The Saab APC  ECU originally controlled boost in the Saab 900 turbo.  I prefer to use it just for a knock light.  How do you do that?

1. Get an APC box and the knock sensor.  Part 7566599 seems to be the most common.  Beware of the earlier gray boxes, they are not as accurate or reliable.  Do not cut the knock sensor part of the harness.  It's a shielded wire and should be left alone.  Snipping about 6 zipties and carefully pulling the connector and wire out of the car is all it takes.  You can cut the larger part headed to the fuse box, just make sure to leave a few inches to patch into.  The knock sensor is under the IM.  A long extension and 12mm socket will help in removal.
2.  Wire it up.  Here's the original diagram.   
Here's what you really need
         

Pin	Wire color	Function
4,16,17	Shielded	Knock Sensor, should be connected already
5	Black	Ground
13	Gray/Red	"+" 12v ign switched.
19	Your choice	LED knock light, connect to "-" side of LED.  Remove an un-needed wire, like Pin 20, and insert into Pin 19 on the connector.  Radio Shack sells 12v LED's with integrated resistors that work great.
25	Blue/Red	RPM signal from ignition coil, use the "-" terminal.  Acts as a noise filter, running without this will cause an over active knock light.

3. Mount the knock light where you can see it, right next to your boost gauge is usually a good option.

Links to more APC sites.

APC with 034EFI Stage IIc
http://www.motorgeek.com/phpBB2/viewtopic.php?t=34&

http://hjem.get2net.dk/bengaard/apc-syst.htm
http://www.geocities.com/tv41mxa/tweakyourapc.html
http://www.saabnet.com/tsn/faq/apc.html


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