a splash guard would only be a band-aid solution, if a solution at all. It won't help if the problem is humidity. Granted, I've had problems with moisture of various types in 240 series cars that are without their 'belly pans', but never a problem in a 120 series.

Another note on 240s... they run two belts. Often it is said that it's a 'safety' issue.... that you can lose a belt and you still got one. But, I tend to believe it's more to keep it from slipping.

Perhaps a single small belt simply ain't enough for your alternator. Maybe a dual belt arrangement could be adapted (you'd need a matching double pulley on teh crankshaft, of course)

Me? Well, first I'd make sure the pulleys are all properly aligned. Second, make sure it's the alternator and not the water pump (and I'm pretty sure it is the alternator...)

Last, and I'm 99% sure this will work... get a bigger pulley for the alternator. Does not need to be a lot bigger... 1/2" diameter increase is a lot of circumference increase.
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-Matt I ♥ my ♂