1992 1227730 VSS re-calibration after a T-56 swap
NOTICE: These instructions are provided for educational and benchtop experimental use only, by advanced hobbyists and the like. Amatwur twiddling with engine control parameters in vehicles intended for road use is not what I am advocating here. Recently it has been discovered that people (imagine!) are selling info like this, found on the net, on Ebay. This info is FREE and you get what you pay for.. no warranties, guarantees, or promises. Steal this info and sell it and rot in the hell of boiling sewage.

ON to the IN-Fooo...

After my T56 swap I discovered that the conversion boxes for getting the speedometer and the computer to read the correct vehicle speed were like $90 and more, and were rumored to be troublesome (although I have no direct evidence that they are). I already have adopted the "fewer parts to fail" attitude so I did not want a little bug to be there and have to stay working all the time for me to drive my car. Given all that, I thought I'd look into making one myself. Well after some research I found that my 92 ECM and my 92 instrument cluster both supported a scaling factor parameter, set on the chip. It took a while to figure out the scaling for the Speedometer factor but trial and error prevailed.
Its trickier thsn most settings on the '730 ECM..since the ECM variable is OR'd with a value that already has meaning in the code, only a certain set of values can be used. For those technically interested, here is the breakdown of the Speedo scaling variable, best guess so far:

The 3 high order bits at 8014 in AXXC (92 305 TPI M5 3.08) control the scaling of the speedo in my application (remember, I do not know at this point, nor do I care to go find out, what other years this applies to. I know the 90 year F car uses a magnetic VSS where mine was optioned for the optical unit. this does not nessecarily mean the re-scaling wont work.. in fact, the scaling bits ARE sent to the external hardware when the MAGNETIC *OR* OPTICAL sensor is used.). This is a hardware divider sent via SCI (serial control interface) from processor to a descreet circuit. from the best I can gather at this point, thru eyeball testing is this: 

00000000     0       =do not divide (aka div by 1)
10000000   128       =divide by 2
01000000    64       =divide by 3
11000000   128+64    =divide by 4
00100000    32       =divide by 5
10100000   128+32    =divide by 6
01100000   128+64    =divide by 7
11100000   128+64+32 =divide by 8


6.17.01
Ignore the "divide by" figures in the chart above... Here is some new test data: 

Drive Testing results:

Observed MPH , 4th gear, @ 2500 rpm

bit 7 set: = 96 mph
bit 6 set = 83 mph
bit 7 and 6 set = 73 mph
bit 5 set =  65 mph
bit 7 and 5 set = 58 mph (closest I can get.. pretty dead-on using a chase vehicle)
bit 6 AND 5 set = 53 mph (amazingly, I still have room to go)

So bit 6 corrects to 86% of what bit 7 corrects to.
     bit 5 corrects to 67% of what bit 7 corrects to

When bit 6 AND 5 are BOTH on, the correction is about 86%*67% of what bit 7
     corrects to. (58% by math, 55% by testing(eyeball speedo))


For reference:..
@ 2500 rpm... 1:1 trans gear, 3:08 ratio  (I'm going about 58 mph.. turns out)

B7 b6 b5 b4 b3 b2 b1 b0 (b4-b0 are not used in the correction)

B7 set =       (10000000) value of 128... speedo reads 96 mph
B6 set =       (01000000) value of 64...  speedo reads 83 mph
B7,B6 set =    (11000000) value of 192... speedo reads 73 mph
B5 set =       (00100000) value of 32...  speedo reads 65 mph
B7 , B5 set =  (10100000) value of 160... speedo reads 58 mph
B6 , B5 set =  (01100000) value of 96...  speedo reads 53 mph

(NOTE made 1-13-02: testing says my speedo is off the mile-markers by 1/16 of a
 mile, per mile, or mine reads 60 when I'm going 56.5 mph)

When I set B7, B6, and B5 all together (11100000).. ths speedo just reads Zero.
When NONE of these 3 bits are set (the factory setting), the speedo pegs 110
when I'm going 15 mph. The SPEED LIMITER will shut your injectors off if you
do not disconnect the VSS from the trans... but disconnecting the VSS makes
then engine stall when you come to a stop, because the ECM does not know
that the car has transitioned from moving to not-moving.

Imagine ASSUMING any of the above!?!  the numbers are way out of order.. but
if you look hard you can see that the significance of the bits is reversed. look
at 128(b7) as being "1", 64(b6) as being "2", and 32(b5) as being "4" and it all
makes sense ;-))

Armed with this vital information, yoo too can be a speedo recalibramator!

If anyone reading this is doing or has done a T-56 swap into a 92 TPI car, and wants to save some money and start the trek into prom burning at the same time... OR you want me to test to see if the fix applies to YOUR year TPI car,...

  • buy a factory memcal for your 92 '730 TPI car and de-solder the chip.
  • Install a ZIF socket in the GM carrier.
  • email me (dzug@eecis.udel.edu) and arrange mailing me the de-soldered prom
  • I'll read it, and change the speedo scaling bits and ECM VSS scaling byte bits for you, and mail it back... for nothing more than a thank you. If it works out for you and saves you from buying a conversion box then send me a bottle of mobile-one synthestc oil ;-)

    • notes:
    You must send a factory GM chip, I will not make one from an image that someone found for you., sorry. You must not be doing this for someone else and getting paid for it. it has to be YOUR car. There are probably plenty of other folks who will accomodate you if you are a shop.... as long as they can understand the changes I outlined above.... and let me know how it works out!