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knock sensors


Recently I've been experiencing the symptom of a bad knock sensor on my 
1990 Coupe.  The check engine light comes on at about 3500 rpm.  The 
"code" idicates a faulty sensor.  On my car I have 2 sensors so I 
measured the resisitance from each pin to the other.  I then made the 
same measurements again except reversing the polarity of the meter 
(swapping the probes) just in case there are semiconductors inside the 
sensor.  In any case, I measure an infinte resisitance between any 2 pins 
on both of the sensors.  I am telling you this because I find it 
difficult to believe that both sensors on my car can fail at the same 
time.  I think it may be a bit more complex to ascertain the 
functionality of the knock sensors with just an ohmeter.  Tomorrow, I 
intend to use my oscilloscope and monitor all of the signals with respect 
to ground and see if there is any signal while the sensor is connected to 
the computer.  My other thought is that these sensors may be 
piezoelectric elements.  They may need a positive and negative power 
source with the output signal being referenced to the negative supply 
(ground) thus necessitating the 3 wires.

I hope this gives you some ideas.  I'll let you know the results of my 
measurements tomorrow.  If you figure anything out, I'd like to hear what 
you find.  By the way, I fired this question off to the "group" a couple 
of weeks and got not one response.

Good luck.