Need guidance on knock sensing mod

[RS68]

I want to install knock sensing on my 85 Saab 900 Turbo which is running MSII v3 2.883 code. The car uses a boost control system called the APC, which is designed to cut boost when it detects engine knock. The APC controller has a spare pin (#5) on its connector that goes to ground when the system detects knock. I have an LED and Piezo Buzzer connected to a 12 volt source, and then connected to the pin 5 on the APC. So when the engine knocks, pin5 grounds out and the LED lights up, and I hear a buzzer. I think I should be able to use pin 5 on the APC to get the knock signal to MS but I wasn't sure how.

I made a little diagram of a mod to the knock sensing circuit that I think will work with this system, but my knowledge of building electronic circuits isn't that great so I could use some feedback on whether it will work or not. If my circuit works like I think it does; when there is no knock, pin 5 of the APC is not grounded and JS4 reads a voltage signal of 2 volts? When knock is detected pin 5 grounds out and the signal to JS4 is pulled low to 0 volts?

[Blown'37]

Megasquirt can use the signal from a GM knock sensor and in the manaul it gives specs on what the knock sensor should read when it detects knock. Do you think you could retrofit the GM knock controller onto the set up you have already so that you get the best of both worlds.

Matt.

[RS68]

I could probably get the GM knock controller to work but I don't want to monkey with the stock knock detection circuit. This car electronically controls boost based on the knock sensor so the knock sensing system was designed and tuned very well from the factory. I think I should be able to get knock detection working with the stock system, but right know the knock sensing project is on hold, as I have to replace my broken transmission, and figure out why I keep killing the direct coil control circuit on MS.

[DME]

Hi,
I'm new to megasquirt but have expiriace in making circuits.
Sorry If this post is to late.
Your circuit is almost OK but You should add a diode between ECU knock input and buzzer/led/knock block to disallow +12v current flows to ms2 input.
see mc9s12f64 datasheet for more info about max voltage on analog input.

[DME]

PS or add current limiting resistor in a place of diod I have ment

[RS68]

Thanks. I got the knock detection working but not with the knock LED or Buzzer yet. Would a 1N4001 Diode work?