Factory Instrument Panels

[Scott Feaver]

After looking over the circuits used on both tachs, I came to the conclusion
that the only difference was the capactor in the RC network.  On the
4-cylinder model, it was a .47uF cap, and the V6 one has a .33uF.  I just
swapped that part over to the turbo tach and all was well :)

The redline on the Turbo was 6000, on the V6 tach, it "orange-lines" at
5500, but red starts at 6000.  Max engine speed (as limited by the ecm) is
6200rpm.

Thanks everyone for their help.

One thing that I noted is, the resistor network was identical on both
boards.  And the circuit is very close to the one in the tech note from
National.


> -----Original Message-----
> From: owner-diy_efi at efi332.eng.ohio-state.edu
> [mailto:owner-diy_efi at efi332.eng.ohio-state.edu]On Behalf Of Ludis
> Langens
> Sent: Wednesday, January 13, 1999 6:51 PM
> To: Diy_efi
> Subject: Re: Factory Instrument Panels
>
>
> "Scott Feaver" <sfeaver at cgocable.net> wrote:
> > The reason I am asking is I need to adapt a tachometer module from a 90
> > Turbo Sunbird to a 91 Sunbird.  The difference is the 90 is a
> four cylinder
> > car, and the 91 is a 3.1V6.  The tach is reading about 50%
> higher in the V6
> > (which makes sense because for every 2 pulses it expects to
> see, it receives
> > three).
>
> As other's have mentioned, GM likes to use the NatSemi chip.  GM's
> circuit is almost the same as the schematic in NatSemi's data sheet.  An
> RC combination calibrates the tach.  I doubt you can find a temperature
> stable capacitor of the correct value.  So you should change the
> resistor.  On the tach circuit board you'll find a thin (thick?) film
> resistor pack (in a DIP package).  The resistor element on the top layer
> is the calibration element.  You'll notice it is laser trimmed/cut.
> You'll need to bypass this resistor with your own.
>
> What's the redline on the 2.0turbo tach?  The 3.1 might need a different
> limit - probably 6000 rpm.  Of course this is just an instrumentation
> issue.  The tach redline needn't agree with the ECM redline.
>
> "David A. Cooley" <n5xmt at bellsouth.net> wrote:
> > Usually there is a resistor to be changed in a tach for the number of
> > cyls... not sure if the dash's had a switch or jumper or
> socketed resistor,
> > but I can't see them using 2 different tachs for the same year,
> different
> > engine...
>
> Wanna bet?  A parts book shows eight different tachs used in one model
> of car over a five year range.  (L4 vs V6, US vs Canada, non-backlit vs
> backlit, w/ oil gauge vs w/ volt gauge vs w/ no gauge)
>
> --
> Ludis Langens                               ludis (at) cruzers (dot) com
> Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/
>