Voltage gauge & alt question - a dumb one, but I've been living in ammeter world...

[jocko]

01 5.3 Z71, voltage gauge reads direct art voltage or alternator output? Assume the later. But started scratching head. While running voltage dropped to approx 10 volts for 5 min with a red batt light. Then it jumped to approx 15-16V and is hangin out there. Kinda high.... Assume it's new alternator time, but just wanted to ask.

Also - do these newer alternators still produce a magnetic pull at the rear shaft bearing like the old ones do?

[turbostar]

Jocko, meter placement was critical in the ammeter days-not so critical with a voltmeter. In the old days, the ammeter would be wired in the system such that all current draws would go thru the ammeter, with the exception of the starter which drew too much current to be reflected on the gage. The ammeter would show you the "net" of the charge and draw in the system. One of the beauties of the voltmeter is that you can almost place it anywhere and get good readings. In other words if you check voltage at the alternator or the battery you will get essentially the same reading. With the readings you mentioned(10-16 volts while running), my guess would be you have a defective alternator(which has an internal regulator). Sometimes a bad battery can give some really weird problems, but I'm betting you need to replace the alternator.

[68c10airstream]

I would suspect if the truck is high miles (150,000) or above what I've heard of and happened to me is the brushes are worn down to the point that they sometimes don't complete the excitation needed. A major rebuilder in my area (45 plus years) says that's the most frequent item needed.

[jocko]

thanks guys, new alternator goin in.

[richards72chevy]

So would a bad alternator give a high reading upon starting then go to 15 Volt's after driving a little bit?I had both the battery and alternator tested at two different places both places tested good.

[turbostar]

Richard, what do you mean by a high reading?

[richards72chevy]

My volt meter used to stay slightly over the middle on the gauge now after I start the engine the gauge goes in-between the 14 and 19.I guess this would be 17.5 Volt's.I have jump started two vehicles before this happened and my battery has been in there since 2011.

[richards72chevy]

Plus some really cold weather here in Oklahoma.I have been told cold weather is bad on a battery.

[jocko]

installed a new alternator, holds steady at about 14V. Once I semi-confirmed that was the problem, I installed a new batt anyway since it was pretty old (but still good). It will go in a tractor and the deader batt from the tractor will be the core trade in. Anyway, all's well that ends well. Thanks for the comments gents, much appreciated. Hope my alternator lasts, it's a Chinese Pep Boys version... BARELY fit in stock location, the through-bolt lands were a few thou too wide to fit in the same spot where the original was. A little wigglin and it's in finally. I think it may be new tensioner time also, almost couldn't get the belt back on - full twist one the tensioner and it just barely went back on, ha. Always something. Anyway, thanks again.

[turbostar]

Richard, as I understand it the alternator should not charge as high as 17.5v, as the main purpose of the voltage regulator is to prevent overcharging. My best guess is that the internal voltage regulator in the alternator is failing. Even a weak battery will result in maybe 15.5v-16v charging. Don't know what kind of test was done on your alternator but sure sounds like you are overcharging your battery and need a replacement alternator. Hope this helps fwiw.

[richards72chevy]

I just had Autozone and orielys test it.So even if my battery was in a state of discharge my voltage should not go that high?

[turbostar]

It is my understanding that even with a bad battery, those real high charge numbers should not happen. At the same time, I know that a bad battery can give some weird symptoms. I recently changed out my battery as it was setting the bat lite on from time to time. A side symptom was that the radio would sometimes lose the time setting. A new battery solved those issues. You said your battery was 5 yrs old so I might consider a new one going into a cold winter. Also how many miles on your alternator? I have heard from more than one source that brush issues crop up on alternators when they get into the 150,000 plus range.

[richards72chevy]

I'm unsure how old the alternator is.When I bought the truck two years ago it had 177,000 miles on it now it's at 230,000.The battery has date on it I don't know how accurate those dates are.I do notice a build up of white film that looks like it's coming from the caps on top. I think will just replace the battery and alternator just to be safe.Thank you.