Hi fellow Merc owners and experts.

Disclaimer I'm new here as I'm a newbie owner of a Merc after owning VWs. I have a lot of experience with those and diagnose, service and repair them myself. As such, I'm already working on getting myself a MB STAR Xentry kit as I've learnt that owning my own diagnostic is invaluable, especially if you can self repair.

Issue I'm getting is the error message: 'Pre Safe Inoperative See Owners Manual' shortly after starting the car, or turning on the ignition - The car radio turns on and sat flips open, then after a few seconds, no more that a minute, the message shows on the satnav display: "Command line shutting down' flickers and few times then the sat/nav flips back closed and the radio turns off. The Air con fans are limited and the seat heaters cannot be turned on.

Now the history is that I bought this car and parked for a few months because I didn't nee to use it at the time so the battery went dead. I didn't really test the car when I bought as it was from a friend(I know, schoolboy error) so I don't know if this car has always had this issue. So anyway I recharged the original battery and fitted it in and started the car ok but car kept going into this pre-safe mode.
However after driving it for a bit, the pre-safe message went away and the command line and radio/satnav flipped back up and full accessories started working again. Then randomly after a 30 mile drive they went off and never came back on. My online research lead me to this advice - Command line shuts down if the sensor detects that there's not enough battery power, in order to preserve your battery and car enters pre-safe mode. So with this I concluded that my battery was the issue as it's cranking deteriorated with each attempt, and bought a new AMG Battery and fitted it in thinking this would resolve the issue and it still hasn't worked since at all. Even after driving loads.

Following further research, also stated that the command line also shuts itself down if it detects higher voltage output that is necessary, possibly alternator voltage regulator issue. So I've taken a multimeter and carried out batter/alternator tests:

battery voltage when car off - 12.6v
battery voltage when car running - 15.20v
tested voltage from alternator to battery and all seems fine.

I'm told that the voltage when car is running should usually be between 13.4 - 14.2v thereabouts and no more. Any more indicates that the alternator voltage regulator is faulty and you could risk damaging your car battery by overcharging. But then again those figures were related to different cars and some reports say that it may vary with cars.

Please can you confirm if that's the expected charging voltage for my car or whether it's too high meaning my alternator voltage needs replacing?
Anyone else have any experience or advice on resolving this issue?

Thanks in advance.

You are right. Change the alternator VR. the charging voltage should be around 14.4.