Dado5

My 18 GS had dead battery, I jump started it and it was fine for 3 weeks and dead again.
i purchase new battery and tested it, voltage keeps jumping up and down, 15, 15.2, 14.7, 14.5, 14.3 then back to 15.
this leads me to believe alternator is bad and it’s over charging and killing Battery.

any input appreciated..

RonC7

Normal - charging voltage can be as high as 15.5 after engine start and then declines as the battery charges. You should see less than 13 volts when the battery is fully charged. Normal charging range is 12.9 to 15.5 volts.

If you see spikes higher than around 15.5 the voltage regulator (integral with the alternator) is bad and the alternator needs replaced. The high voltage has been noted to cause all kinds of electrical gremlins.

Also make sure that the small black sensing wire is attached to the negative battery cable clamp...

Ron

Maxpowers

. Good advice.

Jud Chapin

Before you replace anything, Advance Auto Parts and the like can test your entire charging system free of charge...

xWS6guy

I had mine tested multiple times and every time it said the alternator was fine. Finally replaced the alternator after multiple battery changes and all of my problems were solved. If the test says it's bad, it's probably bad. If it says it's good, it still might be bad.

fatsport

This. That wire is important. It’s a little short and easy to break. I added a crimped connector for some extra length.

Mad*Max

I have had the same experience with batteries.

Kevin Parent

I starting getting the service rear axle, exhaust baffles opening and closing when set to closed, and some IP flickering. I set my IP to tour and saw the volts going as high as 15.8. I first replaced my battery with no change. I then ordered a new GM alternator thru Rockauto which was $260 shipped, I just replaced that and now have no issues and am seeing volts no higher than 15.0. I think it's fixed.

fritz1990

Just traded my 2007 for a 2014 yesterday and on the way home I noticed voltage was 12,9 to 14.3. Glad to see this is normal. I will have questions I’m sure about other things.

Regards, Jeff

Dcasole

Normal ...... the older cars put a constant charge voltage to a battery, newer cars regulate the voltage, 2 reasons for this, 1- better gas milage and 2 extended the battery life

I just rented a brand-new full-size GM vehicle for a business a trip, the gauge never exceeded 12.9 the whole time I was driving it

Dave