ML320 Transmission Problems
 

I was hoping someone may have an idea of what is going on in my Transmission of my 1999 ML320.

My ML320 was experiencing jerks between gear changes and throwing a 110 (speed sensors implausible readings). I changed the connector, replaced conductor plate. New MB ATF and filled to correct level.

Now the car runs fine and no more jerks between gear changes. However as soon as engine gets to normal operating temperature (80 to 90 degrees Celsius) the car goes into limp mode. It now throws a code 109 (intermittent fault on sensor 3). As long as the car is warm it keeps on going back into limp mode. When it is cold it drives fine again until it reaches normal operating temperature and then it goes back into limp mode again.

Any idea why it is doing this when it gets to normal operating temperatures?

I appreciate any help that you can give.

Did you use an actual Mercedes sourced plate or a generic part?

Hang an oscilloscope on the sensor output and watch to see if the wave form to breaks up.

It is an OEM part.

Will the turbines (where the sensors get the rpm from) change in behaviour when the car warms up? i.e. maybe a mechanical issue rather than electrical?

The speed sensor reads from a slotted wheel so any mechanical fault would be consistent. There is a small chance the sensor isn't fully seated on the trans case but I'd have to look at the parts to confirm if this is possible.

I have seen electrical parts breakdown with heat, this is why putting a scope on the sensor wire is needed.

Is the scope able to determine whether it is mechanical or electrical?

It is doubtful that there is a mechanical problem on the transmission side, there just isn't enough to go bad related to the speed sensors. The transmission would be long inoperable before the slotted wheels spin off center enough to affect the sensor operation.

The goal is to look at the wave form ( where the wire goes into the trans computer ) and see if it changes, if the wave form changes when the error code is tripped there is a wiring or sensor problem. If the wave form remains consistent, there is a problem with the computer.

Attached is a close up of the rpm sensors, when I originally replaced the conductor plate due to 110 code.

Assuming you have the 722.6 on that 99 ML

Just curious, did you get a Genuine Mercedes new in the box sensor from the MB dealership parts counter? Lots of counterfeits running around on eBay and the internet in general.

Not shilling for MB, just had a personal experience with this with a Bosch aftermarket crank position sensor, that would randomly shut down my OM606. Dealer parts counter sensor worked like a champ.

It was an OEM part. These parts are usually reliable.

Interesting with your Bosch part as I believe Bosch makes these for Mercedes. They just put on the Mercedes logo etc.

Hypothetically if the turbine wheel was slightly touching the rpm sensors, can one put a thin spacer (aluminium washer etc.) on the rpm sensors to assure that the turbine wheel did not touch it?

There would have to be some sort of drastic mechanical issue that would cause contact that a shim won't fix.

Just an update and hopefully the conclusion to my transmission problems.

I took my car to a transmission shop and they were able to pinpoint the problem to the actual sensor 3 on the conductor plate. The resistance was too high and out of specification to my ML320.

They replaced my conductor plate with another OEM brand and this time my car worked and has been driving fine since. Now over a week.

They did notice some friction material at the bottom of the pan which is worrying as I only changed and cleaned the fluid a month or so ago. It may be as a result of the hard shifting etc when going into limp mode each time??

In any case it is driving fine now.

I would do a proactive fluid/filter change in a couple months just to be sure.

But yeah, limp mode is super rough on a 722.6, and I could see how you shed some material off the clutches during those events.

They use the computer to soften up the shifts by modulating the hydraulic pressure. When you are in limp mode it is just the valve body operating in emergency mode without any computer assistance and you are just getting dumped into second gear.

So the OEM conductor plate though new was bad. Makes you wonder about quality control even in the OEM market these days.

Also be sure the transmission shop used the proper MB spec fluid for the transmission. It is not normal ATF!