I've read about the TCM of the Mercedes gearbox on the XJR not liking a differential ratio change. Does the same apply to the ZF gearbox of the NA cars with the ZF gearbox?

In theory a 3.06:1 in a 3.2 litre would reduce acceleration but reduce cruising revs. Likewise a 3.27:1 in a 4.0 litre would give sprightly acceleration at the cost of a lower top-end.

Is it possible to swap without upsetting the TCM?

 

To my understanding not only do you have to reprogram the TCM, you have to get past the ABS module as well. Options are very limited either without a mountain of work, research time and of course loads of money.

 

It's possible to fool the Mercedes 722.6 TCM (Siemens EGS51) in the XJR by manipulating the ABS wheel speed readings reported over the Canbus to that module (and no others), and so change the ratio without issue.
This works because the EGS51 only sanity checks ABS wheel speed readings against the input shaft speed + ratio selected (there is no output shaft speed sensor on 722.6)
So if you change diff ratio and change the wheel speeds to what it would expect to see, it doesn't know it is being fooled.
Of course the overall program/shift strategy is unchanged even though it is now unknowingly working with a different ratio.
Back in the day it was also possible to get the EGS51 directly re-programmed for a different ratio (at great cost)

I would expect the ZFs to do sanity checks on diff ratio, but I've not researched it personally.
If so, whether the same ABS wheel speed manipulation strategy would work for the ZFs (5hp24, 6hp26) may depend on how the (Bosch?) TCM firmware does any sanity checks to see if the wheels are going around at the speed it expects,
so someone would have to try it out and see......e.g. maybe it also cross checks reported vehicle speed from ABS or instrument pack, which would then also have to be manipulated, etc. etc.
Alternatively TCM could be re-flashed, probably = $$$ per Addicted2boost, if anyone provides a service,
or DIY if there is good reverse engineering info available....

 

Gday
I saw in a post a few years ago that a member changed the diff ratio from 3.08 to 3.27 on his 4.0, as i was looking to do the same, and claimed that after the change he taught the TCM the new ratio by driving and maintaining 2ng gear for a couple of km's did the trick.? i think it gives the TCM time to get past a sanity check and stores the new info??
if i can find the thread ill send it to you.
I understand its a big risk, but anyhow you may see it else where to give you more confidence?
Cheers
John
ps i never got around to trying it as other issues got in the way

 

i found the post....was in 2013... so my recollection was not that great....


https://www.jaguarforums.com/forum/s...ferrerid=57334

Cheers
John

 

Interesting... that thread relates to the XJR and its Mercedes gearbox, so I wondered whether the TCM is common to both, but as a few have said it's likely the ZF has the same or similar checks which would make this a dive into the unknown.

I'm not in a position to "try it and see", so if nobody else has gone this path I think I'll probably stick as I am. I'd like lower cruising revs in my 3.2, I think it would help fuel economy as I'm a motorway cruiser... but it's not sufficiently important to balance the risk that I waste money on a diff that I can't use.

Thank you all for sharing your thoughts - appreciated!
Jeremy