Ok I just replaced my alternator. I have had it on for a couple of days and now I am having voltage problems. At idle after the car has been driven for ANY amount of distance the volts drop down to 11.8. I noticed that the belt was starting to get cut and fray on one side so I got a new belt and put it on and tightend up the crank pulley. I have an afx underdrive but it normally stays around 13.6 volts. As soon as I bump the gas it shoots up then drops right back down. The belt is not squeaking either. I could try to tighten it up a little but my tensioner bolt is gone so I just have to pry and tighten it up. Please let me know ASAP!

Your belt is most likely fraying on the edges b/c your UDP needs to be pushed on a little bit farther. You'll have to get a big mallet for that and give it a few more wacks.

As far as the voltage drop goes it probably doesn't have anything to do with how tight your belt is. Unless it's really loose it's probably not slipping. Do you have any loose connections? Check all the wiring underneath and make sure it's all hooked up tight. Do you have any kind of after market alarm or something else that could be pulling from the system? How new is your battery? Where are you getting voltage reading from - the battery? the alternator?

EDIT: How do you tighten the crank pulley? The crank pulley is force fit onto the crank. You need to rent a harmonic puller or gear puller to get it off. I highly doubt it's slipping.

Its not gonna be his alarm. It would take a GREAT deal of current draw for it to drop his voltage down below 12 volts at idle. Granted, the UDP will in fact make your rpms at idle slower on your alternator pulley but it should stay above 12.6 volts at the very least. Is the pulley on the new alternator the EXACT same size as your old one?

Ok I tightend the belt today at work. I was talking about it slipping around the alternator pulley and not the crank. I think that it is a bad alternator. Its a 90amp and the pulley is the same size. I do have an off of the electircal I have an alarm, SAFC II, two gauges, and AVIC-N2. Nothing else other than the normal headlights etc...

Ok I just replaced the alternator again today and it is no better. Anybody have any ideas....Please?

Do you have cams? Mine does the same thing ... high output alternator, UDP, crower 2s ... at idle under 12V.

No but I have some 12's coming in with the Howell rebuild kit. If it only seems to drop that low when high beams are on. I am going to check grounds and all the simple stuff now.

I JUST delt with this problem on my 2GNT.

First off, do not to over tighten your belt (quick way to ruin bearings / shred a belt)... it will not change the voltage (alternator spins the same speed).

I got a new refurbished alternator from Napa. My idle voltage was ~11.8v, a little gas, it would jump to 14v.

Took the alternator back... tested it... bad, grabbed another refurbished one off the shelf... bad. Had them order me a "NEW-Refurbished" alternator, instead of just a Refurbished alternator... I gave him a funny look for this, but apparently he sold me a line of refurbished alternator that is just basically cleaned up w/ a replaced bearing.

The New-Refurbished alternator turned out to be good, and voltage is now at 13.7v at 800 rpm.

It's not uncommon for those auto part stores to get bad batches of alternators.