Hello all,

I have a 2000 XKR with the Mercedes W5A580 transmission. I've read from posts on this forum, that a differential ratio change is doable with a reprogrammed TCM. I've also read that such a TCM reprogramming (or reprogrammed TCM) is available at around $2K (it's worth it for me).

However, I have no idea where to go or who to contact for such a thing? Do you have any idea what direction to point me in?

Thanks for any help you can offer, Eric

 

I imagine you are looking for a lower ratio? I remember reading that cars sold In Mexico had a lower ratio, nothing huge but definitely lower.

mind if I ask what prompted this? Do you want quicker off the line acceleration?

I bet just about all of us thought of this once or twice, especially those of us with normal aspiration.

it'll be quite a hunt given compatibility, availability, and programming issues, but I hope you have nothing but good luck.

John

 

Hi John,

Yes, you are correct. I've done a number of HP upgrades over the years (SC porting & pulleys, cold air intake, exhaust & cats, etc.), so I'm happy with the motor. However, I like quickness and don't drive at really high speeds. Accordingly, I find the 3:06 stock rear end to be lethargic from a standstill.

Although it has been discussed a number of times on the various forums, my understanding is the naturally aspirated X100 models have a ZF 5HP24 transmission that does not facilitate a final drive ratio change as it would confuse the the TCM/ECM systems due to changes in the speed of some sensors while not changing others (wheel speed vs. others) and will consistently put the car into a "gearbox fault" status.

On the other hand, the SC X100 models (XKR), have a Mercedes W5A580 (722.6, NAG1) transmission that can facilitate this with a reprogrammed TCM (at least as is indicated in various forums).

My search is for a source for such a TCM reprogramming.

Eric

 

I have used a 3.2 liter XJ8 diff (3.27:1) (imported from Europe) on a customer car. I removed the 3 bolt JURID flange and cut the pinion shaft extension. I then used the 4 bolt 'U'joint flange from the XK diff.

It is easier to find a used 3.2 liter XJ8 from Europe than a XK diff from Mexico because of the sheer numbers manufactured.

They are both 'open-diffs' but you can install a 'tor-sen' or QUAIFE aftermarket limited slip to the crownwhleel.

A few forum members have had their TCMs recalibrated to the new ratio.

bob

 

Thanks Bob,

I'm hoping to hear from one or more of those forum members that can point me in the right direction for securing the TCM re-calibration.