Re: Convert 6 cyl tacho for use with 4 cyl

[Shawn Lin <slin01@mail.orion.org>]

black maj wrote:
> 
> Hiya
> 
> Where abouts is this resistor??? as my holden speedo is reading around 25%
> too fast as well and i have almost resorted to building a small gearbox to
> reduce the VSS drive... do you have any idea about which diode to change in
> order to fool the speedo into thinking that it';s going backwards??? as my
> current gearbox spins the VSS the wrong way, hence the odo only works in
> reverse... :(
> 
> Ben

In my '90 Grand Prix instrument cluster, the resistor is on the circuit
board very close to the terminals that the electro-mechanical motor
assembly plugs into.  The resistor is etched onto the back of a 14-pin
DIP which is white in color.  I can't remember, but I think the 14-pin
DIP has no function except as a surface for the resistor to be etched
upon.
I'm not sure how your VSS works compared to mine.  In my car, the ECM
converts the VSS signal into 4000 pulses per mile.  Each pulse is a
ground pulse.  If your car is wired similarly, then perhaps your ECM is
not converting the VSS signal correctly with the car going forward. 
However, if this were the case, then your speedo, if it is indeed
electric, would not work either.  So... I guess it's safe to say that I
really haven't got a clue what you need to do to get your odo working in
a forward gear!
This is just a far-out suggestion, but maybe you could try inverting it
with a 7404 IC or something like that.  Probably won't work, only reason
I even mention it is because I was able to get workstation monitors to
work on a standard SVGA video card by inverting the H-sync and V-sync
pulses in a similar fashion. ;)

> >From: Shawn Lin <slin01@mail.orion.org>
> >Reply-To: gmecm@diy-efi.org
> >To: gmecm@diy-efi.org
> >Subject: Re: Convert 6 cyl tacho for use with 4 cyl
> >Date: Mon, 22 May 2000 01:01:41 -0500
> >
> >Peter Gargano wrote:
> > >
> > > Two questions:
> > >
> > > 1. What's the cheapest way to convert an analogue electronic tacho,
> > > set up for a 6 cylinder, so it reads correctly for a 4 cylinder.
> > > Assuming there is no access to the internal electronics, just the
> > > external signal from the coil. (logically, the tacho requires 3
> > > pulses for every two pulses the coil generates).
> >
> >This seemed easy until you assume no access to internal electronics.
> >
> > > 2. What's the easiest way?
> >
> >IMO, the easiest way is to change the value of the calibration resistor
> >for the tach.  On the early 90's GM cluster I've played with (from a
> >Grand Prix), the cal resistor is laser-etched on the top of a white
> >14-pin DIP.  Less resistance across the terminals, and the tach reads
> >higher.  More resistance, and the tach reads lower.  I noticed my speedo
> >read about 25% too high, so I jumpered a 1 megohm (or was it 10 megohm?)
> >resistor across the original cal resistor and was able to get it 100%
> >accurate (compared to the digital HUD and my multimeter's frequency
> >counter).  I played with the tach a little, and it worked the same way.
> >I think this would be easier than building any kind of conversion
> >circuit.
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