Heat is the enemy of all things electric. Insulation materials break down, especially when they get covered in a layer of grease and dirt as the temperatures get higher and higher. There is an "old rule of thumb" that says insulation life is cut in half for every 10 degrees Fahrenheit increase in operating temperature. So a dirty, oil soaked alternator with road dirt stuck to it is going to run hotter than intended and eventually the voltage will begin to bleed across the insulation and make its way to ground. That usually adds heat to the mix and speeds up the insulation system demise. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)