Convert 6 cyl tacho for use with 4 cyl

[black maj]

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


>From: Shawn Lin <slin01 at mail.orion.org>
>Reply-To: gmecm at diy-efi.org
>To: gmecm at 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.
>
>Shawn
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