It depends on the circuit, but most of them use a simple RC frequency to
voltage converter circuit, where the output can be scaled by changing a
single resistor. Thus, your hand-held tach probably had resistors
connected to the 2/6/8 switch, and many in-dash aftermarket units have a
couple of resistors mounted on the back, where you cut one lead for 6
cyl, two leads for 8 cyl.
Unfortunately, I've never seen a Smith's electronic tach, so I have no
idea where it's scaling resistor is. Nor do I know if a gauge shop
would have this info.
However, even if the info on the Smith's circuit is unavailable, it
shouldn't be too hard to build a new one and hide it inside the case.
Randall
59 TR3A "daily driver wanna be"
George Hahn wrote:
>
> TeriAnn Wakeman wrote:
> >
> > ...How hard would it be for a gauge shop to convert the 4
> > cyl tach to an 8 cyl tach?
>
> Many years ago I had a cheap hand-held tach with a slide switch for
> selecting 4, 6 or 8 cylinders. This leads me to suspect there's a
> simple electronic component that can alter the signal appropriately.
> Now if someone can just tell you what it would be...
>
> Geo Hahn
> 59 TR3A
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