electrical charging issue

1988 GMC C1500
170,000 MILES • 5.7L • V8 • 2WD • AUTOMATIC
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OSBORNZAC99
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i’m having a issue where my alternator is only putting out about 12.8. i had it tested it said it was good but went ahead and bought a new one still having same issue, so i thought it was battery had the old one tested it tested good, but went ahead and bought a new one still same issue.
Aug 21, 2020 at 10:42 AM
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ASEMASTER6371
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Good afternoon,

I attached a wiring diagram for you to view. Can you verify power to the alternator on the 2 wires I circled and tell me the voltage?

https://www.2carpros.com/articles/how-to-check-a-car-alternator

https://www.2carpros.com/articles/how-to-check-wiring

Roy
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Aug 21, 2020 at 11:41 AM
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OSBORNZAC99
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12.8. i didn’t think about the fuse could that cause it to charge under 13.8 if the fuse is blown.
Aug 21, 2020 at 11:44 AM
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OSBORNZAC99
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that 10 amp fuse you circled .
Aug 21, 2020 at 11:44 AM
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ASEMASTER6371
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You stated 12.8 volts at the battery. Is that correct or incorrect.

Yes, if it is bad, the alternator will not charge.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Roy
Aug 21, 2020 at 11:48 AM
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OSBORNZAC99
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yes, 12.8 at battery and 12.8 at alternator.
Aug 21, 2020 at 11:49 AM
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ASEMASTER6371
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Okay, you stated 13.8 in your last response.

The alternator is not charging at all.

Did you verify power at the alternator at the 2 wires I circled?

Roy
Aug 21, 2020 at 11:54 AM
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OSBORNZAC99
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yes 12.8.
Aug 21, 2020 at 11:55 AM
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ASEMASTER6371
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If you have power at both of those wires, the only thing left is the alternator.

You can try disconnecting the white wire. It is for the tachometer. If the cluster is the issue, then it may start charging.

Roy
Aug 21, 2020 at 12:05 PM
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OSBORNZAC99
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i tested the old alternator said it was good, but i went ahead and bought a new one and it’s still doing same thing. i guess i will disconnect that wire and see if that’s the issue and also all my fuses are good too.
Aug 21, 2020 at 12:09 PM
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ASEMASTER6371
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As long as you checked your fuses for power and not visually.

Yes, give it a try.

Roy
Aug 21, 2020 at 12:10 PM
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OSBORNZAC99
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okay, i tried that and it’s charging at basically nothing now.
Aug 21, 2020 at 12:15 PM
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ASEMASTER6371
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Okay, back to the alternator.

What brand did you use?

Roy
Aug 21, 2020 at 12:19 PM
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OSBORNZAC99
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I originally had an O’Reilly Ultima so that’s what i went back to. i had it tested and it tested good, but i went ahead and bought a new one which i had them test straight out of box and it was good. so i threw it in my truck and still same thing is happening.
Aug 21, 2020 at 12:20 PM
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ASEMASTER6371
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How many wires on the back of the alternator?

Roy
Aug 21, 2020 at 12:21 PM
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OSBORNZAC99
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1 just the positive this alternator grounds it’s self through the brackets.
Aug 21, 2020 at 12:22 PM
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ASEMASTER6371
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Then how did you check the other wire I told you about and got 12 volts?

Roy
Aug 21, 2020 at 12:30 PM
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ASEMASTER6371
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Is this the back of the alternator?

https://www.rockauto.com/en/moreinfo.php?pk=3573282&cc=1031106&jsn=486

Roy
Aug 21, 2020 at 12:34 PM