Knock Sensor Output (Again).

[avos at cochlear.com.au]

     Sorry to get back to covered ground.....
     
     I have a question about the outputs of a knock sensor.
     
     My turbo Subaru has a knock sensor connected to the standard ECU. I 
     have pin-outs (from the workshop manual) for the ECU and know which 
     input pin to the ECU is from the knock sensor. Included with these 
     pin-out description is the expected signal (so a technician can check 
     it I guess) for any pin under two conditions: ignition on with engine 
     off, and engine on and idling. For both of these conditions, the 
     output from the knock sensor is supposed to be 3-4 Volts. I would like 
     to connect some visual display to indicate knocking similar to the one 
     that was published on this list recently (well almost published 
     anyway), but I would like to use an LM3914 bar graph display driver. 
     Anyway, my questions are:
     
     * The signal of 3-4 Volts is supposed to be measured with a DC DMM. Is 
     it likely that this signal is DC or AC (i.e. do I need to perform some 
     filtering?) (I do not have a CRO, so cannot check it out myself)
     
     * If an output of x (3-4 Volts) is received at idle what sort of 
     output can be expected when knocking occurs?
     
     I know that a standard knock sensor is just a microphone, and 
     therefore should have an AC output, but is it possible that Mr Subaru 
     decided that it would be easier to filter the signal at the sensor, or 
     is it more likely that it is filtered inside the ECU (JECS ECU), and 
     therefore I must also low-pass filter it.....
     
     Also I checked out the knock sensor indicator circuit posted recently 
     which incorporates some filtering, and I was wondering if I do the 
     same thing as this i.e passive filter, am I likely to stuff up the 
     signal seen by the ECU, or is it safer to buffer with an op-amp before 
     filtering, so that I do not in any way alter the signal to the ECU.
     
     Any advice appreciated......
     
     Adrian
     avos at cochlear.com.au
     
     BTW- Sorry to bring up thi knock sensor strand again, but looking 
     through the archive I could not find the information that I want.