GMC alternator acting funky

[68gtxman]

i have a 2008 GMC Yukon Denali XL AWD (only put a couple 1,000 miles on it so far). I noticed that the volt meter in the dash would sometimes read right at 14.0 volts and other times it drops to 12.3 volts. After a cold start, it immediately goes up to around 14.5 volts, but as the engine warms up, the voltage slowly drops to the mid-12s. I plugged in a digital voltage meter into the cigarette socket and the dash gauge is not showing accurately, but close enough. While driving today, I noticed the same voltage drop to 12.5 - 12.9v regardless of engine RPM, but when I turned on the headlights, it rose and stayed at 13.8 - 14.1v.

Does this mean the alternator is going out (it,s been doing this apparently since I got it in March of this year). Or does it mean that I have a bad ground or ground cable? Any experienced mechanics see this before???

 

[diymirage]

Electronic clutch?
Doesn't it sound like it charges untill the battery is full, then once it is full the alternator disengaged?
Then when the draw gets large (let's say you turn your light on) it kicks back in

I don't know if this is close, but that's what it sounds like to me

Or....it could have one of these Litens OAD™ Decoupler Pulleys

 

[TrailBeast]

It's possible that whatever senses the voltage for the alternator to know how much to put out could be having connection problems, or it could just be normal.
The higher the load the more it'll put out (like turning the lights on) makes it jump up higher, and doing that doesn't necessarily mean it's going out.

 

[Mopar Tim]

My brand new 2016 GMC Denalli does the same thing. Nothing to worry about im told from the Dealer.

 

[abodyfan]

It's normal for that to happen it take load off the alternator and it makes the battery last longer by not charging it when its at full capacity. The alternator pulley(clutch) might go out before the alternator goes out.

 

[68gtxman]

Thank you all for this information! I never heard of these pulleys before. I'll stop worrying about it now.

Do they use these same pulleys on AC compressors? I had a previous 2003 Denali that had the belt tensioner start going crazy like the videos showed, and the repair shop said the compressor (and more) needed to be replaced to the tune of $1,400. I wonder if it just needed a new clutch pulley?