TURBOTRONICS

Dose any one know what the signal is that comes from a knock link sensor
I.e. 0v no activity and 5v lots of activity
Or is it something else some other kind of signal
The reason I ask is I have a gadget im using to monitor other things like
Maps reading
Maf reading
Lambda reading
I should be able to link it to the knock sensor as well I just need to know how to interoperate what signal is coming from it and some idea of what a safe level is and what is not

Sprint Chief

The knock sensor is essentially a little microphone, the output is an audio feed of engine noise. The signal is audio frequency alternating current (AC).

You can't just monitor a voltage; you need a bandpass filter, envelope detector and an integrator (low-pass filter) to turn it into a usable "voltage" - this is what the knock link box does (and what the subaru ecu does internally). The knock event only lasts a few tens of milliseconds as well - although there might be a lot of them to monitor if your engine is getting a hard time!

Don't know if your gadget can do that for you or not? Depends on what sort of a gadget it is!

TURBOTRONICS

its a logic controler you wright programs for it depends on what you want it to do. am i right in that this all hapens around the 15k frequansy

Sprint Chief

I can't remember the exact frequency; the frequency is (primarily) a function of the engine bore (the knock "signal" is generated by block resonances, the engine bore makes quite a good "speaker" when knock takes place). I think the impreza main frequency is somewhere around 5kHz - 6kHz. You usually get a couple of higher frequency harmonics and a lot of general clatter (which are caused by other bits of the engine rattling), so it isn't exactly a clean spike. If your logic controller has an A/D sampling function, you should sample no lower than 10kHz, ideally at 20kHz.

As a word of warning, if you are going to probe the stock knock sensor, make sure you are AC-coupled and make sure not to introduce any earth loops that might trash the signal going to the ecu. Conventional wisdom suggests it is wiser to add a separate sensor, and listen to this, rather than connect to the standard one, to avoid accidentally changing your ecu's ability to "hear" knock.