[Diy_efi] Modify tacho signal.

[Ian_Haynes at amat.com]

Hi everyone,

I am new to this list, so go gently with me...

I have a 1990 3 series BMW that originally had a 2.5 litre 6 cylinder
engine. This has been replaced by a 3.2 litre 6 cylinder engine. Since the
conversion, the Tach has been reading approx 12% high. Not a big deal, but
something I would like to correct.

I have looked into the electronics in the Instrument cluster, and all the
calibration seems to be done with a small 8 pin EEPROM. All the different
models of 3 series have a different module depending on engine type etc. It
is not going to be a case of simply changing resistor/capacitor values. The
EEPROM is labelled as an HML 087. I believe this is some sort of BMW
special, and cannot find any data on it.

The Tacho signal comes directly from the main engine ECU, and I assume is
just a frequency that is proportional to RPM. What I want to do, is modify
this signal before it gets to the Instrument cluster.

Does anyone know if it is possible to make a small circuit that you feed
the Tach frequency signal in one end, reduce the frequency by approx 12%
and then send it on its way to the Instrument cluster. Ideally this would
have some sort of adjustment either by a potentiometer of some DIP
switches.

Would it be possible to use a frequency to voltage converter, reduce the
resulting analogue voltage and then use a voltage to frequency convertor ?

All ideas gratefully received.

Many thanks,

Ian Haynes.


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