Hi, i have this Exact problem on my 2007 L322, 12.8V with engine off, 15V at idle, and 18V going down the road which shuts all the systems down.
I've changed the alenator 2 times now and it still does the exact same thing.
 
There's more American volts to our British volts,:D I'm not into electrickery but I would hazard a guess at the voltage regulator.;)
 
does anyone have a diagram of the wires going into the alternator and which pins the wires go too? the plug disintegrated when it came off, and im not sure my mechanic put them back correctly.
 

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