When I bought the van I installed a USB charger in the dash that has a digital voltage display. When the van is not running but the ignition is on, it displays around 12v. When I start the engine, it goes up to around 14.5 and pretty consistently stays there. I watch this thing alot (because it's right in my field of vision) and it never fluctuates much. The entire time I've owned the van, it's stayed around 14.5. This aligns with the factory gauge in the gauge cluster and also matches voltage readings I've taken at the battery. I have no reason to suspect this gauge is faulty. Last week I was driving and noticed it was at 13.7. The gauge on the dash also reflected this. There were no accessories running that would have dragged down the voltage. I know this is still healthy so I just continued to keep an eye on it. I even stopped and started the engine and it continued to read around 13.7. The next day, it was back to the normal 14.5 area. Last night I noticed it fluctuating between 14.0 and 14.5. Now, I know 13.5+ is normal and healthy. But this has consistently hovered right around 14.5 for the last 2 years and now it's changing. If the alternator and/or voltage regulator is about to go, I'd like to get ahead of it. Does these sudden voltage fluctuations point to a coming failure?

Brand new battery 2 months ago but this didn’t start until this week.

 

Please post your van details every time so people know what they're dealing with. It's even easier if you add it to your profile signature and then you never need to remember.

GM implemented smart charging. Voltage varies depending on battery conditions, and which electrical loads are switched on. All the voltages you listed are in the normal range.
This system is described in great detail in the upfitter manuals (gmupfitter.com)

As long as your van starts normally every morning, I would not worry about the exact voltage too much.

It might actually be a good sign that you see lower voltages. It could mean that the alternator isn't working as hard to charge your new battery.

 

Thank you for posting that. I did not know that it ramped up the voltage for engine braking and lowered the voltage at idle when cold. That chart really helps understand what's going on.

 

my grand prix had digital voltage read out and it reading was always 14.4...then it started fluctuating very quickly from 13.5 to 15.5. this is a ecm controlled system with multiple charging modes. Never left me stranded. Alt and battery tested ok on AVR tests. New alternator and everything went back to normal.

 

I've always had my vehicle info in my signature. Just verified that it's still in my settings and it is - I'm not sure why it didn't get added to my post. In any case, 2013 Express 3500 Ext, 6.0l, 195k mi.

Thanks for the chart, it's fascinating to see how that works. I understand the whole concept now, just not sure why after 2.5 years of consistent voltage readings it's starting to be inconsistent. That tells me that some variable has changed - I'm just trying to get ahead of it instead of waiting for it to break down. I'd rather replace the alternator/regulator now at the first signs of trouble than wait until the charging light comes on and I'm forced to deal with it. For the last week it's consistently been back at 14.5ish, haven't seen it dip down to 13.5 since I posted.

 

Yeah that is weird your signature got omitted.

Anyway, the new variable is probably the new battery.