I recently had a very similar problem. My alternator had gone bad and I replaced it. However unbeknownst to me, the new alternator had a slightly different sized pully on it, which of course caused it to spin at a different RPM than the original. I took it to the place I bought it, they put the OLD pully on my new alternator, wala! - problem was solved. Something to keep in mind. I'm new to this too and have had to quickly obsorb the tidbits of info about the quirks of these trucks. Hope this helps.

 

Its personalities.........welcome to the forum