Cliff,

The battery has a lot of energy, enough to glow the plugs and still spin that high compression engine over. If there was a dead short (V=IR, remember, and the heat generated is related to I squared, so if you have near zero resistance you get all the juice the battery can muster, which is a lot, and lots more than the circuit board can handle, thus the plasma) the fuse should protect the current carrying part of the circuit and the circuit card would become a fuse for whatever parts of the circuit it feeds. The fact that it apparently still works at all is sign of its robustness and a history of abuse. I agree you should change it. 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)