Just a quick post to let all know that I have been successful in finding a way to trick the TCM in my 98 XJR to run a different diff ratio and not go to "gearbox fault mode" and stay in 3rd.
No one has been able to help me with this issue however the tests I have done over the last two days to accomplish this have been successful 100% of the time.
My XJR came with a 3.06 diff and due to lack of sales here in Australia second hand diffs are few and far between. Getting the correct ratio has always been a problem as my car has broken a few diffs in her time.
The last supplier guaranteed me it was the correct ratio and I wasn't even sure how to tell, so I just changed out the diff and sure enough it wasn't correct.
Turns out he sold me a 3.26 and then of course the same old limp mode would come on when the box gets to third gear after about 5 seconds.
Having seen this so many times I was determined to try and get the computer to accept the new ratio and hopefully deal with it as the new found acceleration was fantastic!
After trying a myriad of things I got to thinking it may be a combination.
And that is exactly what it is.
The box appears to do a check and then doesn't try again.
So here it is:
You must lock the car in second gear and take the speed up over 60km/hr.
This will give you rpm's of 3500 or so. I am not sure whether it is speed or rev dependant, I imagine it looks at the rpm.
You now must just part throttle and hold steady for more than 5 seconds. *If you accelerate or decelerate it wont do its test.*
After you have held the beast steady for this short time drop it into third, then fourth, don't rush, and don't rush it into fifth leave it for at least another 10 seconds before you engage fifth and you have a fully functioning gearbox!
She shifts down and up perfectly and accelerates so hard in 1st it will just clip the rev limiter as it changes into second. Sounds fantastic!
The acceleration is a world apart from the old diff and I am sure excited about the prospects of a 3.54 which I believe came in some of the little 3.2 v8's out here.
If anyone out there has an easy fix to trick the TCM in its original test request in third I would be happy to hear about it. However I am sure relieved that this short 15 seconds start to my days travels now results in many more smiles than before.

 

That is absolutely fantastic news. The best part about your post is that you revealed the whole thing without being coy.

I will add a small tidbit.

In a previous gear ratio change thread, I proposed tricking the system into thinking that the original gear ratio was installed by intercepting and modifying the pulses from the ABS sensors since the only way the vehicle can compare the gear ratio is by comparing the output shaft rotation at the transmission and the wheel rotation at the ABS sensor. There are no other possible candidates for suitable signals.

Guess what? Ford Racing sells just such a *adjustable* black box in their catalog for the express purpose of fooling their own PCM when a different gear ratio is installed.

I do not know if on a Jaguar you would get to keep a speedometer that is accurate in the black box scenario. Maybe someone can point to a speedometer calibration facility in IDS/SDD. I suppose that a switch or timer could be added so that the Ford blackbox could be put into passthrough mode after the first 10 minutes of driving to get the speedo back on track.

DOES THE TEST WORK IF STAYING IN FOURTH?

I always keep the shifter in fourth at all times unless on the highway, so that would be great.

 

Thanks Plums it has been a burden of love to get this thing to work.
In answer to your question firstly, if you leave it in fourth it will stay in fourth and not change. that is to say it will start and stop in fourth.
If you drop it into fifth after the 10 seconds it becomes a normal box and you can select any gear at anytime or just drive in D. Its fantastic! I will do a video today and upload so all can see that it actually works. Of course Jaguar being Jaguar they may have changed some software from model to model that may not allow this so I must state again that my car is a 10/98 build and Australian delivered.
But to be honest a diff change takes me only 1.5 hours now as I have done quite a few. So its not a big deal to see what will wake your XJR up like nothing else!
So I think the box to trick the TCM is definitely our ultimate aim and I have read elsewhere here that the input may come from the rear passenger sender. If that is the case it should not be too hard to locate and attempt to trick, however I was told by a jag expert in Sydney that if it comes from the ABS computer it will be much more difficult.
One last note: have you still got the throttle stop kick down switch under your accelerator?
If so unscrew it and remove it. It makes no difference to the way the car changes down or operates it just gives you 100% throttle.
I am not sure whether this is a known adjustment here on this forum but check with a buddy what full throttle is while stopped and you will see what Jaguar stopped you having.
Please keep me updated if you can find out anymore about this ford trick box and I will also chase up on my end.

