Randall, that is exactly what I believed to be true about how tachs
actually convert the "signal"going to them into linear movement!! I
believed that is was voltage, not "frequency" that actually drove the
needle of the tach. I will see if I can get the schematic out of one of
the service manuals, and try to get it to you.
Many Thanks!
Perry
----------
> From: Randall Young <randallyoung@earthlink.net>
> To: 'Perry Robinson' <PROBINSO@flower-mound.com>;
buick-rover-v8@autox.team.net
> Subject: RE: TR7 to V-8 Tach modification
> Date: Friday, March 26, 1999 5:41 PM
>
> Perry :
>
> Many tachs can be switched to/from 4/6/8 by changing a resistor, but it's
> not usually in series with the connection to the coil. For analog tachs,
> there is a simple circuit inside that converts the frequency of the
pulses
> from the coil to a voltage that drives essentially a voltmeter movement
> attached to the dial. The resistor scales the voltage for the number of
> pulses/revolution.
>
> There _are_ circuits that could be scaled by a resistor in series with
the
> coil connection, but they would be sensitive to changes in dwell, battery
> voltage, etc. I doubt you will find a commercial tach that uses them.
>
> I've never seen a TR7 tach (my TR3 tach is mechanical), but if you can
send
> me a schematic I can probably tell you how to modify it.
>
> You might also try asking on the Triumph list. triumphs@autox.team.net
>
> Randall
>
> On Friday, March 26, 1999 2:44 PM, Perry Robinson
> [SMTP:PROBINSO@flower-mound.com] wrote:
> > Ron,
> >
> > Thanks for responding. I understand what you are saying. Is it that
> most
> > "modern" tachometers use a different type of circuitry that measures
the
> > pulses from (or to) the ignition coil, and then converts it to a signal
> > usable by the tach INSIDE the tach itself? How is it that you can
> convert a
> > 280 Z tach (a 6 cylinder) to work with a v-8, by simply adding a
> resistor?
> > And how is it that one can purchase a cheap parts house tach, that can
be
> > used on a 4, 6, or 8 cylinder by simply the movement of a switch on the
> > tach?
> >
> > Inquiring minds want to know!
> >
> > I could just use a "speed shop" tach, but I'd rather keep mine in the
> dash.
> >
> > I have also had more than one shop offer to send my TR7 tach out to be
> > "modified" to work with the 8, for about $75.00
> >
> > Let me Know! :)
> >
> > Perry
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