You may have recognized this problem from a previous post but this time I am having the same problem with a new voltage regulator.

I drive anywhere from 100 to 400 miles a day with work and my battery boiled over today for about 1.5 hrs.

It has done this with two different voltage regulators.

My question to you is what else could be causing the problem?

I know that the alternator belt that is suggested is too long so I had to get a shorter one that would fit over the alternator and could be adjusted properly.

Could my pulley be too small and could this cause my alternator to over charge over long periods of time?

If not, what else could be the problem?

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1993 W124 300D
-297K on the clock as I type this.

If it boils from just one or two cells, you have a sulfated ( failed) battery. If it boils from all cells equally, you have either overfilled it or the regulator is faulty. Any shop can test it. Should never exceed 14.5 volts. Pulley size only controls the amount of time the alternator takes to replenish the lost charge.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

You did not say whether or not you replaced the battery the first time. The primary suspect is the voltage regulator but that is easily checked with a meter, as noted. 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)

Look at post #12 I am not saying it is your voltage regulator but the member was getting normal charging voltage at idle speed and it went up at higher speeds. He was able to test this as he bought a voltmeter that goes into the cigarette lighter.
http://www.peachparts.com/shopforum/showthread.php?t=225820&highlight=battery+charging

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84 300D, 82 Volvo 244Gl Diesel