Transfer case ratio

[ctonton]

Is there a way to adjust the default t-case ratio in EFI Live? I have been looking for this parameter and can not find it. I am certain that the GM Gen III PCM contains this info in the tune for 4WD applications. Can the option be added to the program? Thanks

[turbo_bu]

The standard parameters in EFI Live do not show the transfer case ratio. It can be adjusted, but you will need a CAX file to do it. I have found it for a few GEN III OS's if you need some help.

[ctonton]

If you can point me in the right direction for the CAX file, that would be great. I am attempting to convert a Gen I 383 SBC in a K5 blazer over to a 24x Gen III PCM running a speed density COS and run a 4l80e transmission with my NP208 transfer case. This is my first project working with the new PCM so any help would be greatly appreciated.

[turbo_bu]

You basically have to build the CAX files. Most of us just use some of the examples out there and modify them to match what we are wanting to change. The change part being a hex address in the OS. If you have the gear ratios handy, it might be easier to send me your file and I can make the change. I don't remember off the top of my head what a 208 ratio is :).

[ProperTuningOG]

Where is the speed sender? That determines the need for 4low.

[Big Mike]

Curious why you need to change it. The 208 has a 2.61:1 low range and so does any Gen 3 vehicle I'd presume you would've based your tune off whether it was a 246 or 261/263 T-case.

[ctonton]

I will be using the 12212156 OS from a 2002 Silverado 4x4 with a LQ4 engine.

https://www.dropbox.com/sh/4ydh5wltk3ssxbw/AAB-pmCDYRPohKFwBeBitk4Ia/GM%20Truck/2002%20Chevrolet%20Silverado%20Truck%20Automatic%2 0LQ4%206.0%20Litre%20%28PCM%2012212156%29.tun?dl=0

The truck came with a np246 transfer case with a 2.72:1 ratio in low range. The transfer case I will be using, the np208, has a 2.61:1 ratio. A Dakota Digital 128k ppm VSS will be located on the output shaft in the transfer case. I know it is a small difference in the gear ratio but I am concerned that it will throw DTC codes in low range or possibly put the transmission in limp mode.

[ctonton]

I found something that might be helpful for creating a CAX file.
21326

[turbo_bu]

Yes, the Tuner Pro XDF files are very helpful. I have learned a lot by studying those, looking at BIN files, and reading a disassembly from a 2001 OS. It looks like this XDF is for a 12216125 OS. The address that is listed is slightly different than the one that I have. Do you mind sharing the XDF file, or where you found it?

[ctonton]

I found the xdf here.

https://www.thirdgen.org/forums/diy-prom/674434-xdfs-hack-00-01-a.html

I doubt that it will work for the 12212156 OS that I want to use though.

[turbo_bu]

12212156 is very similar to the 2001 OS's. The flex fuel 2002 OS (12216125) is where the 02 OS's get a little different.

From looking at the binary files from different vehicles, I think that the address that is shown in the XDF file is slightly off. In general though, what you are showing is basically what would need adjusting in your flash file.