The 12SI is older tech and pretty much maxes out the capabilities of the V-Belt accessory drives at right around 100Amps. Even with two V-Belts on the alternator you can run into trouble driving a really high amp SI series alternator. The cooling fan isn't on par with the newer CS and AD designs. New SI series are guaranteed to be Chinese counterfeits. The 12Si was used for a fairly short time span between the 10SI and the CS series.
If you can find and install the Serpentine belt setup that GM used on the T400 chassis you can use the really hot 1999-2005 AD230/AD244 they used on the early T800 chassis trucks instead of the CS130/CS144series alternators. OTOH the CS 130D is a pretty decent unit for a 105Amp alternator.
My 6.5L came out of a 1998 with a CS130D and Sanden AC compressor and the 1997 up high volume water pump and, of course, serpentine belts. I didn't change any of it. If the CS130D gives up the ghost I'm probably going to be updating to an AD244 but the 105A CS130D is working out just fine right now.
Regulator differences between the 2005 and earlier and the 2006 & later units.
xc_hide_links_from_guests_guests_error_hide_media
How to tell the difference between the CS130D & AD series.
xc_hide_links_from_guests_guests_error_hide_media
