KS Attenuation, or Dampening

[thergen at svn.net]

On Mon, 17 May 1999, steve ravet wrote:

> 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.  
> 

Don't some of the ecms look for a particular resistance to ground from the
knock sensor input to ground (from memory, 3.9K Ohms for some sensors)? 
There's probably a range of resistance that will avoid setting a code.  If
R2 in parallel with R1+Rks is too high, will that cause a fault code? 

Thanks,
Tom