After breaking more than my fair share of SAAB T5 Boost Control Valves because of running higher turbo boost, I built an interface that converts the signals from the ECU, that drives a 2-coil BCV used in SAAB’s with the T5 Engine Management Systems, to use the more reliable and less expensive T7 BCV. The T7 BCV is a single coil valve that operates at a lower PWM frequency. There are ways to do this with your existing T5 ECU that involve hardware modifications and firmware adjustments to the ECU. I choose not to modify my ECU but design a small simple hardware adaptor. I’m sure there are hardware adaptor solution available that do basically the same thing as what I have done here. However, having a shop full of parts and equipment, I opted to roll my own and share the design here.
Schematics
The design is based around an ATMEL ATtiny45 MPU, a simple 7805 5V regulator, a couple of opt-couplers to provide level shifting for the input and a N-channel MOSFET to drive the T7 BCV.
SAAB Interface for T5 engine using a T7 BCV
Components were chosen to minimize part count.
Inside assembly view
ATtiny45 C-code
The firmware uses timer0 for the clock base to bit-bang the PWM signal for the T7 valve. Timer1 is used to determine the time and duty-cycle of the signal from the ECU. An ISR for input level changes (PCINT0) drives the input state machine and signals when to compute the output PWM signal and frequency.
Click to download:
boost1.c source code
Installation
Electrical installation involved replacing the stock 3-terminal Bosch style plug that goes on the T5 BCV with an AMP Mate-n-lok plug, for which I have all the tools to assemble. A ground wire is required for powering the interface and a 2-pin plug with some wire from a scrap SAAB T7 engine to mate on to the T7 BCV.
Complete assembly with connectors