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?
Get an APC box and the knock sensor. Part 7566599 seems to be the
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
|Knock Sensor, should be connected
|"+" 12v ign switched.
|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.
|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