# voltmeter going from 14v to 12v and back??????



## tahoeman (Jun 30, 2007)

Has anyone experience this on their car? I would start up the car and the volt meter will start at a little over 14. After driving and coming to a complete stop, the volt meter will decrease as low as 
12v. When I step on the gas and start driving again, it goes back to a little over 14v. Can this be the battery? I just had it replace last year.


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## VW/Porsche Fahrer (Dec 14, 2011)

I am not sure but I would think this is normal. I mean, the charging system should be putting out more than 12 volts ( eg 14) to drive electricity into the battery. Is this something that has recently changed?


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## [email protected] (Aug 25, 2001)

I would more lean towards a dying alternator than the battery, because of the dips towards 12v at idle and then returns to 14v when driving (when RPMs are back up)


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## grohgreg (Jun 12, 2011)

[email protected] said:


> I would more lean towards a dying alternator


or voltage regulator (VR). It's actually what is supposed to control the alternator output. Battery discharged slightly after starting? VR senses that and orders full alternator output; 14.5 volts. As battery voltage tops up, VR tells the alternator to back off proportionately. Battery full, VR tells alternator to remain steady at a nominal 12.2 volts. If the VR is starting to go bad, it's sending mixed messages to the alternator - and show up as unusual activity on the voltage gauge. 

Alternatively - but less likely - there's a significant short somewhere. One that is draining voltage from the battery, to the point where the alternator is working hard to keep the battery topped up at driving speeds. But then can't put out enough when engine RPMs drop to idle speed - and the difference is showing up on the voltage gauge.

Battery - alternator - voltage regulator - short. Take it to an auto parts store for a free charging system checkup to help narrow down that field.

//greg//


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## depiry (Feb 16, 2005)

Check for loose belt or drive coupling,Marty

or loose cable?


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## tahoeman (Jun 30, 2007)

plan to head to suto shop this week. will let u guys know.


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## JTsquared (May 14, 2002)

we just had the same issue with our 08 3.6, the dealer replaced the alternator under CPO warranty :thumbup: although we had no charging issues. volt gauge still varys but not as bad and still no issues.


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## tahoeman (Jun 30, 2007)

had the battery replaced, alternator is fine. the voltage still varys. back to square one.


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## depiry (Feb 16, 2005)

Voltage should remain steady at 14V,unless loose cable ,or bad cable as in TSB for earlier V8's,check voltage drop across cable at starter to battery ,check the TSB,Marty

The alternator is 190 amps and should create steady voltage unless loose drive belt etc.


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## grohgreg (Jun 12, 2011)

depiry said:


> Voltage should remain steady at 14V,


How then does the electrical system prevent over-charging of the battery? I mean, isn't that what the voltage regulator is for? to bring the alternator down gradually from the 14.5v charging output to the normal 12.2v operating output?

//greg//


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## tahoeman (Jun 30, 2007)

it does make sense when you're driving and its at 14 and when you stop it goes down. wish I knew.


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## depiry (Feb 16, 2005)

As the battery is charged the voltages rises and the counter EMF reduces the charging current,nominal voltage of 2.33333 is required to maintain full charge,as load is increased the generator ,maintaining 14 volts produces more current to maintain the battery charge,Marty


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## grohgreg (Jun 12, 2011)

depiry said:


> As the battery is charged the voltages rises and the counter EMF reduces the charging current


Uh-huh. To a nominal 12.2 volts. What's 14.5 minus 2.33? 

Here's the way I understand it. When you buy a battery, it's for the express purpose of starting the car and providing electricity when the engine isn't running. Fully charged, the new battery will produce 12.2 volts. Once installed, the electricity consumed to start the engine must be replaced. This is where the voltage regulator comes into play. It senses that there's now less than 12.2v in the battery, and tells to alternator to increase its output (up to a nominal 14.5v max). That 2.3v boost replaces that which was consumed starting the car AND powers the car electrical system at the same time. As the voltage regulator senses the battery voltage increasing, it gradually lowers the alternator output proportionately. When the once again senses the nominal 12.2v, the battery is switched out of the electrical system. The alternator is now providing all power to the vehicle electrical system, until its max amperage output is reached. Ask any novice who's installed a blinding array of off-road lighting or a massively over-sized audio system without considering a larger capacity alternator. Anyway, at that point, the battery will kick back in again to provide that which the alternator is incapable of providing. 

That said - when alternators start to go bad - it's not unusual for them drop below their original output rating. As normal car circuits are switch on and off, the resulting demand can at time exceed this reduced output capacity. During those times, the battery kicks in to make up the difference. When the load is reduced, the alternator takes over again - but the voltage regulator senses the battery needs to be topped up again. Hence the movement between 12 and 14 volts on the voltage gauge.

Or the voltage regulator could be going bad, and incorrectly sensing battery voltage. 

So as I said, the solution is a simple charging system analysis to pin down a more specific cause and solution. This is a free service provided by most auto parts stores. They want to sell parts, so they're more than glad to find out what's wrong with your charging system for free.

//greg//


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## grohgreg (Jun 12, 2011)

tahoeman said:


> it does make sense when you're driving and its at 14 and when you stop it goes down. wish I knew.


 Yeah, but the cause could still be on a long list of possibilities. A free charging system check-up should be your very next step.

//greg//


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## yellowslc (Aug 29, 2000)

Just went through this. Tried new OEM batteries $4XX.00 and that didn't help.. alternator was dying quickly. Cost the aftermarket warranty $2200.00 to replace. Rock solid 14V once again.


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## depiry (Feb 16, 2005)

A good alternator will produce enough current to keep the voltmeter at 14. volts,check the size of the alternator pulley vs the engine pulley,quite a difference,ratio is at least 5-1,meaning that the alternator is running at 3000 rpm at idle,at that speed the cut in amps are high,if the voltage drops to 12 v the alternator output is off,you are at the standing voltage not charging fully,the regulator or a diode is out in the unit,Marty


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