722.6 and differential change
 

Thinking of changing my differential. Will there be any issues with esp, abs, tranny issues or just my speedometer will be off a bit? thanks

If the ratio is different than the original, the transmission will report an error of gear ratio and go into limp home mode.

meaning - it wont work.

Bad news... i also have 124 with 722.3. I guess a diff swap there will be fine?

On the 722.6. According to an aftermarket trans repair manual, changing internal gearing of the trans will result in an ratio error. ( MB used 2 ratio sets between 4 / small 6 and large 6 / V8 / V12 , Chrysler has 3 more from what I recall. ) In this case the different ratio is interpreted as clutch slippage and the trans will drop into limp mode.

There may or may not be an issue with changing the rear gear. The only info a trans may get in relation to rear gear is a speed signal from the RF ABS sensor ( this supposedly runs the speedo ) and maybe a rear wheel speed sensor / pinion speed sensor.

A really quick way to test is to pull the trans computer from the rear you want to use and install in your current car.

And, if there is an issue, building a CAN filter would solve the problem. A CAN filter looks for a specific information designator and value on the CAN bus and replaces it with a different value. In your case you need to make the trans / engine computer think the car has the old rear ratio. Look at some Euro sites that offer odometer filters for the theory. ( I have not tried a rear swap )

The TCM looks for output from the 2 speed sensor inside teh transmission, but the speed of the vehicle is also always monitored for optimum performance - if the vehicle speed does not make sense to the PCM - it goes into limp mode. Usually with a gear ratio implausible error.

This is also warned in the ATSG manual.

The two speed sensors in the trans are input shaft ( turbine from the torque converter ) and output shaft ( to the drive shaft ) . Looking at road speed is redundant, but that isn't to say it was programmed to look for a rear gear difference and trip a code.

One way to check is to disconnect the drive shaft and see if the trans shifts. Another way is to unplug the ABS computer and / or wheel speed sensors. I'm hoping that the programmer didn't put in a no speed signal OK , some % from factory NG.

97 SL320 suggested some good stuff to try. I see that in ebay you can buy the transmission computer for about 100$. I am thinking this should solve the issue if the computer is from car with the rear gear i want to use. What do you think?

You should be able to get the trans computer for less if you dig around. Also look on car-part.com it is a nation wide ( USA and some of Canada ) salvage yard inventory search where you deal direct with the yards to purchase.

Thanks for the pointers. The longest petrol diff is 2.65. There is diesel one that is 2.47 though. Are the shift points set in the computer? I mean if i buy the diesel tranny comp, will it shift at like 4000rpm or will still hold upto 6000rpm like now?

Shift points are in the trans computer, I don't know of any to change them. Although, if you sued a CAN filter you might be able to fool the trans computer into thinking the engine speed is different than actual. Also, there is a CAN ID from the engine computer to the trans computer that signals engine torque, this is vital in setting line pressure rise so the trans does not shift too softly and burn up.

With the TCM, the none tiptronic cars the module part number starts with 026, and all the others will probably 022.

Tiptronic shifter interface is CAN interface to the TCU
Non tiptronic shifter is Binary interface to the TCU

M119 & M120 are the same module.
M112, M104, M104 & M111 are different
M113 has two versions of the module as the +2000 R129's with a M113 came with a non tiptronic shifter, yet the W220 with the M113 came with tiptronic shifter...

Get your version of the TCU for your engine, and the shifter version, so it works!!

Martin