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About a year ago I was driving around and my radio cut-out. This caused me to scan the instrument panel and I noticed my voltmeter was reading ~12V. I shut-off the engine at checked the battery with a multimeter and saw it sitting above 12v. I don't remember the exact reading now, but it seemed normal to me. When I started the car it jumped back up to almost 14v and stabilized there.

About 6 months ago this problem become more frequent. Intermittently I would notice the voltage dropping. It wasn't every time I drove the car, just on occasion.

About 3 months ago the problem became apparent any time I was idling at a long stop light. I could watch the voltage gauge visibly drop to 12.

At this point, after 15 minutes or so the drop happens every time. However, when I restart the engine the voltage will jump back up to almost 14v for 1-2 minutes before slowly dropping back down to 12v. If the Radio and A/C are both on, it will drop below 12v. After shutting off the engine, the battery's voltage will be around 12.5V.

My assumption is that the alternator stops charging and the car is running off whatever is left in the cells. I assume if I left the car idling, the battery would be depleted and the engine would stall. However, even after driving around for 30 minutes and then sitting in the garage idling for 15 minutes, the car never dies. (How long could the engine idle on just the battery?)

I took the car to Autozone, thinking they had test apparatus that was more elaborate than just a voltmeter. (Granted I think their tool can do things like properly load a battery, but really the information it gave wasn't much more than a voltmeter.) Their machine claims the charging system is perfectly fine, even though you can clearly watch the voltage drop 10mV / sec.

Why is the charging voltage jumping up to almost 14v when the car first starts and then dropping? Why does it stay close to 14v when the car is cold but drops faster as the car warms up? In other words, if I let the car sit over night, I see the same behavior as when I immediately swap the battery with a fully charged battery. This was another test I did with Autozone's help.

I assume the alternator needs to be replaced, I'm just curious why it is acting this way (and if it indicates another problem.)

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About a year ago I was driving around and my radio cut-out. This caused me to scan the instrument panel and I noticed my voltmeter was reading ~12V. I shut-off the engine at checked the battery with a multimeter and saw it sitting above 12v. I don't remember the exact reading now, but it seemed normal to me. When I started the car it jumped back up to almost 14v and stabilized there.

About 6 months ago this problem become more frequent. Intermittently I would notice the voltage dropping. It wasn't every time I drove the car, just on occasion.

About 3 months ago the problem became apparent any time I was idling at a long stop light. I could watch the voltage gauge visibly drop to 12.

At this point, after 15 minutes or so the drop happens every time. However, when I restart the engine the voltage will jump back up to almost 14v for 1-2 minutes before slowly dropping back down to 12v. If the Radio and A/C are both on, it will drop below 12v. After shutting off the engine, the battery's voltage will be around 12.5V.

My assumption is that the alternator stops charging and the car is running off whatever is left in the cells. I assume if I left the car idling, the battery would be depleted and the engine would stall. However, even after driving around for 30 minutes and then sitting in the garage idling for 15 minutes, the car never dies. (How long could the engine idle on just the battery?)

I took the car to Autozone, thinking they had test apparatus that was more elaborate than just a voltmeter. (Granted I think their tool can do things like properly load a battery, but really the information it gave wasn't much more than a voltmeter.) Their machine claims the charging system is perfectly fine, even though you can clearly watch the voltage drop 10mV / sec.

Why is the charging voltage jumping up to almost 14v when the car first starts and then dropping? Why does it stay close to 14v when the car is cold but drops faster as the car warms up? In other words, if I let the car sit over night, I see the same behavior as when I immediately swap the battery with a fully charged battery. This was another test I did with Autozone's help.

I assume the alternator needs to be replaced, I'm just curious why it is acting this way (and if it indicates another problem.)

At a quick glance of this long post, I would think the issue is with your voltage regulator. That will typically cause these odd fluctuations. It will charge, then drop, then charge...etc. Seems very random and haphazard, but it is likely the voltage regulator. That is how they act when they are faulty.....Just my opinion. I would check that out...

Good luck.

DC

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At a quick glance of this long post, I would think the issue is with your voltage regulator. That will typically cause these odd fluctuations. It will charge, then drop, then charge...etc. Seems very random and haphazard, but it is likely the voltage regulator. That is how they act when they are faulty.....Just my opinion. I would check that out...

