OT: Power/Voltage issues on a Mercedes, Looking for the Benz gurus - please chip in Options

[draganc]

guys,

I hope that someone with H2O new car experience can help me out with my problem:

I have a 2009 ML350, which was working just fine w/o any problems until about 2 weeks ago, i'm starting to get a intermittent "key does not belong to vehicle" error message on the display.

Once this happens, it either turns of all the power, starts after 5-10 tries or just starts w/o any problem.

I have the same symptoms on both keys and I already replaced both key batteries w/o any change.

Googling around, I found out that a low battery can also cause those problems with "key does not belong to vehicle", hence, I replaced the Aux battery and TWICE the main battery, once from Autozone and now with a OEM Benz battery.

Observations:

Installed battery voltage with engine OFF - 12.6V
Installed battery voltage with engine ON - 14.2V

Also, I connected a OBD II Bluetooth toggle and have been monitoring the "on-board" voltage via OBD and a cell phone app.
The OBD reading is about 1V less than measured directly at the terminals.

Everything looks OK once the engine starts and while driving; OBD reading ranges from 12.2 to 13.2V, depending of acceleration, coasting or idle, but mostly around 13V.
Apparently the alternator is load sensitive and adjust the voltage accordantly.

The big "aha" moment is when I turn of the engine the OBD readings drops immediately down to ~ 11V, whereas the terminal reading remains at 12.6V.

When I try to restart the engine, the terminal voltage remains at 12.8V BUT the OBD reading drops further down to 10.7V!!!

The dash completely remains dark for about 15-20min and than I'm back to the above situation that it takes be 5-10 tries to start the engine.

Once running, the car/engine works just fine.

- New Aux and main battery!

- It appears the battery gets charged at ~14.2V once the engine is running.

- after ~20min wait-period, the car will start again.

What's the fricking problem??? Could it be that there is kind of a voltage sensor that doesn't read voltage correctly, hence the ECU turns off all the power until it's "reset"?

I wish my teener would drive by now and had AWD.

thanks for your inputs, thoughts and BS!

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