Pita Van 4x4 Conversion

[spencyg]

Ah, so the sensor is looking for a relationship with engine RPM thru the torque converter and transmission ratio gear stacks. That is....unfortunate.
In that case it seems feeding the ECU an alternate wheel sensor signal while in low range is likely the only option aside from hacking the ECU and trying to find the code that controls these functions (in German...!). You likely don't care about ABS working correctly in Low range anyway, so maybe disabling ABS (yet another trick I expect) and building a circuit that converts the existing wheel sensor output to one that aligns with the low range relationship between the transmission output and the wheel rotation might work.

MGMetalworks is, as we say up here "Wicked Smaht" with ECU hacking....might want to ping him.

SG

 

[BajaVan]

No we're getting some thinkers in here. I love it. I'll be the first to admit that this kind of thing is not my strong suit. So those of you with strong electrical backgrounds, please keep chiming in.

We've tried running it with the ESP off, made no difference.

Here's where we are at with getting the computer happy. Right now, the transmission has the Sprinter's valve body in it and Sprinter transmission control module (TCM) still controlling it. We've manually switched the t-case into low range to see what the vehicle does. It will downshift manually only when the vehicle is first turned on. The computer quickly realizes something is not right and defaults to 5th gear. The transmission is clearly confused and has issues going into gear. ABS and traction control still appear functioning, but it's clear the trans is not happy. Shifts are very hard and delayed as well.

The ML has a transfer case control module which links to the CAN bus system. The goal is to link this in with the hope that the low range traction control works the same as it did in the ML so that it will apply brakes to slipping wheels to help the van go even farther off road. I've found a great full ML wiring diagram which has been a big help in understanding the ML's system. I can share it too if it helps. The Transfer Case ECU looks for the following conditions from the CAN:

-All four wheels at zero speed.
CAN bus data from the 4-ETS/ABS module must show the speed of all four wheels at 0 km/h (signal from the wheel speed
sensors).
-A/T in Neutral.
CAN bus data from the ETC (N15/3) must show the transmission gear selector lever in the "N" position.
-Engine at idle speed.
CAN bus data from the engine ECM must show engine speed between 400 and 1500 rpm.
-Hi-Lo shift motor (A55) in position.
The transfer case shift motor (A55) must be in a proper position to shift (e.g., high range) and providing a valid code to the shift
motor position encoder in the Transfer Case ECM N78

I believe I've found the correct CAN bus wires to splice the transfer case control module into. The L&H CAN bus wires which plug into the TCM match voltages of those which were going into the ML's transfer case control module. But I'm not 100% sure it's that simple, again computer circuits and binary outputs are not my strong suit. We've mocked up the connections and are getting a constant 12V to the t-case motor which is an issue as it should only be seeing a momentary 12V when the T-case control module sends the ok for the motor to shift to low. The motor on the t-case is a simple 12V motor which turns a lead screw. 12V to one side of the motor turns it one way, reverse polarity and it turns it the other way.

However, after getting the T-case motor shifting properly, this still likely won't solve the communication issue between the transmission and wheel speed sensors. I'm starting to lean towards going back to the ML's valve body and swapping in the ML's TCM with the hopes that the TCM makes the connection between the two. But it's possible this will cause issues elsewhere as I'm not sure the ECM will be ok with the TCM change.

 

[luthj]

The ECM gets only limited information via the canbus from the TCM. More likely the TCM will have issues with the ECM communication. The TCM gets engine load data, throttle position etc from canbus, as these are necessary for shift control.

Is there a factory automatic ABS module and TCM module available? These units might solve your problem, and be fairly plug and play. The most elegant solution is to modify the firmware on one or more of the modules to report a consistent (as far the TCM is concerned) output from the wheel sensors. Maybe just a reflash of these modules would solve the problem? The actual hardware may be the same, and just the firmware differs.

One thought might be to have a canbus connected micro controller that would spoof the ABS modules reported vehicle speed. This is the value the cluster shows, and the value the TCM is likely freaking out about. The ABS and traction control would still work in theory. The cluster and TCM would simply see a much higher (and consistent with the current gear) vehicle speed.

