you already mentioned that input is ac, so to make it square wave, all you need is a bit of signal conditioning and Schmidt trigger is just the right device for this.
there are many ways to make this, for example using gates with Schmidt trigger or just good old 555:
http://www.tradeofic.com/Circuit/771-SCHMITT_TRIGGER_SINE__SQUARE_WAVE_GENERATOR.html

next step is to reduce frequency. in digital world this is cone by counting.

So, given a variable AC input of 0 - ~2,000Hz, what might I construct that would modulate that signal to 0 - 212Hz? The 2KHz number is a rough guess.

To go from 2000 to 200 Hz, you need to divide by 10, so you need counter that counts to 10. there are infinite possibilities but one of them could be also very popular 4017 for example (note: output here is not 50% duty cycle without additional components).
common task in digital circuits is to come up with a counter that counts to some N.
http://www.electronics-tutorials.ws/counter/count_2.html

 

Here in Australia there was a Kit for correcting mis match betwean sender & speedo. I have built one some yeas ago for a vehicle that didn't have a standard dif. No idea if its still available, but it was in Silicon Chip magazine Dec 2006 as Speedo Corrector MK11. Was as a kit from Jaycar KC-5435.

 

Here in Australia there was a Kit for correcting mis match betwean sender & speedo. I have built one some yeas ago for a vehicle that didn't have a standard dif. No idea if its still available, but it was in Silicon Chip magazine Dec 2006 as Speedo Corrector MK11. Was as a kit from Jaycar KC-5435.

Excellent! However, their correcter won't take the AC signal my VSS puts out. I just sent an email to them to see if they might have another solution, perhaps their device could be modified to suit my needs. At least this shows me that such a thing exists, and its name gives me different search terms. The correct catalog number is AA0376.

 

Im not into design, but all you need is a circuit to change the sine wave to a square wave. That's what the Kit uses as a drive signal.

 

maybe this is helpful.

 

Okay, after reading all the replies, for which I thank you all, which have pointed me in all sorts of wonderful directions and actually think (Heaven forfend!) about what sort of device might fulfill my needs with absolute minimal design and construction, I've come up with what will probably work: a frequency modulator. if modulator is the right term. I already have an AC signal in and a digital (again, if that's the right term), signal out. All the PCM needs is a digital signal at the proper frequency, so all I need to do is slow the GEM's digital signal down. A proper vehicle speed signal is approximately 2.2Hz/mph, so 10mph = 22Hz, 60mph = 132Hz, and so on.

So, how do I build a device to do that? As I've mentioned, I've never designed a circuit, so I need very specific instructions, as in what attaches to the 555 chip and what resistors go where. From what I've read a 555 will do what I want, but I don't really know for sure. The diagram I posted in response to roadey_carl's request shows that the GEM signal is sent on a single wire to the PCM et al, if that makes any difference.

Thanks,
Bob

 

you already mentioned that input is ac, so to make it square wave, all you need is a bit of signal conditioning and Schmidt trigger is just the right device for this.
there are many ways to make this, for example using gates with Schmidt trigger or just good old 555:
http://www.tradeofic.com/Circuit/771-SCHMITT_TRIGGER_SINE__SQUARE_WAVE_GENERATOR.html

next step is to reduce frequency. in digital world this is cone by counting.



To go from 2000 to 200 Hz, you need to divide by 10, so you need counter that counts to 10. there are infinite possibilities but one of them could be also very popular 4017 for example (note: output here is not 50% duty cycle without additional components).
common task in digital circuits is to come up with a counter that counts to some N.
http://www.electronics-tutorials.ws/counter/count_2.html

See my answer toward, or at, the end of the thread. Your suggestion steered me toward a simpler solution. The circuit you show looks a bit complicate for my skill level. Is there something easier about? Also, the frequency change isn't a straight multiple, though it is linear.

Thanks,
Bob

 

maybe this is helpful.

Oops. I didn't realize your "this" was a link. And that device would be perfect . . . if it could divide the signal by more than whole integers (if that is the correct description). I don't know if you've read my new post, but I've discovered (stumbled across?) a simpler solution: All I really need is something to drop the frequency of either signal, though I'd prefer the GEM's digital output. The problem is that the necessary drop has to result in a 2.2Hz/mph ratio. Is there a device that can condition the signal to a decimal number? At least the ratio is linear, which, I would think, make such a drop relatively easy.
Assuming my solution is possible, might you have a circuit in mind that a barely-above-novice-circuit-builder could put together? Well, maybe an actual novice.

Thanks for the help,
Bob

 

Dear Friend
I can now tell you i am living in the same of your problem from one year ago and more
And just today i fixed this bad problem
The reason of this problem that the Vss send the signal to ABS and the ABS make the converting the frequency to the readable signal to the ends like the PCM, Odometer, GEM and so
All signal out from the ABS to all ends
All read the signal correct only the PCM which read the frequency more than it and duble of it
Today I removed the wire which comes from the ABS to the PCM
The wire must remove from the PCM plug which in number 58 place in the PCM plug
Do that and tell me what you feel when your problem fixed
You will find after that the check engine indication light on do not care
It will give you code P0500 VSS problem
That because the PCM not see it
Feel the Freedom and have the fun with your Ford