Howdy folks, new here!
Back in January, or there abouts, I backed into my driveway and put the ML350 in park and then it sat for a few days.
My battery was showing wear and if it sat for a week it would be dead.
4 days later I went out to take a trip to the store and my battery was dead so I boosted it and went to throw it into D to head out when I got the dreaded "do not shift gears head to shop" message.
I had had this before when my aux battery was low and swapping it out (twice) fixed the issue.
I went & bought a new AGM battery & AUX battery (to be safe) but nothing changed...tried doing a remove and re-add to see if that would correct it but nope.
A friend came by and scanned it with his STAR and it seemed to show the ISM as the issue as it would read the shifter going from P, R, D, & N.

So today I crawled under (finally) and removed the ISM (I didn't even need to remove the drive shaft or bracing whew) and when I got the ISM out into my hands I noticed the ISM is in the P position and thus the transmission should be as well.
Just to note: when I would put the key in the ignition before starting it I could hear the ISM whirring and could as well when shutting off the car so info is being sent to it and it's motors are doing something.
Issue is, why does the car think it is NOT in P but rather in gear?
If I open my door while running it tells "vehicle not in park" but the dash shows P.
Could it be the shifter then or has the ISM had a gear/belt failure internally and is not able to read it's in P?
Cheers for any knowledge.

This was the code I got after it initially happened:

ISM-Intelligent
servo module
Fault[2] Code number:1904

Describe: The internal mechanism in
component A80 (Intelligent servo module for
DIRECT SELECT) for engaging selector lever
position P is not in the correct position.
Code number:1974
Describe: Selector lever position "P" was
engaged due to a fault occurring in
component A80 (Intelligent servo module for
DIRECT SELECT) or in the vehicle
communications.

 

Took the ISM apart and when I opened it I realized that the emergency park motor had engaged and released the locked in cam to push the vehicle into Park.
Not sure why it would have done that when I originally parked it in the driveway in reverse but it did hence why I couldn't get it to go into gear because it had released it. I have now compressed that spring and locked the cam back in place and I'm about to go out and connect it to the vehicle and start it and see if it actuates properly via the shift lever. Fingers crossed




EDIT:

No dice...I did the actuations via the Fox scanner and same errors come up, the ISM flips the emergency park motor when trying to change gear.
Seems these are prone to low voltage (battery) issues that put them into this state...either by greedy design or pure engineering malpractice.

 

It's essential to check the electriical connections_ of the ISM and and the condition of the wiring.Sign/al transmission isssues often stem from corrosion or damage.Also, consider recalibrating the ISM after replacing the batteries, which may restore proper functionality

 

Thanks for the reply.
I did the actuations on the ISM via the Fox scanner and same errors come up