Knock Sensor Funny Business

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Mark Dalton
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Knock Sensor Funny Business

Post by Mark Dalton »

I was checking voltages on a Knock Sensor, but found that when I was measuring both circuits of the Knock Sensor at once, one of the circuits was being pulled down by 300mV, which caused a DTC to set. Measuring the circuits one at a time, was fine. What going on when both circuits are being measured that allows one circuit to be pulled down. Is it something to do with the impedence of the two chanels in parallel is lower than the impedence of the circuit in the PCM?
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P0327.pdf
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Steve Smith
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Re: Knock Sensor Funny Business

Post by Steve Smith »

Hi Mark, thank you for the post and the PDF file

To summarize the PDF and measurements result:
Channel A (Blue wire) at 2.626 V and Channel B (Pink wire) at 1.381 V when measured individually
Channel A (Blue wire) at 2.521 V and Channel B (Pink wire) at 1.063 V when both measured at the same time
Channel B has most certainly been affected by the simultaneous measuring of both knock sensor circuits suggesting the design of the sensor/circuit is sensitive to changes in impedance

Some knock sensors seem to be biased to approx. 2.5 V, typically by using two 1 MΩ resisters within the PCM.

If we now apply the scope to the knock sensor circuit, we introduce another 1 MΩ resister (The impedance of the scope channel)

This did not appear to affect the circuits when measured individually (tolerated by PCM) based on the results in the PDF, but I would like to repeat these tests using a High Impedance Probe as per our Guided Test below:
Image 1
Image 1
As you correctly mentioned Mark, if we now measure both circuits simultaneously (with x1 test leads) we have introduced a second 1 MΩ resister in parallel.

Lowering the entire circuit impedance results in a change in voltage to such an extent as to influence the monitoring system and so trigger the DTC

The solution is to use the High impedance probes https://www.picoauto.com/products/test- ... be-adaptor or https://www.picoauto.com/products/test- ... 00-mhz-bnc (set to x10 attenuation) where the impedance increases to 10 MΩ, so having minimal influence on the knock sensor circuit

You can see the effects of “intrusion” and the benefits of high impedance probes here topic21591.html

You may also find a number of floating crankshaft sensors will require the use of high impedance probes or indeed, a Differential probe if you wish to measure across the sensor

I hope this helps……take care……Steve

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Mark Dalton
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Re: Knock Sensor Funny Business

Post by Mark Dalton »

Hi Steve,
Thanks for the information. Didn't think about using my High Impedence probes at the time. But it will definitely stick in my mind now for the next time I come across something similar.
Thanks again for the assistance.

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