You need to run the test as described. No loads like lights, A/C, fans, heated seats, etc.. etc. Then, with the engine running at normal idle speed you should have 13.6 V minimum, DC and very little AC voltage, definitely less than 1 Volt. If you don't get both of these, you have a problem with the diodes, which are devices that either work or they don't, and when they fail you have no choice but to replace them. A good rebuild shop can handle this for you, but from the sounds of what you did I would expect the insulation is damaged. I would just replace the alternator. 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)