[Diy_efi] Modify tacho signal.

[Scott & Jennifer Peitzsch]

> 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 This has been replaced by a 3.2 litre 6 cylinder engine.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.

For a simple and readily available solution, the Dakota Digital SGI-8 would
solve your problem nicely.  Check out:

http://www.dakotadigital.com/Detail.cfm?Category=122&PartNumber=SGI-8

For $80, it fixes the problem and can be at your doorstep in days.  I've
used
these in applications where I had to correct a speedo for a gear swap (and
couldn't do a custom PCM) and they work well.  Only drawback is that they
can sometimes be a bit noise sensitive.  They require a VERY clean power and
ground feed to function well - which can often mean using one of those
cheapy
Radio Shack car stereo noise filters in some cases.  Either way, a small
price
to pay IMO.

-Scott


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