'02 Mercedes ML320 with transmission "limp mode" issues...
 

My '02 ML320 will randomly go into what I hear is a "limp mode" that keeps it in whatever gear you are in, whether going 20 through town or 50 on the highway. For example, when coming to a stop from 50mph it will try and start itself out in 4th gear and will just accelerate very slowly. This can be fixed when we put it into neutral, turn the engine off, restart, put back into drive and it's wiped clean and good to go.

I have seen and read about similar issues in other ML320s... here is another example of a similar issue from a different person- http://www.peachparts.com/shopforum/ml-gl-g-wagen-r-class-unimog-sprinter/104862-99-ml320-shifting-problem-please-help.html


Where is the best place to start when troubleshooting this issue?


I appreciate any assistance. Thank you very much!

I will post the Codes asap.

Thank you.

Best place to start is to get the codes read. Report back what you find out and we can go from there.

Here is a list of the codes currently showing on the ML320:

PO335- Crankshaft Position Sensor - A Circuit
PO720- Output Shaft Speed Sensor Circuit
PO600- Serial Communication Link
PO400- Exhuast Gas Recirculation Flow

To describe in more detail…

After starting the ML and driving it operates normally until triggered into “Limp Mode.” There doesn’t seem to be a certain speed, gear, amount of time or anything noticeable that makes the ML go into Limp Mode, it just happens when it happens. The ML will beep 6 times, all of the dashboard lights come on, the speedometer drops to zero and the ML stays in whatever gear you were driving in before Limp Mode starts. Which means I can keep driving in that same gear until I come to a stop, in which case it is easy to put into Neutral and restart, which resets Limp Mode and allows the ML to run just fine until it goes into Limp Mode again.

Usually in a 20 mile drive I will have to restart from Limp Mode at least once or twice. During an 80 mile stretch yesterday I had to restart 4 times before I got home. The transmission shifts well and doesn't make any noise. There are no leaks.

Does anyone have any ideas or troubleshooting steps for me to pursue?

Thank you for reading! I appreciate any feedback.

Do you have access to an MB SDS computer? 722.6 diagnosis can be quite simple using SDS but if you don't have one it's just a guessing game.

Okay, here's how I would proceed. The other codes you pulled may be legacy ones (not active in other words, the algorithm stores them for a while and unless the trigger is correct the light doesn't come on) but the salient one is the P0720.

Crawl under and on the front right of the transmission you'll see the harness connector. You may have to remove a bolt from a heat shield on some models.

Take a drip/drain pan with you when you go under.

The harness connector slides into the pilot bushing. On the pilot bushing you'll see a ribbed ring, turn that (can't recall the direction one way or another) and it will unlock the harness connector.

Pull the connector straight out. If you're greeted with a big belch of fluid, we probably have found the issue. Let it drip out into the pan (it will drip both from the pilot bushing as well as the harness connector), then plug the connector back in (remember it goes in and comes out straight, don't twist it!!).

Now go to the TCM (E-TCU), the transmission control module. It's in the electrical box under the hood. Unplug the connector and check the plug socket (this is the other end of that harness connector you unplugged below). If it's wet with ATF, then snap the plastic cover off the TCM and bath both as well as the socket with some aerosol electronic parts cleaner spray (non-residue type). Let them dry thoroughly.

Your truck will be parked for a bit, too. You'll need to source a new pilot bushing (part has been redesigned several times for just this reason, it leaks fluid into the harness where it fouls the TCM) from the dealer network, you don't want an aftermarket part here. Get a liter of ATF as well and you'll need the dipstick tool to set the level when you're finished.

When you have the new part, unplug the harness connector again, then unscrew the tiny screw in the center bottom of the pilot bushing. Pull STRAIGHT OUT on the pilot bushing to remove it. Lube the O-rings on the new one with some clean ATF, then align it properly and push it STRAIGHT in. This will take some effort and you have to resist the urge to turn it or you'll damage the conductor plate.

Once the new pilot bushing is fully seated, snug up the little screw in the center. It's only like 8nm or something, I use a nut driver and just go a bit past snug. Don't crank on it or you'll break it or strip it.

Align and slide in the harness connector, then turn the ring to lock it in place. Replace the heat shield if it was removed.

Head to the top and remove the cover for the AT fill tube. Start the truck and let it idle, insert the dipstick tool and read/set the level for the 25C range. The goal here is to have it as close to the minimum line for the 25C range.

Drive the truck a bit and then let it idle until the electric cooling fans cycle off, then check the fluid level on the 80C range. Target the middle of the range since the fluid may be a little cooler or warmer than 80C.

Remove the dipstick tool and replace the cover, you can fit a new locking tab if you like.

Take the truck to the shop and have the codes cleared, garden-variety OBD-II scanners won't clear the code from the TCU (that's the P07xx series) and until that happens, you're going to continue to have issues even though you fixed the underlying problem.

Good luck.

Reading 1

Can't Know, thank you for your tip.

Here is how we proceeded. We have replaced the conductor plate and pilot pushing (aka transmission plug). There was NO atf fluid along the line to the TCM, nor was the connector wet with atf. That was fine and in-tact. Replacing the conductor plate has cleared a few codes that were previously showing.

Now the only code showing is for the Traction Control Module.

It looks like part number 1635458032

Does the Elec. Stability Program need to be programmed with a Star machine after it is installed?

Do most after market Star machines have the ability to program Mercedes?


Does anyone know where to buy a good one? Please advise, I need one asap.

The '02 ML320 is still going into Limp Mode, same as before.


Generic Code Reader shows the following:
P0720 -Output Shaft Speed Sensor Circuit
P0600 -Serial Communication Link

ICARSOFT i980 codes found in manual mode
Traction system
c1000 - Control Module

This thread seems to have some similar issues that I'm having

Help!!! Limp Mode in the Mornings P0720 P0600 - Mercedes-Benz Forum

Seems this 'wet' connector problem of ATF causing 'Limp Mode' is getting traction.

Most late model trucks and cars have the TCM in the transmission ! So I do not see why the ATF in the connector from the plate to the loom would cause problems. After all, the shift solenoids all operate submerged in the ATF.

The problem I see most ...( especially the V8 ML.) is the TCM loom runs down next to the pre cat and the wire insulation melts and wires cross short. MB only installed a little metal shield on the front corner of the transmission ....not good enough.

On my own 430, I teased out the TCM loom wires and insulated them and made a heat shield .
30,000 miles and no more problems.

So, open up the TCM loom where it leaves the conductor plate connector and check for melted insulation.



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