 

What I meant was if you pull the lever into fourth at first drive off, will the test still pass? As far as I know, the transmission will still cycle between first and fourth as required. It is only fifth that is locked out.

Or is the test specifically performed in second gear hold?

I would suspect that no change was made in the TCM since the W5A580 was used in only the SC vehicles and only for 5 years. Changes would have involved the particpation of Mercedes engineers and not free of charge.

But, the 98 might differ from the later years in the ECM since it moved from the AJ26 generation to the AJ27 generation.


Can you please hop a plane or list the procedure?

Especially where it differs from the factory procedure. Short cuts 'r us.

The Ford box looks like it intercepts and modifies the signals from all four ABS sensors and applies a change. For example the output pulse train for your application might be: pulse*(3.27/3.08) which would return the same pulse frequency for a given transmission output shaft rpm as the stock 3.08 ratio. By varying all four wheels, ABS stays functional as well as traction control.

If you were to modify only one wheel, both ABS and TRAC are going to get upset and start kicking in to get all four wheels rotating at the same speed.

The easiest would be to just buy the Ford box as long as the signals are compatible. It's already been engineered and packaged.


Bone stock so it is still there.

But bear in mind that it is adjustable. So, it should be possible to get it to be at the end of its travel when the pedal is at the position where the throttle is at 100 percent flap opening.

In theory.

The ford box is in the Ford Performance Parts catalogue.

 

I hope someone tries this (different diff+driving) on a more recent (different PCM) car and then posts back.

hmm, changing the diff is fairly painful just to try the driving part. How about using the Ford Racing box so that it looks like the diff was changed, check it causes unhappiness if no special driving, then see if the driving trick works? Much cheaper and easier than starting with the diff swap.

 

Gday Plums, Yes I did a quick 4th gear start and it just went through the gears normally till it hit 3rd and dropped into limp mode.
I have had a little more chance to do some refining of the technique and it appears to work under 60km/hr.
I ran it for 20 seconds at 50 km/hr in second and she dropped through the gears no problem.
In answer to posting my quick way in and out for the diff change I will be happy to however I think it will be some time even though I have 3.06 gear and pinion on its way wild horses couldn't make me change the car back at the moment! And they are plenty wild.
I will talk to my computer guy that does engine conversions for us here and he says he can find the output and also what type of wave it is. I will then see if it is all compatible to black box it with ford.
Does anybody know the correct coloured wire/input/location for the gearbox check?
This will make it much easier for me.

 

I am glad you were able to find a way around the $2000.00 I have paid 3 times to modify the TCM. The increase in acceleration is quite noticeable as you have already seen. If I can offer you any advice it is go straight to the 3.77 LSD from a 6cyl XJR & install the blue top solenoids Transmission solinoids solved all my problems! - MBWorld.org Forums
This I promise will keep a smile on your face for a long time.

 

Thank you for your comments JgaXkr, I will call them tomorrow and order some.
I had a look around for some different ratios from the 6 cylinder cars and I found this on ebay in England.


Differential Rear Diff Assembly Jaguar X300 XJ6 3 2 1994 1997 JLM11882 | eBay


I didn't know they made a 4.09 but I think that may be what the doctor ordered! Yeah!

 

I think the 4.09 would be great on an XK8 because it revs to 6500. I will tell you the one issue I have with the 3.77 is that when in sport mode so starting in 1st gear the car will shift into second when going slowly in traffic. With the 4.09 you may have the shortest first gear known to man.

 

@Koss, would like to see the process of foolery from the drivers seat - can you upload a video of the trick process from key on? Ad's been running a 3.54 and doing the same thing for over 12 months.
A 4.09 would do my head in, top speed would be dependent on the rev counter I'd guess if you mashed it, it would just hit the limiter then change up, all within about 20 yards!

There's a few ratio's, the 3.2 3.27 you fitted would be enough then the x300 range. Personally wouldn't go further than a 3.54, Jag made them from 2.88 to 4.09...

