Repairing the Saab 9000 ECU

By Saabben

Note

This procedure was carried out on a 1997 Saab 9000 Aero. I expect there will be no differences between this and other post-93 cars with the Trionic engine management system. This procedure will not apply to the 3.0 v6, light pressure turbo’s without a boost performance control solenoid valve or any pre-'93 cars with the APC system.

Introduction

Most post-93 9000’s employ Saab’s own Trionic engine management system together with a boost performance control solenoid valve (AKA BPC Solenoid) to control turbo boost. The ECU sends a sinusoidal waveform to the boost control solenoid which in turn opens the waste gate via the waste gate actuator to limit boost. Due to the BPC Solenoid being a mechanical device, it is prone to wear and occasionally the solenoid will jam which normally causes one of two of the coils inside it to burn out and short circuit. In this case, fuse 5 in the fuse box will blow to protect the ECU however; I and others have discovered this is not always the case.

I first discovered the problem when my car was not boosting much over the basic boost pressure (some have also found that the car will continually over-boost) and after some investigation I discovered that fuse 5 had blown. I replaced the fuse, which blew again within a short period of time, which pointed me to the BPC Solenoid. I purchased a new solenoid, replaced the fuse and checked that it was humming on ignition which it was. Shortly after I did this however I noticed that fuse 5 had blown again. I tested the resistance between the pins on the BPC Solenoid (circa 3 ohms between pins 1-2, and the same between pins 2-3) and confirmed that one of the coils had short circuited.

I discovered later that the reason the ECU had blown the second BPC Solenoid was because one of the three MOSFET’s in the ECU which control the BPC solenoid was damaged (due to fuse 5 not protecting it?) and instead of supplying a sine wave was just supplying ~ -13v which burnt out one of the coils very soon after the ignition was switched on. The below procedure is how I repaired the ECU in my car to restore the which I so sorely missed!

Time required

Including removing and reinstalling the ECU in to the car, the job should take about 45-75 minutes to complete.

Tools required

To remove the ECU from the car

• Torx® (T-25) key
• 8mm Socket

To repair the ECU

• Small flat bladed screwdriver
• small craft knife
• soldering iron
• solder
• de-soldering pump
• ohmmeter
• Philips screwdriver

Procedure

1. Firstly, buy the replacement MOSFET’s, I actually used MTP3055V but as these are no longer available an RFP12N10L or a similarly specified MOSFET should do, you can buy these from RS stock number 295-703 here in Table 13: MOS Power - Low Voltage 10 to 20A. It is advisable to buy a few of these as you may have to replace all three (I only needed to replace one) and it's always good to have a few spares!
   
   
2. Once you have the MOSFETs you need to remove the ECU from the car. As a precaution disconnect the battery and then remove the Torx® T-25 fixing screw and 7x8mm screws holding the top of the false bulkhead in place. Once removed, disconnect the washer jet hose and lift the false bulkhead cover free and put it to one side.
   
   
   
   
3. You should now see the ECU on your right hand side (facing towards the front on the car), unclip the ECU from its brackets and disconnect the ECU from the Saab wiring connector at the back of the unit. Disconnect the earth cable and remove the ECU from the car.
   
   
   
   
   
   
4. Once you get the ECU out of the car, unscrew the 6 screws from the top and bottom of the metal casing and pull out the circuit board.
   
   
   
   
5. You will see that on the right hand side of the PCB there is a bank of 3 MOSFET’s and on the other side there is another. The side with three is the side which controls the BCV.
   
   
   
   
   
   
6. Gently remove all three of the metal clamp covers from the MOSFET’s with a small flat bladed screwdriver, then with a craft knife or similar, gently prise the back of the FET away from the heat sink, they are only held to it with a conformal coating, which the whole board is dipped in at the factory to insulate it from moisture.
   
   
7. If you have an ohmmeter, measure the resistance between the base or metal back part of the FET (you may have to scrape away any residual coating to get to the metal back) and one of the three pins. The reading should be in the range of mega-ohms on all tree pins, If you find that the reading is in the kilo-ohms range then the likelihood is that this is the FET that is giving you the problem. Continue to check the other 2 MOSFET’s just in case. All three MOSFET’s are identical, so the resistance on each of them should be near enough the same. If you do not have an ohmmeter, replace all three MOSFET’s.
   
   
8. Once you have identified the faulty MOSFET’s remove them by desoldering them from the reverse of the board. Once this is done the replacement MOSFET’s can be put on the board in the same orientation as the old ones. Solder the MOSFET’s from the reverse of the board.
   
   
   
   
9. Once you are happy that the three MOSFET’s on the board are now ok, put the metal clamp covers back on the MOSFET’s and reassemble the ECU back in to its metal box.
   
   
10. Replace the ECU back into the car in the reverse order that it was removed. Check fuse 5 and check the Boost Performance Control Solenoid is circa 3 ohms between pins 1-2, and the same between pins 2-3. If not, buy a new one before testing the ECU, otherwise you run the risk of blowing it again!
    
    
11. Test it out!

Thanks go out to Scaero who supplied the pictures of the inside of the ECU.