[Diy_efi] Modify tacho signal.

[EFI Dude]

Going F->V, then V->F seems like an unnecessarily lengthy way of getting the
job done, though it can work.
If you can get a clean square wave tach signal, then the conversion should
not be a problem using a simple PIC or other microcontroller.  Should be a
simple matter of counting input pulses and outputting a modified count of
pulses.

Cheers,
-Neil.




> -----Original Message-----
> From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On
> Behalf Of Ian_Haynes at amat.com
> Sent: Saturday, November 23, 2002 3:40 PM
> To: diy_efi at diy-efi.org
> Subject: [Diy_efi] Modify tacho signal.
>
>
> 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 This has been replaced by a 3.2 litre 6 cylinder engine.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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