 

Thanks Sean B. It is good to know someone else has been able to fool the damn Mercedes computer.


I will upload a video in the next couple of days when I get a chance as the festering season is in full swing here, and so are the police. we have a new law here in Australia that you can not even touch your phone whilst driving so I will have to mount the GoPro somehow.
I think the 3.54 too would be a super fit.
Does anybody out there actually know where the pick up for the ratio check to the TCM is derived from? (1 ABS sensor or from the entire ABS unit?) I have a computer gut here in Aus that can check its wave form and build a box that will adjust the ratio but first I have to find it.
Cheers.

 

Did you mean this one Plums? SPEED-DIAL SPEEDOMETER ADJUSTER | Part Details for M-4209ADPT-AC* | Ford Racing Performance Parts
I don't think that will help...since the speedometer is running on the CANBus.

Don't know about the X308, but I couldn't find any means of adjusting the speedometer apart from selecting a wheel/tyre size.

The problem is if you start to play around with the wheel speed sensors, it'll probably throw up other error codes for the ABS and DSC systems.

Looking at the X308 XJR wiring diagrams, there are two internal speed sensors in the W5A580, and the signals are fed back to the TCM. From what I can tell, you'd have access to those wires.

I guess the sensors are measuring the input shaft speed, and the output shaft speed. That might be something to investigate, maybe a converter box on one those sensors? Or feed a dummy signal to the TCM to get it through the test?

I don't know, just throwing some ideas out there.

One other thing, according to the CAN Message Matrix Table in the back of the wiring diagram, the signals of all four wheel speed sensors is sent out by the ABS/TCCM modules, and received by both the ECU and the TCM. The right rear sensor goes additionally to the Instrument Cluster.

But it's all on the CANBus...

 

What about modifying the reluctor rings on the hubs? Turn the old teeth off all 4 reluctors, get custom rings machined up with the tooth count adjusted by the change in diff ratios, shrink fit them to the reluctor bodies, and you should be away. Not a complicated job for any shop with with a CNC mill, so shouldn't be horribly expensive.

Only issue is your speedo will now be wrong.

 

@cambo351

not that one

the one i suggest is listed next to the differentials and gearsets in the pdf catalogue.

the two signals involved in the gear ratio check are the transmission output shaft speed and the abs sensor. there can be no other way.

modifying only one abs signal is useless because it will kick in both abs and trac. modifying all four leaves the system happy and funtionally intact.

the likelihood of compatibility is high since it is interposed in the abs sensor circuits and they are almost universally hall effect sensors.

the signals are changed before ever reaching the abs module or canbus. they are not aware of the lie.

ps. the reason for two sensors in the transmission is to detect internal ratios.

 

or are the reluctor rings different on say a 3.2L equipped with a 3.27 diff?

 

Please be so kind as to post up a picture or a link, the only one I could find in the pdf was the same one I linked to before.

 

"Sensor Rotor" is the same for all engines/diffs by the look, p/n CBC4705

 

Hey sean b, I have a better solution for the trick. Plums asked what happens if you start the car in 4th gear as it should think it is just in 4th. The problem is it cycles through from first and as it reaches third does its check and goes to 4th. What I do now is simply engage 2nd drive off as the car cycles 1st to 2nd I simply pull it gently through to 4th and changes to 3rd then 4th and cant do its test. Drop it into top and she is away. No more holding it at any speed or any revs.
Pretty cool.

 

That's a very worthwhile update.

Play boy racer for the first upshift sequence and you're good to go

Especially since I habitually have it in fourth and sport anyways ... it wouldn't be much of a change.

Are you changing your diff by just sliding it out the front after removing the front pendulum?

BTW, is being at 60+ km/h in second still a requirement?

 

Nope. No revs at all. Just drive normally but slide it past 3rd. Job done.
As for the diff change I undo the frame and drop it down with a trolley jack. Pull it out the back of the car and then rattle the frame apart and back together. Then just a matter of reversing the procedure back in. I think having air tools for everything speeds the process up immensely. I have a friend with a shop that specialises in suspension and to watch how fast he is has probably rubbed off on me.