The Saab APC system is a boost control system that regulates boost based on a knock sensor and a pressure transducer.  For a compete description of the operation, required parts, and wiring please see John Bertram's old APC Site (no longer).

Locating the Knock Sensor:

On the 700 series cars there is already a knock sensor used by the EZK ignition module so piggy backing the APC knock sensor is easy.  On the 240s there is not such a convenient place to mount the knock sensor.  On the B21FT block there is an undrilled boss located between #2 and #3 on the intake side near the top of the deck that is essentially the same place as the 700s.  Since I was rebuilding my engine and it was at the machine shop, I had them drill and tap the boss for the knock sensor.  The boss is very close to a head bolt hole and breaking into this would be bad, so keep the depth to 1/2" max.  The threads on the bolt that comes with the sensor is M8x1.25 but a M8x1.0 bolt could be used also.  You do not have to have the engine out of the car or even remove the head to put this hole in.  The boss can be acessed by removing the intake manifold.
 
 

Mounting the Control Module:

The engine bay of my 240 is rather cramped.  I didn't feel that I could mount the control module a place that would be protected from heat, accessible for adjustments, and allowed me to use the knock sensor lead as is (no splicing needed).  Because of these 'restrictions', I decided to mount the control module under the passenger side dashboard on a piece of aluminum sheet stock .  To keep my wire runs to a minimum I also mounted the pressure transducer with the control module.  The a tight fit, with the control box plug touching the top of the firewall.  I got power for the whole system from the fuse box and ran the wires for the knock sensor and the solenoid out the engine harness grommet on the passenger side.  I initially had the RPM signal coming from the tach signal from the coil but later disconnected the RPM signal for the APC (see below).
 

 

Solenoid:

I mounted the solenoid right next to the fuel distributor and the overflow tank.  This seemed to be the best place to keep the hose lengths the shortest and not be exposed to too much engine heat.  For the "R" port that needs to be plumbed back into the intake, I just drilled a hole into the metal elbow before the turbo and shoved the vac line in.  I made the hole tight so it wouldn't pull in unfiltered air.
 

 

Knock 'Light':

If a LED is connected to power and the ground leg of the LED is connected to terminal 19, the LED will light for 0.5 seconds when knock is detected.  I mounted my LED in the little blank plate above the headlight switch.  I used a 12VDC LED from a local electronic parts store.
 

 

Performance and Troubleshooting:

When I initially installed my system, the boost curve had a 'step' in it.  If I stomped on it below 2500 RPM, the boost would jump to the max I had the APC set for.  If I stomped on it and the RPM was above 2500 RPM, the APC seemed to completely open the solenoid and I would only get the 5 psi that the wastegate is set at.  This was really annoying, I wanted all the boost all the time!  John Bertram helped me swap a few parts and nothing seemed to work.  Because I have an MSD ignition I thought that it might be 'interfering' with the RPM signal and causing the APC to act funny.  I tried all kinds of combination of MSD, stock ignition, and RPM pick-up points and it still had the 'step'.  In a moment of frustration, I unplugged the RPM signal all together and, viola!, it worked, max boost at any RPM.  I have talked to John Bertram about this problem and he is taking a look at it.  If you install an APC in a 240 and get a good RPM pick-up point, please let me know.

At one point during part swapping to try to find the problem, the solenoid would cycle for a very long time at idle.  It made a noise like a loose engine valve   It was found that the pressure transducer was bad and causing the APC not to see the manifold pressure, thus causing the cycling.  I replaced the pressure transducer and the extended cycling went away.

UPDATE: I have actually disconnected the knock sensor amd am running the APC just as a fancy boost contoller. The sensor was just too sensitive to my engine. I just keep an ear open for detonation and adjust the timing on my MSD BTM when it is hot or I run regular (rare)

Page reformatted and hosted by John Laughlin on 4/30/09