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Re: Electric Tach question

To: Randall <randallyoung@earthlink.net>
Subject: Re: Electric Tach question
From: Joe Curry <spitlist@gte.net>
Date: Sat, 03 Jul 1999 18:36:33 -0700
Cc: George Hahn <geohahn@azstarnet.com>, TR <triumphs@autox.team.net>
References: <199907031514.IAA18305@blackie.cruzers.com> <377E8FA8.21E0@azstarnet.com> <377EB754.4011B4D3@earthlink.net>
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


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