The ABS module may not require and data from the TCM, if it doesn't maybe it can be isolated from the system? Just bouncing stuff off the wall.

 

[spencyg]

Hacking CAN bus systems is way outside of my wheel house but it sounds like you at least see the path here. I would be surprised if just hooking up the right wires makes everything start working between a TCM and ECU that weren't originally intended to play nicely with each other. What would be the drawbacks of just putting the ML valve body and control module(s)?

SG

 

[Haf-E]

I think a better approach might be to figure out where the output shaft speed sensor is on the regular sprinter tranny and see if there is a way to have a similar signal be produced by the ML tranny. This would result in the control of the low range shifter having to be done manually - which is better in my book.

Its possible that there is provisions for that sensor in the ML tranny and the just omitted it since they have the transfer case / low range to deal with. That would be the easiest solution - the computer wouldn't know it was in low range then.

 

[Haf-E]

I just remembered that this same error occurs if you change the rear diff ratio in the axle - which would suggest its not a sensor in the tranny - but it is using the wheel sensors to estimate the tranny output shaft's speed.

I know there must be some solution since white feather had conversions working - although I have heard there where issues with their conversions in low range also.

 

[Haf-E]

Just found a post from the top sprinter guy in the USA:

The transmission output shaft speed is not measured directly but rather is calculated by the TCM from rear wheel sensor signal data and the programmed rear axle ratio.

If you changed the rear axle ratio without changing the programmed ratio you would most likely set an immediate 'transmission slippage' DTC (2502, on a DRBIII) which will put the vehicle into a "controlled" limp home mode, and light the MIL.

A controlled limp home mode (as opposed to a 'permanent' limp home mode) in this case, would likely have the TCM place the transmission into 3rd gear and restrict it to 3rd gear only, for forward drive. Doktor A

 

[BajaVan]

The ECM gets only limited information via the canbus from the TCM. More likely the TCM will have issues with the ECM communication. The TCM gets engine load data, throttle position etc from canbus, as these are necessary for shift control.

Is there a factory automatic ABS module and TCM module available? These units might solve your problem, and be fairly plug and play. The most elegant solution is to modify the firmware on one or more of the modules to report a consistent (as far the TCM is concerned) output from the wheel sensors. Maybe just a reflash of these modules would solve the problem? The actual hardware may be the same, and just the firmware differs.

One thought might be to have a canbus connected micro controller that would spoof the ABS modules reported vehicle speed. This is the value the cluster shows, and the value the TCM is likely freaking out about. The ABS and traction control would still work in theory. The cluster and TCM would simply see a much higher (and consistent with the current gear) vehicle speed.

The ABS module may not require and data from the TCM, if it doesn't maybe it can be isolated from the system? Just bouncing stuff off the wall.

We went to the junkyard this morning and pulled both TCMs from the donor MLs. I just gave them a try to no success. Trans is not happy even in high range. Likely needs the ML valve body for proper communication. Same problem we encountered prior to swapping the Sprinter valvebody into the ML transmission.

The canbus micro controller might be a good solution. I don't think it'd be a big problem having the speedo read incorrectly in low range.

Hacking CAN bus systems is way outside of my wheel house but it sounds like you at least see the path here. I would be surprised if just hooking up the right wires makes everything start working between a TCM and ECU that weren't originally intended to play nicely with each other. What would be the drawbacks of just putting the ML valve body and control module(s)?

SG

There really isn't a downside to using the ML valvebody. It's simply a different part number as far as I can tell and I'm guessing has one or two differences electronically. What those difference are, I have no idea. They look identical and have the same number of pinouts to the electrical plug.

I think a better approach might be to figure out where the output shaft speed sensor is on the regular sprinter tranny and see if there is a way to have a similar signal be produced by the ML tranny. This would result in the control of the low range shifter having to be done manually - which is better in my book.

Its possible that there is provisions for that sensor in the ML tranny and the just omitted it since they have the transfer case / low range to deal with. That would be the easiest solution - the computer wouldn't know it was in low range then.

The ML shifted into low very easily and quickly with its electronic control. Really seemed like a good system. That and the traction control in low range would make the van so much more capable. I don't see the low range 4 wheel traction control working if we trick the system. The standard traction control will likely just apply brakes to the rear when it starts slipping, leaving the front to just continuously send power to the wheel with the least amount of traction.

