• If you currently own, previously owned or want to own an Avalanche, we welcome you to become a member today. Membership is FREE, register now!

Is it normal for the Alt guage to read below 14v when running the A/C ??

CASD57

Full Member
Joined
Dec 22, 2020
Messages
93
Location
Meridian, Idaho
I just notice this..since I've had to use the A/c in the last week... I'm more worried that this is a precursor to a problem starting?
it stays in the white below 14v probably around 12v moves up to 14 and back to 12v slowly
 
My bet it is th new age computer controlled alternators and they do no try for 14 volts all the time...

Few threads in here about it.
 
Get the battery and alternator load tested at a parts store if you're worried or just curious. However it is normal to be below 14v, it only goes 'full-tilt' when required. The only thing that catches my attention from your post though, how long does it take to go from 12v to 14v and back? Mine doesn't fluctuate much, but might drift down over longer trips as the battery is topped off.

P.S. How old is the battery?
 
Not sure...I'll check the top but it looks newer, Truck is new to me Last Dec
It never really gets back to 14v for any length of time...hovers around 12-13v or whatever the mid point is in the white marking thats before and after 14v

Okay I have a tester...its says replace battery 336 CCA battery is rated at 730 CCA
 
Last edited:
But would a weak battery cause the -14v only when the A/C is on? or do I have an Alternator and Battery problem...lol dang
 
Since you have already confirmed that you current battery is dying, I would go ahead and replace that first.

A battery, along with the alternator, will provide the power needed to run everything electrical in your vehicle.

If the battery is weak, it can not hold up it's end of the tasks and the alternator must do double duty.

That could certainly show itself as a lower than expected voltage value.

The alternator is working harder just to keep up and can not produce the full voltage is normally does.
 
Replaced battery.....Did it again when the fans kicked on but it recovered back to 14v plus a lot quicker and playing the radio without the truck running I used to get the warning light within minutes the "battery was low start engine" I did it for over 10 min yesterday and no warning..So I'm happy the battery was the problem....it was a 4 year old autocraft gold ..
 
Still doing it.....Maybe it's just normal? have not had any problems other than Alt. gauge showing below 14v at times. So I'll live with it until a real problem shows up
 
Can you post a video? I don't think mine fluctuates as much as you are describing, but it's hard to tell via text.

Edit: Did you get the alternator load tested? If you have a few extra minutes, just about any part store will test it for you free of charge.
 
Yes I'll get it load tested this weekend and try to catch it doing it on video..........This morning it went to 14v and stayed there usually in the mornings and when it's not below 14v its always above 14v... It could be I'm just more in tune to watching it now...?
 
I have the same problem with my 2007 LT Avalanche, most of the time I see voltage reading at 14. Hot days when I run the A/C it usually keeps showing 14 mostly at stops and idle drops to 13 and 12 when at 12volts kicks in ABS light, stabilitrac problem codes. As soon as I start driving usually voltage goes back up and acts like nothing happened.
No codes stored from it saying stabilitrac,abs problems only happens when voltage drops around 12?
I bought new AGM battery thinking same problem my old lead acid battery was bad. When found out problem wasn't the battery put original in extra battery tray as backup hoping one day having double battery system. Waiting on an isolater to keep both charged, drain only one when engine not running.
But I'm now thinking my alternator is faulty or the part that tells alternator when to engage or disengage.
Or possibly A/C compressor problem? Slowing belt and alternator? IDK? It's definitely a head scratcher...
I'd like to know if you found the cause of your power dropping below 14V maybe mine has similar problem.
 
Mine has been fine, but it does seem to happen with longer trips, 15 plus miles. I'm to the point, where I'm trading it in when the Ford Maverick or Hyundai Santa Cruz hits the lots... I don't like the problems or MPG at this stage of my life...
Nice truck, maybe if gas was cheaper, and I was younger and wanted to tinker with it all the time I'd keep it but I don't need a fullsize truck(As you see in the Avalanche Bed now) But I need a Small one to carry homeowner stuff, So even the Avalanche is overkill and throw in 13-14 MPG while babying it...Well, Nice truck for someone else :)

The battery did seem to help, But I feel it is the Alternator giving me a heads up...that it's getting weak, Sounds like yours is farther along that road. So I'd replace it.
 
One last thing... It is based on the Engine cooling fans... As soon as they kick on it drives the voltage down to 12v +/- when they aren't running 14v+
 
Newer battery, replaced in March 2021, alternator original since I bought the AV in 2019. Alt meter shows 14v at start up.
After truck is at operating temperature it drops below 14v.
When I turn the AC on it drops ever so slightly.
Radio on it drops a little more.
Truck always starts right up.
Lights don't dim when excessive load is placed on the system but the ALT meter never reads 14v consistently.
When I hook up my volt meter to the back of the alternator, I get a fluctuating reading between 14.1v and 14.4v.
This leads me to believe either the ALt meter isn't reading correctly or the wiring has more resistance due to its age causing the meter not to receive the correct voltage.
Wires do increase their resistance as they age, so I am going to upgrade the power and ground wires for the alternator and battery using 2 or 0 guage wire.
Don't forget that temperature affects the resistance in a wire and could also be a factor , especially in hotter climates like south Florida or the west so take that into account as well.
 
You folks may want to look and see if you have a heavy duty alternator rpo code KW1. It's a 160 amp versus stock 145 amp.

Also there is a 2nd designed battery current sensor that maybe causing some problem, they redesigned it for a reason, that I don't know.
 
I figured id drop this in here despite most of you having issues with your newfangled 2nd gens. Came across this diagram while looking for HVAC stuff and remembered this thread. It might help to visualize how GM has the charging circuit set up. Which includes associated grounds.
 

Attachments

  • charging circuit.pdf
    173.1 KB · Views: 8
I did forget to mention in my last post.... It goes below 14v when running the A/C and the cooling fans kick on....Never did it in the winter
 
Still haven't gotten around to installing my high output alternator and the new wiring for the charging circuit, but today I noticed that while running the ac on full and raising two windows at the same time, my alt guage dropped to below ten volts. It went right back up when I released the window buttons.
I never saw that before in any vehicle I've owned. I get they're high draw items but damn it made me concerned for a second thinking my alt just died.
 
It has never gone below the white line which I assume is 12v by the 9v and 14v markings...So as you say No light ..no problem,
 
Back
Top