

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Lance

Reply to
Lance Morgan

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