New alternator only works part time why?

red2go

It's 1998 s10 p/u. Replaced battery and alternator in dec. the alternator only works part time? I've got more details if you need.

Mr_Shiftright

yes more details. What are you basing your observation on? A light in the dash, a gauge or a voltmeter? Does the charge light go on and off, or....? What do you mean exactly by "part time"?