My 2009 SS is intermittently tripping the charging system light in the dash. It seems to be prevalent on cold startup. Ohio temps are single digits (°F) these days, which may be a contributor. Sometimes the light will go off a little later after revving the engine, sometimes not. At those times when it goes off, I'll hear/see the electrical accessories spring to life (e.g., brighter headlights). O'Reilly's tested the starter, battery, and alternator. It was coincidentally done at a time when the dash light was off (I.e., systems operating normally). No issues were found. The battery reported no dead cells; engine off, its voltage was about 12v where it needs to be. Engine on, the alternator was charging properly with the system at 14v. The battery is name brand and about 3.5yrs old. Connections at the battery are tight and corrosion free. Because of the light going off after revving the engine sometimes, I'm suspect of the field exciter. According to my manual, three wires go into the alternator:

1. Battery positive (i.e., big red wire for the starter)
2. Generator Turn On Signal
3. Generator Field Duty Cycle Signal

Are the latter two supposed to have 12v at them? If so, is it continuous or only on startup?

The car has 180K miles, and the alternator is original, so I realize the alternator may be dying. But I'd like to do a proper diagnosis to conclude that. And yes, I realize electrical problems can look like the wild, wild west with a possible cause hidden virtually anywhere. But I'm trying to look at this logically to rule out the big things first, before hunting for a mysterious hidden short.

 

12 volts is a very dead battery. Even 12.6 volts is suspect.

 

It would be best to remove the alternator and have it bench tested.
Ive had similar symptoms, turned out to be the alternator in Sweeties 2003 Sunfire GT and a dead cell in Oldblue back in 2011, then that battery was dead at 12.4 volts in 2018. As you might have read here before, I have my battery and charging systems checked every spring.
min a side note, have you considered the serpentine belt and tensioner, have worn out, the tensioner spring does fail after a decade or so. I replaced the BlackSwan’s during the timing chain swap in Feb 2020, I noticed that the arm of the tensioner required much more effort to move it to install the belt, so that indicated to me they were tired.
I ordered the AC Delco matched set from RockAuto.

 

12v is very dead battery? Really? When the engine is not running, 12v is perfectly normal. I'm unclear what you're trying to convey.

Yes, I'm going to look at the belt too. That's a good point.

 

The belt is not squeaking, but it may be dry and losing friction. What can I spray on the belt to test if that's the problem?

 

Here are the numbers for battery state-of-charge vs voltage (remember, this is measured directly across the battery terminals):


A battery that measures right at 12.0V is at less than 50% charge.

Steve

 

Great chart Steve, the AAA guy showed me a similar chart back in 2011.

The computer systems require the voltage, this isn’t my fathers Oldsmobile

 

He is trying to convey that a 12 volt lead acid battery is actually a 12.5 volt NOMINAL with a peak of 12.73 and won't take a charge under 11.96. Common misconception.

The way batteries discharge is they slowly go from 12.73 to 12.5, they will stay at 12.5 for quit a while then rapidly fall to nothing. Your alternator is doing a good job keeping the battery at 12V, very soon it won't be able to do anything for it. The alternator should be putting out around 14.8 volts at times, if it is lower with the battery in that condition the battery is sucking everything out of the alternator and it will soon die.

 

Serpentine belts don’t usually squeek, just stretch and the idler pulley gets glazed and the tensioner spring gets weak. Adding up to a failed belt and alternator.
I found that just replacing the serpentine belt alone , wasn’t effective, as the tensioner was worn out.
I found replacing the serpentine belt and tensioner as a matched set was better and is also recommended by GM

 

I feel like we are getting off track. My statement about the battery being at "12v where it needs to be," was a generalized "off the cuff" statement to imply that the battery is okay. Automobiles have run nominal 12v electrical systems for decades and we all know this (keyword "nominal" as sometimes it fluctuates a few tenths up or down). The title of the thread already infers that it might be undercharged at the moment due to a charging system fault. But I didn't realize we were going to get off track talking about tenths of a volt and its impact on battery vitality. Bottom line, I have strong reason to believe the battery is okay based on 1) its age thus far, 2) it being tested 3x recently and showing "good" each time, and 3) the system's sudden response to spring to life when the fault disappears (infers an alternator problem, not a battery). So I'd still like to hear back on my original question...is anybody able to shine light on the "generator turn on signal" and the "generator field duty cycle signal?"