It is possible that the ML's valve body did not have the output causing the issue for exactly this reason. Might be why it would go into limp mode when we had first completed the swap with the ML valve body and Sprinter TCM. Again, not sure though. It's not a huge job for me to swap the valve body and TCMs and see what it does again.

 

[Haf-E]

The only solution I can think of that would work is putting a new tone ring on the rear output of the transfer case that would have four additional wheel speed sensors on it. When you switch to low range you would have to switch from the four real wheel sensors to the new ones. Since the ratio of the tcase is almost the same as the rear diff ratio - it might work - or you could make a slight change in the tone wheel to make it match the slight difference (I.e. a couple of teeth more or less).

You'd loose the ABS and ESP benefit - and the speedo would be incorrect - but that could be lived with perhaps. Not ideal....

 

[luthj]

I would investigate the differences in the valve bodies and electronic parts. Maybe the internal tone rings in the transmissions have different numbers of teeth? Could also just be a simple difference in solenoid or sensor resistance. If you could get the ML TCM to function, that might eliminate your problems. Its a long shot though. A local Mercedes automatic transmission expert might be able to provide some insight.

You might consider posting in an ML related forum. The expertise lurking about could be surprising.

 

[mgmetalworks]

Maybe use a CAN Triple to spoof the data packets you need? www.canb.us Easy to program, monitor one bus, broadcast on another...really simple to use actually.

I haven't read through all of your posts on this yet but as a general rule, introducing a "new" module into an established network can sometimes cause communication problems if everything is not configured for it. Do you have the MB diagnostic tool? Star/Xentry?

 

[luthj]

Maybe use a CAN Triple to spoof the data packets you need? www.canb.us Easy to program, monitor one bus, broadcast on another...really simple to use actually.

Damn, that is fine piece of hardware. The challenge will be decoding the various packets. Should be pretty easy for any of the hardware developers and computer engineers out there. Really nice user interface as well.

 

[dlabl0]

Any updates? I am interested in purchasing this kit.

 

[BajaVan]

Not a whole lot to show / update. We've been very busy with developing our other product offerings. I've just been driving it a lot and putting it through the paces. Just got back from a trip up to Spokane Washington and back. So now I have over 3500 miles on it with the AWD set-up and no issues to report. Still touching 20mpg with the AWD set-up as well. The torsion bar front end needs to go. The spring rate just isn't there. So I'll be converting it to a coilover in the very near future. I'll have some photos of that soon. Hoping to finish up the van more or less by the end of this month and moving onto the next one.

A little while back, we finalized the design and ordered a set of brackets from our laser cutter. Everything went together really well and they welded up with ease. Ee are really excited to get these brackets under my business partner's 158" WB van. Hopefully we will be converting that van in October and writing instructions in the process. The finalized brackets have integrated bump stop mounts and are set-up to run a 5" stroke Fox 2.0 coil over.







We are still trying to work with a tuning company to help us develop the electronics so that Low range works just as it does in a stock ML but we have not had much time to devote to the 4x4 conversion. For now, I've just been clicking the t-case over into low range manually and keeping it in 1st gear when I really need Low range. The trans stays happy as long as I'm not shifting it through the gears.

I picked up some 17" Porsche Cayenne wheels for the Pita a few months back because a buddy gave me his Jeep JK Rubicon takeoff tires in exchange for building his JK. The wheels required a bit of machining to fit the over the Sprinter hub in the rear, but I'm really happy with the look and the ability to run stock JK take off tires since they are very easy to come by.

I also built an engine skid plate and threw one of Rackworks new 72" long racks up top. Rackworks will be making a lot more van specific racks and mounts in the near future so stay tuned for some cool stuff from them! Their racks are all aluminum and fully modular with all sorts of mounts for rotopax, Yakima / Thule accessories and more. Super cool product. Well designed, very light weight and first class fit and finish.

 

[thebigblue]

Looks great - I´m amazed that you can alter a vehicle this much, where I live (Denmark) you´re nearly not allowed to change a single bolt in a vehicle for it not to pass biannual inspection... :/