Generally this means the brushes are worn out. On my car you can just replace the regulator which has the brushes in it. However I think that has been "improved" on that car so you need a whole new alternator. But I don't have that car so I can't say for sure.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra