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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