I have a 91z28 and a t56 tranny and as you all know the speedo dont read right. I now have an auto meter programmable speedo to fix the calibration problems but I still need to tell the ECM the correct PPM rate. Can I have the prom recalibrated for the t56's PPM rate? I really dont want that SGI 5 box again.

 

maybe.

you have to adjust the speedo divisor bits and also the pulses per mile.

divisor bits:

http://www.eecis.udel.edu/~dzug/92z2...0-vss4T56.html

I don't have anything with me right now to tell you what values I used, but I have 3.23's on 245-50-16 and was able to get it perfect just about (approximately 1% difference in odometer reading over a 100 mile trip). I am NOT sure though, that it has enough range to handle your 3.73's. IF i remember correctly, I was already at the limit of the divisor. with your deeper gears, you need to slow the speedo down even more.

EDIT: i just re-read your post. I didn't realize you had an aftermarket speedo. Since you ditched the factory speedo, the answer to your question is YES; you need to adjust the pulses per mile and increase them. I don't have the value I used with me, but it's significantly larger (the t56 vss is 17 pulses per rev!)

 

ok with that beig said, How can I go about getting this gone? Is it something that has to be a trial and error thing or can I just find someone to ship my prom to to have it modified?

 

it's not really trial and error..

the information is all available... have to search through the archives. thats what I did when i went t56 and wanted to use the stock speedo without dropping $100 on some converter box.

Since you don't have to worry about the speedo, all you have to do is get the pulse per mile setting set...

you just have to set a new VSS divisor in the prom. default, if memory serves, is right about 4000 (4013 I think?). i don't have any of my equipment to look, i'm at work...

one of the principles this board tries to promote (and I support) is to learn to do this stuff on your own (hence the DIY in the forum name). being able to do changes and tuning like this yourself is an incredible thing... you should read some of the stickies and get involved, you certainly have enough modifications to make it worth your time! Heck, I just have a stock L98 with a t56 (no cam/gears/heads/etc like you) and the increases in drivability from minor tuning are incredible. The more people who get involved, the more who benefit - including yourself!

anyways, i'll get you started...

https://www.thirdgen.org/techbb2/showthread.php?s=&postid=1641621

https://www.thirdgen.org/techbb2/showthread.php?s=&postid=1647972

that should get you started at least understanding the differences.

I will post tonight the actual value I'm using (3.23 car) that is dead on (for me). There seems to be a bit of confusing in the archives about how many pulses per mile etc. the t56 vss gives off...

but none the less, it is doable and i would encourage you to get into prom tuning and do it yourself!

 

I am going to get into it this winter. My car hasnt been driven in 2 years due t a lot of mods being done. I had a custom prom burnt for me from fastchip just to get the car running so I can at least have a couple monthes of fun with it this year. Once I get all the equip., I am gonna learn it and make a copy or 2 of my stock prom and custom piggyback prom so I have something to fall back on in case I mess up. thank you for the info

 

cool.

i'm using 43886 pulses per mile for my road speed constant with 3.23 and 25.5" tires..

SABLT194 in one of those threads I posted above, is using 54198 pulses per mile with 4.11 and 26" tires.

I would suspect that you're looking at a value of about 50,000 for your setup.

 

ok great, I am gonna try and see if someone can help me out with that in my local area. I just dont have the equipment to mess with the prom right now. I wan to learn this prom stuff so bad I just get lost when I start reading about it in the DIY section. Im sure I iwll get it soon or later.

 

by the way, reading the stickies is very educational, but it's also very helpful to buy the software and hardware and once you see it in front of you it helps a lot to actually _see_ it.

the solution to this post is really this simple:

(1) load tunercat
(2) file | open, and open the .BIN
(3) Tables | ECM Constant table
(4) scroll to "road speed constant"
(5) type in a value (say 50000)
(6) File | Save
(7) File | Exit
(8) load pocket programmer software
(9) pop a chip in the programmer
(10) click "load buffer" and select the .BIN
(11) click "program device"
(12) wait 10 seconds.
(13) place chip in car, enjoy.

 

I just read one of those links and I notice that Dyno Don had posted some pics of what looks to be a modified VSS housing on the T56. Can I just modifiy the VSS hole to accept a T5 VSS and then just plug the stock connector into it?

 

no clue. I did not even research any mechanical solutions. I wanted an all electronic solution (thats why I got into this DIY PROM stuff!) so that's the path I took. I have the factory original speedo and ECM for my '91 working exactly right with whatever factory VSS came with my t56.