Should the volt meter drop to 13 volts when the batt. is charged while driving

2007 GMC SIERRA
85,000 MILES
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TBROWN1024
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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
Jul 16, 2012 at 5:38 PM
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ASEMASTER6371
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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
Jul 16, 2012 at 5:40 PM
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TBROWN1024
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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 ???
Jul 16, 2012 at 5:45 PM
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ASEMASTER6371
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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
Jul 17, 2012 at 12:32 AM
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SCORD5
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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.
Jul 19, 2012 at 7:03 PM
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ASEMASTER6371
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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
Jul 19, 2012 at 11:45 PM
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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
Jul 21, 2012 at 1:10 AM
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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
Jul 21, 2012 at 6:15 PM