7730 using $88 mask road speed constant to Speedo question

I'm tuning a 24000 vss to a fiero. I'm running 14" tires and have had to adjust the road speed constant in $88 to about 26000 to calibrate to actual road speed. No problem there.
The problem is:
The speedo is fed not from the vss, but from the ecm (7730) which converts and then divides the signal to 4000 so my Speedo can use it. However, when I calibrate the constant, the Speedo still reads the same, no matter what. So it always reads 8% more than actual.
I don't understand this because the signal is generated by the ecm....or at least i thought it was, but it looks instead to be simply filtered via hardware.
Question :
Is there a way to adjust the ppm out from the ecm to make my Speedo read correctly?
Ron

If it helps, i made a buffer from the attached schematic. Could i simply add a pot to adjust something?

Howdy,

There is no circuitry in that buffer to adjust the pulse count so no, you can't just add a pot. You can adjust the 4000 pulse output though. What's confusing is that there are two speed related constants to adjust. One is for internal math which creates the ecm calculated speed. The other is for the speedometer output and is only able to make a limited amount of change. It sounds like you've adjusted the internal math but still need to work with the divisor. The divisor has an 8 bit value but only three bits make a difference (REF Ludis Langens, GMECM):

The '64992 can divide the VSS frequency before sending it to
these outputs. The Divisor[ABC] bits select the divisor. Note that the
ECM never sees this frequency division internally. Here are the divisors:
A B C
0 0 0 Divide by 1 (ie for a 4000 ppm VSS)
0 0 1 Divide by 9 (36000 ppm VSS)
0 1 0 Divide by 7 (28000 ppm VSS)
0 1 1 Divide by 11 (44000 ppm VSS)
1 0 0 Divide by 6 (24000 ppm VSS)
1 0 1 Divide by 10 (40000 ppm VSS)
1 1 0 Divide by 8 (32000 ppm VSS)
1 1 1 Divisor disabled, no output

This might help further.
(A B & C are the 3 most significant bits of that byte value, a byte can represent a # from 0 to 255, eight bits add up as such: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255, the lower 5 bits are ignore by the chip in the '7730 that controls the speedo output)

So in theory, you can't control the Fiero speedo with the '7730 and F40 6speed ... HOWEVER! I have confirmed that no matter what code mask you run that the BC1 pin is always outputting the 2000PPM signal. So you would need a divisor of 15 if the ECM supported that but it doesn't. However, switching over output from the 4000PPM out to the 2000PPM out on BC1 gives you a free division of 2. So using a divisor value of 64 (div of 7 and div of 2) you get a divisor of 14. Using 192 you get a divisor of 16. Basically a 6.67% margin of error either way. Given I have oversized tires that puts me 2% faster going from a diameter or 24.5" to 25" then my margin of error is down to 4.67% ... and I can live with that rather than having to wire up a Dakota Digial SG-1 unit.

Note: you still need to convert the square wave(digital) signal output back to a sine wave(analog) signal for the Fiero speedo as before.
from http://www.fiero.nl/forum/Archives/A...-2-122835.html

Wow! Thanks for the explanation! I've already got the Speedo working from the ecm. It's just not "perfect", and that's what I'm looking for.....or less I wouldn't have gotten into tuning, right ;)
Basically, what I'm thinking, based on your explanation, is I need to update my wheels. It has factory 14" rims (which are hard to find tires for now and $$s!). Need to upgrade up to 17's. That's probably closer to what that trans originally pushed around anyway.
Ron