VSS and freq converter circuit (real long)

[Andris Skulte]

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 was just about to post something similar, but I need to convert the
signal. I'm converting from my T5 to a T56 transmission, which has an
electric sender. The biggest problem is that the pre-90 camaros have a
mechanical sender. The firebirds had electric after 86, I think. I'm
picking up a firebird electric speedo and VSS box, which will wire right
up. My challenge is to make the cyberdyne calibration box. This box
converts the 40 pulse +12V signal per shaft revolution to a 2000 pulse
per mile signal (what my VSS Buffer expects). Here are some emails I've
gotten before, but I'm a MechE, who knows just enough electrical stuff
to get in trouble. The technical stuff will have to be explained, so I
can go to Active Electronics, buy the parts, solder them together, and
off I go :)

Peter Paul Fenske wrote this:
>The truck 40 tooth reluctor system needs a DRAC ie digital ratio 
>adapter. This converts the high res VSS signal to 2000 and 4000 
>pulses per mile. To use this system raid any later S10 or
>PU and steal the drac. Mind you they are gearing tire spec.

GM made a piece tha converts the 40 to a 2000 signal? And then I would
just change the nylon reluctor drive gears in the tranny to get the
right ratio, right? Where is the DRAC located on the car, and what does
it look like (Is the DRAC the VSS buffer box?)

Shannen Durphy wrote more on the DRAC
>5) 91 to present pickup truck has DRAC, (Digital Ratio Adapter Controller),
>which can be programmed via jumpers to multiply and divide speed signals.
>DRAC also has outputs for cruise, ABS, and speedo.  If interested, I can
>furnish some info, especially on programming DRAC.  Speed sensor for this
>type vehicle is 40 tooth.

Yes, more info would be good :)

Eric Schumacher wrote some circuit stuff:
>This is very straight forward to do electronically. In years past I made a
>few of these things to use 6cylinder Porsche tachs in 4 cylinder VWs. They
>are most easily implemented in 4xxx CMOS logic. Doubling is almost trivial
>(That's an engineering term :)) to if all you want do is double the
>frequency. The scheme for doubling goes like this: Get the input to
>approximate a square wave.  Edge detect the nominal square wave. Stretch the
>edge detector output pulse with a oneshot to a pulse that is just wide
>enough to look like a squarewave at the maximum RPM that you want to
>resolve. That's it. At the most this takes two 14 pin dips. An edge detector
>can easily be made by differentiating the input signal with a RC and feeding
>it into one input of an exclusive of gate while the original signal goes
>into the other XOR input. the XOR output will pulse once for each transition
>of the input wave.

What is CMOS logic, and can I learn (be told) what to do with it easily?
I sortof follow what Eric is saying, but don't know how to accomplish
this.

If you read all this, you must be bored ;) Seriously, thanks for any
help. I know Cyberdyne makes a black box that lets you adjust for ratios
in addition to converting for $70, but if I can make it myself for $10,
it would be nice. Later!

Andris
PS-I ordered the turbos for the IROC yesterday :)