My friend's 2003 ML500 has an issue that he can't resolve himself, the battery dies if the vehicle isn't driven every 2 days or so. He says he's checked everything and has had the dealer check it out but so far nothing has come up. He said the went through the vehicle thoroughly checking everywhere that a light could be left on or a sensor could still be left on and has had no luck. He's out of the country at the moment and I told him I'd take a look at it. I figured here is a good place to start since this might be common theme among ML owners.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

Do a test to see what the vehicle-off current usage is....and then narrow it down by removing fuses one at a time to find what possible system(s) might be the culprit?

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I assume someone has tested the battery itself already? If not, a 6-7 year old battery could easily be causing that problem.

Had similar problem with near new battery ! It was the boot /trunk light staying on , [think about it ]

His battery is new and shouldn't be the issue, I think he's replaced a couple of them just to be sure. I think I'll have to do what pawoSD suggested and go through the whole system and isolate what the drain is.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

those mercs like the C class and all new mercedes cars have a Control unit called SAM Signal Acquisition Module and it controls almost all the electrics of the car. they sometimes go bad and cause batteries to go flat. in the C class as one of the forum membs has found out it cause a new battery to die in 48 hrs. the c class has 2 SAMs one under the fuse box one in the trunk, you need a scanner to be sure thou.

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Jay,
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-1995 Black W140 S500 (Lady)
-1992 Black W124 E300 (Dima) (Ex-Mosselman
Twin turbo Kit).
-1988 Black W124 300 E 4-Matic.(Nadeen)
-1983 Brown W126 500SEL.(Old Lady)(Sold)
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http://www.youtube.com/user/jayrasheed

I had the same problem in my C32 AMG , and it ended being the from SAM module was faulty. It can be changed or it can be fixed if you got a smart car electric man.

So I just tested the vehicles off current usage and it's a bit unusual. For the most part the reading from the multimeter was about 30mA which I think is the norm. When I opened the doors the dome light would come on and the draw would jump to just over 2 amps and would fall back to 30mA once off. I still decided to pull some fuses just see what would happen and a couple of them affected the draw, lowering it a bit, but since the general reading was 30mA it was safe to say that everything was fine.

The usual part came up when I opened to rear tailgate and then closed it, the draw never when down to 30mA, it stay around 200mA at its lowest point after letting it sit to 10 minutes. Before going down to 200mA it stayed for a minute at 450mA. I disconnected the battery and reconnected to see what would happen and it went back to the normal 30mA. I played around with opening and closing the doors and tailgate over again and the symptom came up eventually again, staying at 200mA.

It seems to me that this excessive drain happens at random. What it is I have no clue, but hopefully based on the power it's using someone will know.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

There is a TSB on this:

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1973 280SE 4.5 (Sold)
1984 LWB 280GE (Sold)
2000 G500 NMLE - "Deep Throat"
2007 ML320 CDI (Sold)
2010 Porsche Cayenne S Transsyberia TipTronic (Sold)
2010 Audi Q7 3.0 TDI (Sold and Sorely Missed)
2014 ML350 BTC
2015 Audi Q7 3.0 TDI (On Order)