Had a no crank no start condition, replaced both batteries. Car starts in the morning, once. Then when you shut it off all the dash lights will come on but back to no crank no start.
code reader show in live view that it's charging 13.6-14.8. While it's running. Temp outside was 20 degrees. Ran 2 hours sitting. Shut off, no start. Unhook both batteries wait 30 min hook up and car starts fine and several times throughout the day. Next morning cold outside it begins again. Initial start then all dash lights come on but no restart.
Any ideas ?

Have a look in the rear floor/spare tire area for water infiltration. The description sounds like flakey electronics which in 164 models can come from water intrusion via the rear taillamp gaskets.

On our R500, being unable to start after just shutting it off was due to the starter going bad. Replaced the starter and it starts perfectly every time.

All of the starter electronics are at the front of the car and under the hood. I don't see water intrusion in the rear causing starter issues unless it's causing a major short (unlikely given that it starts just fine 30 minutes later).

See no moisture, hardly any draw on batteries big or small. No lights on, battery reads 13.4 to 13.9 when shut off. Next morning reads 12.9-13.2 and starts right up. But all the lights come on again. Repeat process, unhook battery after 20 min rebook then it starts and runs fine all day ?

What do you mean by all lights come on?

All the dash lights, warning lights, SRS ECT.

Is it the normal sequence of lights or are extra lights turning on and staying on when they shouldn't be? If normal, see my comment above.

Starter is new already also.

This is definitely cold weather related. Been in the 40's all week car starts everyday, 17 this morning. No start again.
unhooked both batteries wait 10 min rebook both. Starts and drives fine for the rest of the day. 🤔
Agrivating.

All connections to it and the battery are secure?

Yes, battery, under the hood at fuse boxes and all.

Could still be a bad starter from supply, that seems to be the only variable that has changed with this car.

Why would starter change just by unhooking the battery ? Took it in, it checks out fine.

No it wouldn’t change reasonably because you unhook and re-hook the battery, what I’m saying is that unless it’s a Mercedes OEM bought parts you might’ve gotten a bad starter. Because all the symptoms you’re describing is that of a failing starter. I’ll either crank weakly or not crank at all after they get hot.

Bye the bye, one way to know that you have a failing starter is that one will not be able to initiate the engine compression check in Xentry/MbStar, as it uses the electrical load measured by the starter as it compresses each cylinder to calculate the compression in each.

Have you had any under body damage or anything of the sort that might have nicked the wiring harness somewhere?

This unit won't start after sitting all night out in the cold. Temps under 30 degrees. The starter never has the chance to get hot.
I just checked the underside of the car nothing seems out of order no scrapes dents holes. It's been checked for moisture in the spare tire compartment.

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And you have an AGM battery of decent quality?
Frozen battery?

Have replaced both batteries large on and small one.

Yes, we got that - is the main battery OEM quality and Absorbed Glass Mat type (NOT lead/acid)?

Have you tried putting the car in maintenance mode to read the battery voltage when cold, and when starting?
That'll tell you what voltages are present in the system when dormant, cranking and running.
Anything less than 12.2 at rest is suspect, below 10.9 it won't crank. If it does crank, what it dips to will tell you the health of the battery.
You did state charging voltage is around 14 volts which is good, but that just tells you your alternator is fine. It says little about the battery's health.

Did you ever find the fault? Replaced my starter on my x164 320 and worked fine first crank, but after the first crank....wont crack, can crank it by shorting the starter relay in the fuse box....starter works fine. When i turn key to start , only a Click from the starter, but no crank ..

Try swapping the relay. The GL uses a bunch of the same relays; it would be good to have a spare on hand anyway.

the exact same thing is happening to mine. Mine is a 2010 GL450 with 167k miles. I replaced both batteries, i got all lights flashing, then mechanic did a reset of the ecu/memory then work for a day or two then again it happened, this time it said to drive to workship without shifting gears. The ISM is suspect is what i am being told, however my sister n law just replaced hers as it was doing the same thing as mine and the problem came back. Hers is a 2010 ML550. I am waiting to see what else they find with hers before i take mine to mechanic. I am hoping the ISM they installed was faulty.

On another note, if i leave my batter connected after an episode, it will drain the batter whatever is happening keeps something on such that it drains the battery. Have yo you experienced this? I feel as if our cars came with some sort of expiration date in the ecu..so frustrating. Hope we can help each other. I have babied my car its whole life only to have something like this happen, would hate to scrap it.

Need to trace the stray current. Lots of time spent with a multimeter or take it to either an indy shop or the dealer.

Most drain events are related to doors.
if you have keyless go, just touching a door is a wake up event. If not, opening a door is a wake up event. If a door or keyless go sensor is reading an input to wake up, this will keep all the systems alive and drain the battery. So start reading out door and tailgate sensors. Pull the tailgate breaker and see if the problem persists.

if not; there’s your problem.

Did you ever find the fault to your no start/start issue when you unplug the batter? I am also dealing with this issue on my 2010 GL450 and also my sister n laws ML550, both have over 160K miles, but taken care of otherwise.