Electrical charging issue

Tiny
OSBORNZAC99
  • MEMBER
  • 1988 GMC C1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 170,000 MILES
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.
Friday, August 21st, 2020 AT 10:42 AM

17 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
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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Friday, August 21st, 2020 AT 11:41 AM
Tiny
OSBORNZAC99
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  • 11 POSTS
12.8. I didn’t think about the fuse could that cause it to charge under 13.8 if the fuse is blown.
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Friday, August 21st, 2020 AT 11:44 AM
Tiny
OSBORNZAC99
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That 10 amp fuse you circled.
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Friday, August 21st, 2020 AT 11:44 AM
Tiny
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
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Friday, August 21st, 2020 AT 11:48 AM
Tiny
OSBORNZAC99
  • MEMBER
  • 11 POSTS
Yes, 12.8 at battery and 12.8 at alternator.
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Friday, August 21st, 2020 AT 11:49 AM
Tiny
ASEMASTER6371
  • MECHANIC
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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
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Friday, August 21st, 2020 AT 11:54 AM
Tiny
OSBORNZAC99
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Yes 12.8.
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Friday, August 21st, 2020 AT 11:55 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
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
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Friday, August 21st, 2020 AT 12:05 PM
Tiny
OSBORNZAC99
  • MEMBER
  • 11 POSTS
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.
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Friday, August 21st, 2020 AT 12:09 PM
Tiny
ASEMASTER6371
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As long as you checked your fuses for power and not visually.

Yes, give it a try.

Roy
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Friday, August 21st, 2020 AT 12:10 PM
Tiny
OSBORNZAC99
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  • 11 POSTS
Okay, I tried that and it’s charging at basically nothing now.
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Friday, August 21st, 2020 AT 12:15 PM
Tiny
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Okay, back to the alternator.

What brand did you use?

Roy
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Friday, August 21st, 2020 AT 12:19 PM
Tiny
OSBORNZAC99
  • MEMBER
  • 11 POSTS
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.
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Friday, August 21st, 2020 AT 12:20 PM
Tiny
ASEMASTER6371
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How many wires on the back of the alternator?

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

Roy
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Friday, August 21st, 2020 AT 12:30 PM
Tiny
ASEMASTER6371
  • MECHANIC
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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
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Friday, August 21st, 2020 AT 12:34 PM

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