VSS

[Clare Snyder]

Charles Morgan wrote:
> 
> At 09:21 AM 3/3/98 -0500, Christopher Bruno wrote:
> >I was curious what some people have done
> >to add VSS output to older cars that didn't have
> >them initially.  JTR sells a VSS 'box' that attaches
> >inline with the speedo cable and will output the
> >standard 2000 pulse per mile required for the 85-89
> >GM computers, but it is $70. I would like to keep
> >my stock speedometer if possible, if for nothing else,
> >just so that my dashboard matches.  What are other
> >people's expereience in this area?
> 
> Chris, I believe the JTR device uses a signal generator made by Rostra
> Precision Controls.  It is a rotating magnet device that fits inline with
> your existing speedometer cable.  As I recall, there are two versions--one
> outputs 2 pulses/rev, and the other 8 pulses/rev.  The speedometer cable
> connects to the generator, and then a short extension cable connects to your
> speedometer, leaving its operation intact.  If your speedometer is rated at
> 1000 rev/mile, then the first generator would produce 2000 pulses/mi, which
> is what your computer needs.
> 
> The generator output, however, is a psuedo sine wave, whose amplitude and
> frequency both vary with rotational speed.  This psuedo sine wave must be
> digitized (turned into a square wave).  I don't know if the JTR device does
> that or not--you need to ask them.  If not, there are a couple chips
> designed to do exactly that--the LM2907/LM2917 and the HIP9020.  These chips
> can drive transistor switches or solid state relays if an on/off signal is
> required rather than a square wave.
> 
> I installed a Chevy 4.3L V6 in a Corvair, using the Corvair transmission.
> Therefore I needed a speedometer cable driven VSS.  Unfortunately, the
> Corvair speedometer is driven by the left front wheel, and the speedometer
> is rated at 825 rev/mi (based on a nominal tire size).  I used the 8
> pulse/rev generator, and designed a dividing circuit to reduce the
> 825x8=6600 pulses/mi down to 4000 pulses/mi (for the cruise control) and
> 2000 pulses/mi (for the computer).  The dividing circuit is adjustable using
> a potentiometer, so that I could adjust for varying tire sizes (similar to a
> DRAC).
> 
> Charlie Morgan
How about a simple da converter to change frequency to voltage, then an
AD converter to change back to frequency, with a pot and/or bias control
to scale the critter?
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