my friend had the same issue with his 560sel. Everything checked out ok, except mech found the starter to be grounded and alternator too old. changed both of those and the car runs fine. See if you have a ground anywhere, or maybe your battery just need replacing?

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6