frwilk@the944.com

Motronic Math

This applies to model ML1.2 only unless otherwise stated. The Motronic digital fuel injection systems uses an 8-bit processor. The word length is 8-bits. Each 8-bit word contains an executable instruction or a piece of data. The data is arranged in Hexadecimal (Hex) format for use with computers. Hexadecimal is a base 16 system much like the decimal system is base 10. If you are not familiar with Hex, letters are used to display numbers 10 and above as shown below.

These 8-bit words will vary from 00h to FFh (0 to 255).  Hexadecimal numbers are 2 digits wide followed by the letter "h". All numbers herein are decimal unless followed by the letter "h" to indicate hexadecimal.

Angular Measurements

The computer gets engine rotational information from two sensors. They are the engine-speed sensor and reference-mark sensor.

The reference-mark sensor detects a single set screw on the outer edge of the flywheel. All engine angular measurements are made with respect to the crankshaft's top-dead-center (TDC). The reference-mark sensor produces a single pulse corresponding to engine TDC.

The engine-speed sensor works much like the reference-mark sensor except that it senses the teeth on the starter gear attached to the flywheel. For the 944, there are 130 teeth on the starter gear and the engine-speed sensor produces 130 pulses for each crankshaft rotation. Counting these teeth gives the angular information to the computer.

360/130/4 System or Quarter-Tooth

Angular measurements using a 130 tooth starter motor gear limits the accuracy of a Motronic system. One full crankshaft rotation of 360° using a 130 tooth gear gives a disappointing 2.77° per tooth. Ignition advance angles would have to be multiple of 2.77° which is not adequate. BOSCH introduced the 360/130/4 system or quarter-tooth system. Timers are setup by the computer to divide each gear tooth into 4 even segments. This gives the value of each quarter-tooth 0.692° which gives the Motronic its accuracy. This is for a 944 with a 130 tooth starter gear and can vary with car model as shown in the following table.TDC has an advance value of 00h.

Changes for the ML3.1

Negative numbers for advance are allowed for the first time. A value of 00h stands for -20° advance. Do to lack of referance material, this number -20° is close but may not be exact.

Frequency

Frequency data values can vary from 0 to 255. Each digit is valued at 40rpm. The largest engine rpm that can be understood by the Motronic computer is 10,200rpm.

255 X 40rpm/digit = 10,200rpm

To calculate the Hexadecimal code to set the maximum engine cutoff at 7,000rpm for example, we divide the 7,000rpm by the conversion factor as follows:

7,000rpm / 40rpm/digit = 175 or AFh

Example

For an 1986 944, lets raise the rev limit to 7,000rpm. The address for the rev limit is at 0171h and the value is A2h. A2h converts to decimal 162. Multiply by 40rpm to get rpm and we have 6480rpm for the stock value of the rev limit.

Now converting 7,000rpm to hex, we divide by 40rpm to get 175. converting to hexidecimal, we get AFh. We now store AFh at memory location 0171h and our rev limit is now 7,000rpm.

Changes made to newer models
Porsche 944S, 944S2, and 968

Starting with the Porsche model 944S, a third DME change was made to the BOSCH M2.1 version of the Motronic. The starter gear is no longer used for speed sensing but a dedicated timing wheel has been added to the flywheel. It requires only a single sensor that detects both reference-mark (TDC) and engine speed.

The new timing wheel are designed for 60 equal sized teeth and spaces. Each tooth is 3° wide and the gap between teeth is also 3°. There are 2 missing teeth so there are actually 58 teeth. The reference-mark is the first tooth following the 2 missing teeth. The waveform can be seen below indicating the reference-mark or TDC.

The new sensor is now ±edge triggered. This gives 120 rising and falling edges with a resolution of 3°. This 3° is divide up into a quarter-tooth of 0.750° just like with earlier Motronic types.