This was annoying as hell to figure out. To keep others from the same snakepit:

Car is a 2000 trans am.

Replaced my factory alternator with a Powermaster 48242. I needed the factor-ish casing to avoid interference with a procharger head unit. The Powermaster unit shows up with a 2" pulley and a big tag on the top that says if I spin it past 18,000 rpm it voids the warranty. I did the math, 7" pulley and 6,800 rpm redline and realized I was going to be spinning it well past 18,000 rpm (about 30% past). So, as a good citizen, I ordered a bigger 2.63" alternator pulley from ASPWholesale and installed it.

I put the alternator on and everything runs fine - at first. After about 20 min from start, I start getting an intense voltage dip at idle with lots of electrical oddities coming into play. After some testing of the alternator, and conversation with friends, I was able to generate some data that shows the lower rpm limit of the alternator. I'm not certain exactly what is going on inside the alternator (others here probably know better), but there appears to be some sort of limit that comes into play around 2,500 rpm alternator speed once things are warmed up.

I called Powermaster technical to talk through the problem. They were quite helpful. They disagreed with the 2,500 rpm issue, but acknowledged that the alternator needs to be kept about 2,200 RPM. I think the data speaks for itself - it needs to be spun faster than 2,500 rpm (below graph). I put a tune in the car that idled it high enough to stay above the min limit on the alternator and everything runs just fine. Voltage stayed above 14 consistently. So, the alternator seems to work just fine, but it is just very particular about staying above 2,500 alternator speed.

I will be putting on a 2.3" pulley and idling the car a little higher than I normally do (~900 rpm vs. my preferred 800 rpm). This should fix the idle issue, but it does mean that I am going to be overspinning the alternator ~15%. It also raises the larger issue that the operating range of this alternator is 2,500 rpm to 18,000 rpm - in other words, if you want to avoid idle voltage issues, you are going to overspin the alternator, or at least spin it past the 18,000 rpm limit that Powermaster seems to like.




TLDR: Powermaster needs to be spun faster than 2,500 rpm (alternator) or you start to have weird voltage dips/issues - which translates to having to overspin the unit.

 

I am running the Powermaster 58242 with my procharger head unit and not seeing those issues. Weird that you are having issues and I am not as the only difference is the BLACK vs Grey case

 

I introduced the problem with the larger alternator pulley. If you just run it with the 2" they send it with, you won't have any problems at idle. However, you will likely be overspending the unit - or at least above what Powermasters says max RPM is.

If you ignore their directions and just install as delivered, I don't think folks would have this idle problem - but they would overspeeding it.