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Should the Volt meter drop to 13 volts when the batt. Is charged while driving

Tiny
TBROWN1024
  • MEMBER
  • 2007 GMC SIERRA
  • 85,000 MILES
When I am driving I notice the volt meter charging 13.9 volts then it drops to aprox 12+. Is this normal. The dealer checked the system & sad it was ok. I asked the same question to him but did not get a straight answer. I thouhgt the vole meter always read aprox. 13.9 volts while driving. I have made sure that all loads were off, but I still get the same readings. I have deen told that when the batt. Is fully charged that the system will stop charging fully. I don't think I belive this.
Thank You
Tom Brown
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Monday, July 16th, 2012 AT 5:38 PM

7 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
No, fully charged it should be 14 to 14.5. When it is charginf, it will lower to 13.9 or so till the battery is charged.

Roy
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Monday, July 16th, 2012 AT 5:40 PM
Tiny
TBROWN1024
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  • 2 POSTS
So your saying the volt meter should not go below 13.9 or so. It does go down to aprox 12.5. I don't thine the daeler is testing it long enough to see this Right or Wrong?
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Monday, July 16th, 2012 AT 5:45 PM
Tiny
ASEMASTER6371
  • MECHANIC
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Makes sense. Lets put it this way. When the voltage is low, like 13.8 to 14, the alternator is pumping out high amps. When the voltage is high, like 14, 2 to 14.5, the alternator output in amps is low.

It should never go below 13.8 or so. If it does, there is an internal issue with the alt.

Roy
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Tuesday, July 17th, 2012 AT 12:32 AM
Tiny
SCORD5
  • MEMBER
  • 2 POSTS
SAME PROBLEM. Last week My battery died, change the battery and got a new alternator and now the amp starts off at 16 amps then drops to 13 and alittle below. It always seem to run higher before I had work done on it. Is this normal? If everything is charged what should the amp be? This is driving me crazy. I feel like I got a bad alternator.
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Thursday, July 19th, 2012 AT 7:03 PM
Tiny
ASEMASTER6371
  • MECHANIC
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You are referring to volts, I assume.
Fully charged battery should have 14.5 volts at the battery when running. If you turn on headlights, wipers ac and defogger, the voltage will drop to about 13.9 or so but the amps will increase to the battery.

I would have a voltage drop test done from the battery to the alternator to see if there is any high resistence causing this issue

Roy
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Thursday, July 19th, 2012 AT 11:45 PM
Tiny
SCORD5
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Can I do a voltage drop test on this vehicle myself or do I need to take it to a mechanic. Battery is reading 12.6 volts with car off. Thank you
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Saturday, July 21st, 2012 AT 1:10 AM
Tiny
ASEMASTER6371
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What is it when it is running?

Yes, if you know how to do it. Do you have expierence at this type of testing?

Roy
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Saturday, July 21st, 2012 AT 6:15 PM

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