Old and new Alternator quits charging and on restart will begin charging again for a short while.

[Tim White 1965]

I've looked at so many post on alternators, I've started drinking.  On the way home the other day the battery light and battery not charging message came on and voltage on dash dropped to below 12.  I have a Chevy 2005 2500 HD 4x4 6.0.  At the time I didn't think to pull over and try to restart so I tried to make it home and eventually the battery went dead and I was dead in the road.  Thinking it was obviously the alternator, I installed a new one.  Dumb me.  Any way the new one worked fine for several miles and then the same thing happened, voltage drop, battery not charging and then it would charge a little.  Pulled over and cut off and on restart it went back to 14 volts and was charging.  Increased rpm's to charge battery and it maintained charge.  Then it dropped again several minutes later.  Seems to do better on Interstate at speed but even then at times it would do it again.  Drove 90 miles and it was fine until I got in the city at slower speeds, stopping and starting.  I've even checked the outside temp of alternator and at 110-120 it would quit charging.  I've turned off lights, a/c radio and same issue.  I put volt meter on it when I crank it and it's 13.5 - 14.1 but after a minute or two decreases to 12 and then less.  Charged battery to 95%, pulled non essential fuses in groups and restart and monitored voltage.  Same result after pulling every fuse in groups and starting it would eventually quit charging.  Checked my grounds all seem to show no signs of corrosion or being lose.  Battery is holding charge at or above 12.5 an 70-90% even with all the starting and cutting off to check fuses.  I'm so frustrated it's not even funny.  I did take it to a shop today and they told me it was the alternator until I explained the above and informed him I had already been to the alternator expert here in town.

 

It would at times come back on while riding down the road, battery light go off and the message would go off.  It was off and on.  Volt meter would show it going down to 12.1 and then gradually to below 12 if I let it sit there and run.  I've had the truck since it was new and lots of repair over the years but this one has me stumped.

[garagerog]

GM implemented their RVC (regulated voltage control), basically a smart charging system in 2005, although I'm not sure if it applies to the HD trucks. If you're not familiar with RVC, you might want to read up on it. If your new alternator and the battery tests good, the only thing left that I can think of is your battery cables, they could be corroded or even broken internally and causing an intermittent connection. I suppose you could test them by putting a voltmeter on the battery with the engine running and yanking on or flexing the battery cables one at a time and see if you get any voltage fluctuation. Make sure and be careful of the fan if you have a mechanical one. 

[richard wysong]

Try running jumper cables or wires between the alternator output and the battery and between the alternator body and battery negative and I bet 1 of them cures the problem. That will point you in the right diagnostic direction

[garagerog]

16 minutes ago, richard wysong said:

Try running jumper cables or wires between the alternator output and the battery and between the alternator body and battery negative and I bet 1 of them cures the problem. That will point you in the right diagnostic direction

 

Better method than mine

[richard wysong]

Thanks Roger, years of experience. I ran into a car a few years back that the alternator bracket lost it's ground despite being tightly bolted to the block, took a couple of new alternators and a bunch of head scratching to figure it out so now I check every time before condemning the alternator. A quick test with a test light is usually all thats needed

[Tim White 1965]

Thanks for the responses Richard and Garagerog. 

 

I've checked voltage between battery negative and casing on alternator running and .01-.02.  Checked voltage from positive battery post to output on alternator 14.1 -13.9 max drop.  What I have found if this makes any sense is that brown wire of the 2 pins has about 10.5-11 volts when I turn the key on and it runs for a while it may have been 12 once I started it but when it seems to get warm or has run for a while and truck is hot the voltage on the brown wire drops to 9-9.5 or less.  I ran it for 30 miles at 45-70 mph and it was fine, when I slowed down and did some stopping and starting, it dropped to 12 or less battery light came on and battery not charging.  When I stopped and put volt meter on output it wasn't charging.  Three things come to mind.  I think the EMC is causing the internal voltage regulator to shut down and I may have a faulty EMC.  I have for years had the Reduce Engine Power error come up from time to time sometimes 6-8 months nothing and then 4 or 5 times in a day.  Second it's a 105 amp alternator and I believe my old one was too.  Could the alternator need more amps and the lack of causing the EMC drop in volts in the excite voltage brown wire, causing the alternator regulator to either not have enough voltage or it over heats.  Would a 145 amp be better?  Lastly what about a self-exciting alternator?  I can't tell if Auto Zone has any.  I really don't know if that is something typical retailers carry.  Where would I get one?

[Tim White 1965]

I've cleaned both battery cable ends, cleaned wires in junction box, pulled and wiggled on every wire I can get my hands on.  I did find some wires under my fuse box under the hood were worn and eaten by a mouse or something.  I cleaned those up, put some spray liquid tape after separating them and taped them up but non seemed to be anything related to the charging system.  I'm wondering now if repairing those may fix my intermittent reduce power error.  We'll see.