Dead battery...or so it seems!

I've got an '04 Avalanche that twice in the last month wouldn't start due to a dead battery.It's got 86,000 miles on it, and being 4 years old I have no problem replacing the battery. However, when jump starting, it immediatly starts. No slow cranking or clicking, it just starts up and runs fine. The first time it happened I didn't think much of it but I did some checking to make sure there there wasn't a light on or some kind of electrical draw. Now it just happened again so I checked a little more. I ran a multi tester across the battery terminals prior to jump starting and had zero voltage. After jump starting I shut down the engine and the battery measured 12.7 volts. I started the engine again a few times and each time it started flawlessly. While the engine was running I disconnected the positive battery cable from the battery and the engine did not stall, even with all the lights on. The dash guage shows just below the 14v mark & does fluctuate a little. I'm leaning towards the alternator as the culprit. I had a '77 GMC years ago that did the exact same thing and the alternator was the problem. Is there any other test I can try to narrow down my ( guesswork) diagnosis? Has anyone else experienced this problem? Any feedback I'd really appreciate. Thanks

Do not remove cables with engine running or you may blow a grand in computers and alternators.

If battery is 4 years old have battery and charging system tested.
A weak battery will run down sitting.
MT

Do not remove cables with engine running or you may blow a grand in computers and alternators.



Can't stress that enough!!! If you haven't already, make sure your battery cables are clean and tight at the battery and where they attach to the vehicle. A bad connection could give you that zero battery reading and then start working after you have hooked cables jumping it.

Thank you very much, especially about removing the battery terminal with the engine running

You may also want to check and see if you have a light staying on in the truck, I do not recall if there is a glove box one though I recently had a customer with this issue, You may also want to use a multimeter and see how many amps the truck is pulling with it turned off and all the doors shut

1. corroded battery terminals/poles. 4 yrs for as modern battery is not that much
2. faulty battery cable
3. faulty relay that kicks in when you turn ignition to on.
remove battery, bring to any major parts store, and load test. keep in mind, they are prone to "sell you" on this stuff, so they might be biased on judgement.

"being 4 years old I have no problem replacing the battery" you also have driver's license at 4?:iceslolan :iceslolan :iceslolan

Your discription of your problem is excellent. Based on what you write the problem is the altenator. its not gone but its on its way out.
Your altenator should charge at 14.5V.
Even under a heavy load it should not drop below 14V.
You should not worry about harming the computer by removing the cables while the vehicle is running. The charging system won't divert the charge back into the computer.
The worse that could happen is the car would stop and that would verify ( as I am sure you know ) the alternator was completely shot.

Your discription of your problem is excellent. Based on what you write the problem is the altenator. its not gone but its on its way out.
Your altenator should charge at 14.5V.
Even under a heavy load it should not drop below 14V.
You should not worry about harming the computer by removing the cables while the vehicle is running. The charging system won't divert the charge back into the computer.
The worse that could happen is the car would stop and that would verify ( as I am sure you know ) the alternator was completely shot.

It may not devert it back but that is a good way to blow a couple grand :swear: in computers.
A spike in voltage can wipe them all out.
MT

Thanks very much for that information ErickTR. I do appreciate the time you (and everyone else)took to respond to my questions. I did check all the connections from the battery cables, and all are tight & surprisingly clean. I think at this point I'll have the battery & charging system checked with the proper equipment and make a decision based on the results. Thanks again !...

Dave

you can go to your local autozone, advance auto parts store, checkers... and have the charging system checked on the parking lot/ on the vehicle for free!

but since your on-board gauge reads that your're under 14V while running seems to be that the alt has worn brushes...