At 100,000 miles my 1988 240 was in dire need of new brushes. The brush/regulator unit is around $80 from a dealer, maybe less from an on-line place like fcpgroton. Lot less than another alternator.

Before replacing the alternator, I would remove and clean and re-connect all the wires to the alternator (there are three) and then check results. If still bad, I would replace the brush/regulator unit.

If it's still the alternator then swap the old brushes into the old alternator before "trading it in". May a well have a known good spare.

Brush failure due to wear-out (to below 5mm long) is much more likely than alternator failure, IMHO.

Good Luck,

Bob

:>)