[TR] Adding a VSS pulse generator inline with speedometer cable

[John Innis]

Ok, so a little googleing shows that most of the GM VSS output a 0-5vdc
square wave at 4000 pulses per mile.  So at 60mph, you are going to get a
square wave with a 66.6hz frequency.  Assuming you are running about 3000
rpm on a 4cyl engine at the same speed, your tach pulse will be a 0-12v
irregular spike at about 200hz.  The two frequencies are close enough that
you might just have to reduce the voltage down to get the tach signal to
work.  I'd use a Schmidt trigger to reduce the signal to 5v and try it.
Once you get down to 5v, there should be no risk of damage to your cc
module.  If it doesn't work, then you will need to add a frequency devider
to requce it to a point that it operates.  Should only take 3 or for
components to build the circuit.  You could probably do the same thing with
just a transistor but you might not get a clean enough square wave to make
the CC module happy.

On Wed, Aug 15, 2012 at 9:04 AM, Brad Kahler <bkahler1 at gmail.com> wrote:

> No, I don't.
>
> Since the new cruise units are designed to work with modern cars I would
> imagine whatever is typical for a modern VSS would be appropriate.
>
> Just shows you how little I know about modern cars :)
>
> If it was made after about 1973 I don't know much about them!
>
> Brad
>
>
> On Wed, Aug 15, 2012 at 10:02 AM, John Innis <jdinnis at gmail.com> wrote:
>
>> You could also get (or build) a converter that will take ignition pulses
>> and give you a VSS signal.  Do you know what the Specs are for the VSS that
>> your unit wants?
>>
>>
>> On Wed, Aug 15, 2012 at 8:59 AM, Brad Kahler <bkahler1 at gmail.com> wrote:
>>
>>> Geo,
>>>
>>> This one doesn't allow connecting to the ignition coil as a speed source
>>> either.  It seems that all of them on the market now require a VSS
>>> signal,
>>> pulse generator in the speedo cable or else magnets on the drive shaft.
>>>
>>> Also the one you've listed is a vacuum unit and I prefer the electric
>>> servo
>>> type.
>>>
>>> I installed an SCS/Frigette cruise control with electric servo unit on my
>>> Spitfire back in 1998 and it worked great.  Bud Rolofson then installed
>>> one
>>> on his TR6 and it's been working great all these years.  Now that I want
>>> to
>>> install one in my TR4 they don't make the style that will sense speed
>>> from
>>> the ignition coil.
>>>
>>> The two simplest options are a VSS in the speedometer cable or the
>>> magnets
>>> on the drive shaft.  I haven't given up on the speedometer cable option
>>> just yet, still a few more places to check with.  Barring that I'll
>>> probably end up with the magnets on the drive shaft.
>>>
>>> Thanks,
>>>
>>> Brad
>>>
>>>
>>>
>>>
>


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