I have examined the insides of a Spit's Smiths electronic tach and it is
controlled by a Texas Instruments integrated circuit. This circuit is a
custom product made specifically for Smiths to incorporate in their
Tachs. Therefore, I doubt anybody will be able to locate any specific
details on the function of the device not the input/output parameters.
Therefore, the best approach is to intercept the signal going into the
Tach and change it at that point. Instead of having 4 pulses each
revolution, you would be getting 8. So it stands to reason that for a 4
cyl tach to be accurate on a V8, you would have to divide the number of
pulses coming from the coil tap by 2 and feed the remaining pulses to
the tach.
That would satisfy the requirement of the tach and not necessitate
opening the unit at all.
Dan Masters can probably whip out a circuit to supply that function in
his sleep! Right Dan?
Dan! Dan, are you listening?
Regards,
Joe
Randall wrote:
>
> 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
--
"If you can't excel with talent, triumph with effort."
-- Dave Weinbaum in National Enquirer
|