KS Attenuation, or Dampening

[steve ravet]

GARY wrote:
> 
> >
> > First of all, isn't the KS a piezo sensor?  If so, I think DC resistance
> > doen't really matter.
> 
> Yes Ord, it is but the ECU freaks out if it notices that the KS circuit
> is not present so I assume it is looking for a certain current flow with
> an applied voltage.

The knock sensor is a microphone essentially -- it produces voltage.  It
is not a driven sensor like a temp or TPS.  Some GM ECMs test the knock
sensor when certain requirements are met:  Engine warmed up, throttle
open a large amount, RPM low (high load condition).  The ECM briefly
advances spark and listens for knock.  If it doesn't hear it it sets an
ESC code (40 something).  This test only happens once per startup, if it
happens at all.  This applies to 16136965 ECM ('91 Caprice, straight
from the service manual), but probably applies to lots others.

> 
> If you want to attenuate, you could build a divider
> > around it - two resistors in series accross the element, leave one wire
> > connected to the KS, connect the other to the junction of the resistors.
> >
> > If it is really damping that you want, instead of just attenuation, then you
> > need to build a low pass filter.
> 
> I am unclear on the deference between the two.  Attenuation is reducing
> the signal's amplitude (desired), dampening is reducing the signal's
> magnitude?  What's that?  I don't know what I want!  I want to see of
> the ECU will stop retarding my timing every time the piezo reacts to a
> non-detonation engine noise.

A piezo sensor is very high impedance.  In that sense it's like an O2
sensor, you have to be careful what kind of stuff you hang on it's
output or you'll degrade the signal.  If you build a voltage divider
you'll have to use really large resistors, like 10x whatever the
resistance of the sensor is.  You can measure the sensor with a digital
meter.  I think piezo elements are in the megohm range.  To hook it up
get 2 big resistors and place them in series.  Hook the knock sensor
output to one end, ground the other end (at the block, close to the
knock sensor), and hook the ECM/ESC module to the middle connection:


                    R1        R2
   KS output -----/\/\/-----/\/\/------gnd
                         |
                         |-------------to ECM

If the resistors are large enough the attenuation is given by
r2/(r2+r1).

If you can find a high resistance potentiometer (again like 10x the KS
resistance) it's even easier and adjustable.  Connect the end terminals
to KS and gnd as above, send the middle (variable) connector to the
ECM.  

--steve


> 
> I don't quite follow your description of how to build the divider
> either. Your help is appreciated!
> 
> --
> GARY       mailto:hobiegary at earthlink.net

-- 
Steve Ravet
steve.ravet at arm.com
Advanced Risc Machines, Inc.
www.arm.com