Replacing a Porsche 944 AFM With a Modern MAF
Simple hack cuts costs, modernizes L-Jetronic engine control.
Early electronic fuel injection was a major step forward in car engine control. Their systems however often wear out and cause glitches over time. There are simple solutions however, like this one that recently impressed website hackday.com.
Air Flow Meters, also known as an AFMs are common in older engine control systems. They’re usually based on a flap that moves a potentiometer wiper across a carbon trace. This trace is prone to wear out over. AFMs also restrict air flow in some cases, limiting performance, but that is not part of this discussion.
More modern Mass Airflow Sensors, otherwise known as a MAFs instead measure the amount of air flow through with a hot wire. The amount of current required to maintain the temperature of the wire indicates the amount of air flowing through the sensor. They’re less restrictive and are readily available as they’re used in many modern cars today.
Rob Replaced his Porsche 944 AFM With a Modern MAF
Reader Rob owns a Porsche 944, which had an old-fashioned mechanical Air Flow Meter. It had worn out and caused the old 944 to cough and splutter. Rob decided to replace his Porsche’s AFM with a modern Mass Airflow Sensor.
Running a MAF in place of an original AFM requires a circuit board to emulate the AFM’s output. Rob built a rig to blow air through both devices in series and measured their output on an oscilloscope.
He used a STM32 Cortex-M0 to read the MAF, outputting the voltage to the Porsche’s engine computer via Pulse Width Modulation and a low pass filter. Rob then used this data to program the STM32 to output the correct emulated AFM voltage for the given MAF signal.
Rob Built His Own Rig to Measure AFM Output
“My 1989 Porsche 944 project car has a Bosch L-Jetronic fuel injection system,” inventor Rob explains in his report. “Its Air Flow Meter measures the volume and the temperature of air entering the engine, to enable the ECU to meter the correct amount of fuel.
“The original AFM potentiometer has a carbon track that wears out over time. “A Porsche replacement AFM is expensive, so people resort to hacks such as bending the potentiometer arm. “I fitted a modern VW/Audi MAF sensor to do the same job in my car.
“To do the conversion, you need some plumbing and a ‘mapper’ that converts the MAF voltage level to what the ECU was expecting. “I used an STM32 Cortex-M0 microcontroller with a built-in Analogue to Digital Converter for the voltage input.
Rob’s Porsche Now Runs Happily on New VAG parts.
“I used Pulse Width Modulation for the voltage output and built my own Printed Circuit Board with the schematics inspired from here. “The finished ‘mapper’ with the red PCB in the picture now successfully runs the engine. “I kept it in the family too – the MAF is a VAG group one from an Audi TT!“
Hackday.com has seen similar hacks done to other cars before, but Rob now has his Porsche running happily on new VAG parts.
Images: Rob, Porsche