alternator mystery


Old 03-05-15, 05:41 PM
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Join Date: Feb 2015
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alternator mystery

Tell me what you think of this if you are an electrical auto tech.
My alternator started going weird today. Oddly it did the same thing exactly a year ago on the same route from Florida to the Rockies. But not at any other time.It fixed itself then and has not been a problem since then.
I am driving along on the Interstae and
It suddenly shows 12.5 volts ,down from 13.9 v. Then it stays from 11.9-12.4v.And of course the battery light goes on.
But if I take an exit on the interstate and go to the bottom of the off suddenly goes back to 13.5 and works it way up to 13.9v which is normal voltage. ( I have a ScanGauge so I can read the voltage any time.) THe normal voltage 13.9, will stay for about 10-20 miles and then do the whole thing again.
I wondered about my 5 year old battery so I put in a new one. No change.The alternator was rebuilt 2 years ago. THe reason I mention the road trip is I wonder if the elevation difference and the temp difference has anything to do with it..??
If I had to guess I would guess the solid state regulator is bad. If it was a loose wire why would it change from just slowing down on an offramp??I cleaned all connections.
I thought maybe the semi conductor material in the regulator was intermittently breaking down depending on rpm and temp and maybe elevation. What do you think???
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Old 03-05-15, 05:49 PM
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It certainly could be a problem with the regulator.

Without knowing year, make, model of vehicle it would be hard to offer more advice.
Old 03-05-15, 06:50 PM
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Moved thread to auto forum. Will move again to trucks if vehicle is a truck.
Old 03-06-15, 03:04 PM
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I don't know whether your problem is carbon brushes inside alternator or not. I had 1990 Honda Accord and never had alternator problem for 18.5 years (192K miles) since I owned it new. When I starting found the alternator can't charge battery (kind on and off for a while and then die), it is due to two carbon brushes inside alternator worn out. In stead of changing a new or rebuilt alternator which costs much more, I bought two carbon brushes from alternator rebuilt shop for just $15-$20 and they gave me two extra carbon brushes for free in case I can't solder them back without breaking them. It has been almost 6.5 years with 63K more miles on it and still working great. If your rebuilt alternator two years ago didn't put a new pair of carbon brushes, who knows how long of that old carbon brushes has been used. Since you have problem with your alternator, it is worth to check them now.
Old 03-06-15, 03:25 PM
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What is the make & model of the vehicle? Is it one of the engines that are installed sideways? What interstate is it & what's the difference in elevation between the intestate & the service road?
Old 03-07-15, 04:23 PM
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Join Date: Feb 2015
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it was the brushes....this pair would have about 110K on them. A shop in Florida put some cheap ones in there a couple of years ago.The original Toyota brushes lasted for 300K. Anyway its quite a job to dismantle a modern alternator and reassemble it.
So I bought a new reman for 200$. I barely made it into a remote town in the desert but nowadays you get next day parts even in the boondocks. Especially if its within 6 hour drive of Vegas and Phoenix. THe hubs of distribution.

If it was the solid state regulator it would not have been charging at all.

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