Voltmeter Reading with A/C On
07-08-2009, 04:59 PM
#1
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Voltmeter Reading with A/C On
Do I have a problem? Was enjoying this afternoon's lovely 103 degree temps ... with both front and rear A/C going saw the gauge drop just under 12 volts while idling at a stop light. Once moving, it stayed right at 12 the rest of the way back to work.
Have never before seen higher or lower than 13 since I've had the car.
When my wife was in the car the other day and she cried out for max A/C front and rear, the gauge fell to 11 volts while creeping through some slow traffic.
Battery is just over two years old and never had any problems, but with this Texas heat I know I need to get it tested.
Have never before seen higher or lower than 13 since I've had the car.
When my wife was in the car the other day and she cried out for max A/C front and rear, the gauge fell to 11 volts while creeping through some slow traffic.
Battery is just over two years old and never had any problems, but with this Texas heat I know I need to get it tested.
07-08-2009, 05:12 PM
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Haha this happens to many people, although I can't say why it has never been a problem before for you and now it is. Typically it is a combination of low alternator output, old wiring (which has high resistance), and corroded ground points. You can try cleaning the ground connections and perhaps that will improve it, although I would be hard pressed to come up with a "sudden and rapid ground point oxidation" theory. Other solutions include an alternator upgrade or battery-to-alternator cable replacement.
07-08-2009, 06:51 PM
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As always, make sure that the alternator belt is REALLY tight. It should have virtually no "give" when pressed - none of this "half inch in the middle" stuff on this one.
07-08-2009, 06:52 PM
#5
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Well it has nothing to do with the battery - when running the alternator is supposed to be supplying all the juice.
Alternators struggle most at idle - when its very hot & when the current loading is high.
Hot days (end especially similarly hot nights) are a double whammy! its hot and due to the heat you will pretty much always have high loading: AC, blowers, cooling fans & lights.
Idling after a high speed run gets you the max alternator temperatures.
So partly its just the heat - over time regulators get worse - so a new one may help. Its also possible you have lost a phase diode bank (especially at 11v) so checking the alternator at a good shop (off the car) is probably a good idea.
Its also possible the alternator belt is just not tight enough (needs to be super tight) and its slipping at idle. Most folks find this unintuitive - the alternator belt is much more likely to slip at idle than at high rpms - and reving the engine just a little may cause it to stop slipping altogether unless its rediculously loose...
Also - this clearly has nothing to do with the battery to starter or starter to alternator cables or connections. This is also true for most ground points, (of course checking all connections is probably always worthwhle - but it wouldn't cause this).
It could however have something to do with the front jump post connections or alternator main terminal connections and the engine ground strap may be worth checking too.
Also verify all voltage assumptions with a reliable DMM - don't rely on the dashboard voltage gauge
Alan
Alternators struggle most at idle - when its very hot & when the current loading is high.
Hot days (end especially similarly hot nights) are a double whammy! its hot and due to the heat you will pretty much always have high loading: AC, blowers, cooling fans & lights.
Idling after a high speed run gets you the max alternator temperatures.
So partly its just the heat - over time regulators get worse - so a new one may help. Its also possible you have lost a phase diode bank (especially at 11v) so checking the alternator at a good shop (off the car) is probably a good idea.
Its also possible the alternator belt is just not tight enough (needs to be super tight) and its slipping at idle. Most folks find this unintuitive - the alternator belt is much more likely to slip at idle than at high rpms - and reving the engine just a little may cause it to stop slipping altogether unless its rediculously loose...
Also - this clearly has nothing to do with the battery to starter or starter to alternator cables or connections. This is also true for most ground points, (of course checking all connections is probably always worthwhle - but it wouldn't cause this).
It could however have something to do with the front jump post connections or alternator main terminal connections and the engine ground strap may be worth checking too.
Also verify all voltage assumptions with a reliable DMM - don't rely on the dashboard voltage gauge
Alan
07-08-2009, 07:50 PM
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Burning Brakes
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That's pretty much exactly what I thought (and said). I only included the ground point business because my understanding is that low voltage conditions can be caused by resistance in the lines, so it got thrown in as a matter of course. Alan is the pro, listen to him.
