Jared

was thinking about putting a dvom on it, but i'm not sure what the range is and what is deemed acceptable.

Bottoz

Its a 5volt sensor, 0 - 5 volts.

Bottoz

Jared

So it creates 5V ac or dc? and what threshold is considered knock?

John_at_J&S

The knock sensor is a piezo-electric microphone that generates its own signal. If you were to connect it to an amplified speaker, you would hear your engine running, though it sounds a little different, because you don't hear the exhaust.

In general, knock detectors look at the amplitude of the signal, after running it through an am detector:
http://www.st-andrews.ac.uk/~w....html

The output of the am detector is then compared to a threshold. If the signal is bigger, then it's assumed to be knock.

The MSD Knock Alert takes an average of the signal, and uses that as a threshold. As the RPM's increase, the engine noise increases, automatically raising the threshold.

Sometimes engine noise is louder than knock, so that method doesn't always work on a noisy engine.

The J&S knock detector is in software, running on a Motorola processor. The software uses signal processing techniques.

Jared

since you're comparing it to speaker output, I'm assuming it is an AC waveform that will show up on any oscilliscope. Do you happen to know the min and max it creates?

Bottoz

The OUTPUT of the knock sensor (as read by the ECU) is in volts.

Here is my Knock Noise table, but if using an AEM you must first set up your own custom knock table by following their instructions on the AEM board. Each car makes its own noises, depending on your mods.

Bottoz

Jared

Very helpful. Can anyone confirm that this is an AC signal like lots of other knock sensors out there? I'm assuming it is because of the source.

John_at_J&S

Sensor output is about 500mv/g, where g is the acceleration of the sensor, due to vibration. The signal is ac, audio frequency, usually between 5khz and 10khz.

At 5000 RPM, the engine noise could easily be a 10vpp. Audible detonation spikes can double that.

NegativeLift

So the knock sensor determines whether or not there is knock and sends a high or low signal to the ECU, or does it simply gather the frequency data and send it to the ECU where the ECU then determines if there is knock?