VSS

Charles Morgan cmorgan at efn.org
Tue Mar 3 21:22:13 GMT 1998


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     




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