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knock sensors

Thanks for the response.  I made some measurements this morning using an 
oscilloscope and this is what I found.  Upon peeling back the connector 
hood on the knocks sensor plug, I found 3 wires.  One of them was the 
shield which I assumed was signal ground and power ground.  I'm assuming 
that the other 2 wires are for the output signal and power.  I measure 
the 2 wires with respect to the shield and measured a signal on the 
middle wire.  As I increase the engine speed, I could see an actual 
signal develop from the noise floor.  The signal was measured on a 50 
millivolt / division scale and there was not a whole lot of output, even 
at 3000 rpm.  Perhaps 100 mV pk-pk.  This same type of signal was present 
on both knock sensors so my theory is that they are both good or bad.  
Since I find it hard to believe that both of them could fail at the same 
time (although, one could have died a long time ago and the ECU was 
operating only with 1 sensor and when this one, the last one died, the 
fault got indicated), I'd say they were good.  I got back in the car and 
verified the performance and guess what, no CHECK ENGINE light upon 
taking the car up to 5000 rpm.  Maybe the connectors had to be reseated.

Other things that I have noticed is that I did not measure any excitation 
voltage going to the knock sensor.  Does anyone know what I should see?  
Should this be some kind of bias voltage (developed from ECU?) that 
changes according to output?  I have a theory of how this whole feedback 
loop operates and would like some feedback.

The ECU attempts to keep the timing advanced as much as possible up to a 
preset limit.  If the ECU never detects an output from the knock sensor, 
the timing curve will be that preset limit and will indicate a fault on 
the check engine light.  If the sensor is good, then the ECU will never 
achieve the preset limit and will vary the timing curve according to the 
sensor output.  By functioning this way, if the sensor fails, you should 
not be able to damage the motor.

How does that sound?

1990 Quattro Coupe
1984 4kq