Saabscene Saab Forum - Saab Technical Information Resource banner
1 - 7 of 7 Posts

·
Registered
Joined
·
1,219 Posts
Discussion Starter · #1 ·
Hi,
I am wanting to rig up an LED from my APC ('87 9000T) to light up when there is engine knock.
anyone able to tell me which pins to solder the wires to and how many volts the knock sensor outputs? (cant get it to knock when out of gear to test with a voltmeter, and not going to try and test while driving!!)
thanks for the help.
 

·
Registered
Joined
·
14,305 Posts
There won't be enough current available from the sensor, which is a high impedance piezo device, to illuminate an LED.

It's also not a digital signal but analogue, so there is no sharp transition from knock to no knock. The ECU works that out. It should be possible though (if somewhat complicated) to hook up a high impedance amplifier in parallel with the ECU though to drive some sort of display.
 

·
Registered
Joined
·
8,216 Posts
Mark, you are correct - the knock sensor picks up all sorts of vibration and it is the APC system that figures out from that signal whether there is knock. However, I believe the APC system (pre-DI) has a knock signal output which can directly drive an LED.

I believe you need to connect the cathode (-ve end) of the LED to pin 19 of the APC connector and the anode (+ve end) to +12V. There is likely no terminal in the connector at that point. The terminal is available from Saab as part number 91 20 668 and you can simply crimp it onto the wire running to the LED cathode, then slide it into place on the connector.

There is also a knock output on the DI/APC unit (pin 26) which is reputed to provide 0v for pre-ignition, 6.5v for normal running and 12v for knock. I don't know of anyone who has successfully used this to drive a warning light.
 

·
Registered
Joined
·
1,219 Posts
Discussion Starter · #4 ·
thanks for the help guys.... i did a search earlier and all i could find is that i had to solder to pin 19 (no reference as to whether the cathode or the anode goes there) and no other info on where to solder the other pin.

another question.. . if i disconnect the rpm signal wire, i'm guessing that i wont get boost taper up at the higher rpms???? (i dont think i will do this, but it would be maybe something to do once i get a different car way into the future and i decide to race this one... )
 

·
Registered
Joined
·
1,219 Posts
Discussion Starter · #7 ·
Originally posted by gyro:
[qb]why do you want to know when the engine is knocking???? [/qb][/b]
My engine is knocking below the base boost limit, therefore APC cannot do anything about it, so i want a light to show me when i have to back off, basically so i dont have to listen for it.
today with +40°C temps outside and 91 RON unleaded, ACC on full, the knock was terrible, so as of now i have an LED hooked up so that i can see when there is knock.
 
1 - 7 of 7 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top