Building on the canbus decoding efforts of crbass, I thought it likely that by changing the values of the wheel speeds reported by the ABS module to the TCM module, the TCM can be fooled into accepting a different rear axle ratio.
This would provide a cheap alternative to reprogramming the TCM, which is really expensive if you can find anyone to even do it.
My 2002 XKR has a 3.27 differential (was available on some UK and Mexico XJ8s I believe) and a reprogrammed Mercedes TCM (for the 722.6 transmission).
By buying a breaker yard XJR 2001 TCM module which is programmed for the standard 3.06 ratio, I was able to test out the concept of mismatched TCM and axle.
I built an Arduino based canbus repeater to intercept the messages between the TCM and the rest of the canbus (ECU, ABS, INST).
I used two MCP2515 based canbus modules, one facing the TCM side of the bus, and the other facing the ECU/ABS side of the bus.
Since the Mercedes TCM module is conveniently located next to the the ECU in the 2002 XKR, it is easy to snip the canbus wires at the TCM connection, and bridge in the repeater module.
The repeater module receives the messages on the TCM side, and sends then onwards on the ECU/ABS side, and vice versa.
There is a small delay because the whole message needs to be received before it can be sent on, but this seems not to disturb the correct operation.
The ABS wheel speed messages towards the TCM are intercepted, and the wheel speed values for all 4 wheels are scaled appropriately so that the TCM is not aware of the different axle ratio, and then sent on.
I driven a fair few miles, and I'm happy to report there are no codes and no errors so far, everything seems to work, smooth shifting, etc.
Note I did check as a control, and without the conversion the TCM throws a P730 (incorrect gear ratio) within about 30 secs, and in car display reads "Gearbox Fault"
I'm thinking about changing to an HU15 rear axle since it is stronger than the stock HU14, and maybe a 3.58 or 3.77 ratio, but I needed to figure out if the electronics issues could be solved first.
So for anyone considering a rear axle ratio change, this seems to be a viable route based on successful testing so far.

 

Details archived here
https://github.com/chirpy8/canbus_x1...xleRatioChange

 

Great effort!

 

I did this ratio change (3.27:1) for a member mjlaris years ago. (Last Activity: 10-08-2018 02:11 PM)
He bought a Quaife unit to install in his XKR (4.0 liter).

https://www.quaife.co.uk/quaife-prod...-differential/

He had to send his TCM out for reprogramming while I installed the diff and a bunch of other upgrades to the SuperCharger system.
The car had improved acceleration (wow).

 

wow this is a game changer. it allows you to use the JK ring gear for maximum strength.

if you go 15HU remember that the stock XJR6 powerlok clutches are a joke

 

Motorcarman - Yes, this is Mark's old car. SC bearings went bad, so I was 'forced' to call Avos. Goes a bit quicker now, hence concerns for flaky HU14.

Xalty - Are all the HU15 powerlocks questionable? (various years - XJ6, XJ12, XJS, ...)
If so, what are the upgrade routes - Dana44 + Torsen, Auburn, etc.? I've not looked into that yet, I guess research is needed........

 

yes they barely worked as an LSD when they were new and even less as they wear out. i think the 15hu still uses the dana44/4hu clutch packs so you can rebuild it and put a more aggressive set on, ask this guy.


https://youtu.be/TBVOn-ls9XY

 

I have a 15HU (1996 XJR) being readied at the moment and first thing I noticed when I opened it up was it is NOT a Powr-Lok in the sense of all previous Jag diffs. While it is still an LSD, It does not have the ramped two piece cross pin arrangment and more closely resembles a Dana Trac-Lok 80. After some research online it appears this is the case with all the 15HU LSD I could find.
It is still going to be superior to the 14HU in the car at the moment.

 

So my dilemma is whether

a) get an XJR6 3.58 locker and bracket (I have a line on one, but not sure whether to pull the trigger - is that a mixed metaphor...), and then have to rebuild an unknown amount of it - seems at least clutch packs

Or b) risk life and limb at the local pick a part crawling under an '95-'97 XJ6 balanced on 3 wheel hubs (plus the 4 axle stands and 2x4s I'll bring along to supplement), pull a non-locking carrier and front bracket, dump the guts, and start from scratch with all new internals, a bag of shims, and endless hours of fun - more expensive but I'll get a 3.73 or 3.77 in the end, and an 8.9" would be kind of cool.

I want to do a) because I'm lazy, but I don't want to start at a) and end up at b) anyway.
I guess I'll plough ahead with a) and cross fingers. (Knowing me, I'll do both in the end)

 

l will be doing (a) myself Chirpy. If not happy down the track l have a few actual Powr Lok carriers l believe l can swap into the 15HU but still run the 3.27 gears.

 

nitro just put a 19 spline torsen on the market. dual drilled and ready for the 15HU ring gear. uses normal d44 carrier bearings of coruse

https://www.vividracing.com/-p-153399121.html


Inner pinion bearing set: 12252 (HM89446/HM89410)

Outer: ATA7166 (M88043 + M88010)

Seal: CBC6923

Crush sleeve: 12456

Coated washer: JLM10782

 

Great info.
I did not realize the 30 spline dana44 was much more common than the 19 spline until I started looking into it more.
I was interested in Torsen T2 andT2R but they all seem to be 30 spline. And procuring custom 30 spline stub axles seems to run a lot of money....
(Some earlier XJS - maybe 85-87 - seem to have a 30 spline Dana44 carrier, but the output bearings seem to be unusual, and not sure whether the carrier would be easy to adapt/fit)
So I have been researching possibly putting an Auburn posi (clutch) or Eaton Detroit Trutrac (gears) into an 15HU carrier, as alternatives to getting/refurbishing an original 15HU posi.
- annoyingly the 30 spline dana44 trutrac seems to be considerably cheaper than the 19 spline.
This is another nice option. I could not find any info about the torque bias ratings on a quick scan through though.

 

https://www.speedmaster79.com/DANA-4...p-Differential

found the source, unit comes out of australia. everybody else rebrands and marks them up. stick to 19 spline, the strength gains are negligible