My thoughts were moving towards the regulator as well. I just lumped it in with the alternator because I thought the regulator was internal on the 996's alternator. So in order to replace it, I have to replace the alternator anyway.

Is it true the regulator is integrated into the alternator, or am I incorrect?

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About a year ago I was driving around and my radio cut-out. This caused me to scan the instrument panel and I noticed my voltmeter was reading ~12V. I shut-off the engine at checked the battery with a multimeter and saw it sitting above 12v. I don't remember the exact reading now, but it seemed normal to me. When I started the car it jumped back up to almost 14v and stabilized there.

About 6 months ago this problem become more frequent. Intermittently I would notice the voltage dropping. It wasn't every time I drove the car, just on occasion.

About 3 months ago the problem became apparent any time I was idling at a long stop light. I could watch the voltage gauge visibly drop to 12.

At this point, after 15 minutes or so the drop happens every time. However, when I restart the engine the voltage will jump back up to almost 14v for 1-2 minutes before slowly dropping back down to 12v. If the Radio and A/C are both on, it will drop below 12v. After shutting off the engine, the battery's voltage will be around 12.5V.

My assumption is that the alternator stops charging and the car is running off whatever is left in the cells. I assume if I left the car idling, the battery would be depleted and the engine would stall. However, even after driving around for 30 minutes and then sitting in the garage idling for 15 minutes, the car never dies. (How long could the engine idle on just the battery?)

I took the car to Autozone, thinking they had test apparatus that was more elaborate than just a voltmeter. (Granted I think their tool can do things like properly load a battery, but really the information it gave wasn't much more than a voltmeter.) Their machine claims the charging system is perfectly fine, even though you can clearly watch the voltage drop 10mV / sec.

Why is the charging voltage jumping up to almost 14v when the car first starts and then dropping? Why does it stay close to 14v when the car is cold but drops faster as the car warms up? In other words, if I let the car sit over night, I see the same behavior as when I immediately swap the battery with a fully charged battery. This was another test I did with Autozone's help.

I assume the alternator needs to be replaced, I'm just curious why it is acting this way (and if it indicates another problem.)

That's strange, my lawn tractor battery charging system acts the same way. Immediately after starting it the charge rate, current flow and voltage, goes up. Within a few minutes the charge rate declines as does the alternator output voltage.....Strange....?

NOT...!! SOP....!!

And a "cold" battery will accept/require/indicate a higher charge rate and voltage.

You undoubtedly have a dead cell in your original battery.

terminal

Edited by wwest

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Charging voltage is dependent on several factors and will change over time. Those factors some of which are battery condition, and ambient temperature. As the battery charges the overall system voltage will back down a bit. Generally it will maintain around ~13.7v .. It may go as high as 14.4v if the battery is low and the ambient is low as well. Engine speed vs load is a factor as well. If the engine isn't turning the alternator fast enough to maintain the system load then the voltage will tail off. On newer cars they generally have small enough pulleys on the alternator to maintain charge at idle with the A/C on which is absolutely necessary for the electronics. Sometimes 3rd party add-ons like high power sound systems over tax the charging system at low speeds.

I've seen the action you mention here with bad batteries, bad voltage regulators, worn brushes, and lose / high resistance grounds as well.

HTH

--CC

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That's strange, my lawn tractor battery charging system acts the same way. Immediately after starting it the charge rate, current flow and voltage, goes up. Within a few minutes the charge rate declines as does the alternator output voltage.....Strange....?

...

You undoubtedly have a dead cell in your original battery.

The battery was less than 1 year old when the trouble began. Since it was still under warranty, I swapped it for a brand new one before making my original post. Needless to say, it wasn't the battery. I have a new regulator coming so I am going to try that next.

Edited by CMiYC

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That's strange, my lawn tractor battery charging system acts the same way. Immediately after starting it the charge rate, current flow and voltage, goes up. Within a few minutes the charge rate declines as does the alternator output voltage.....Strange....?

...

You undoubtedly have a dead cell in your original battery.

The battery was less than 1 year old when the trouble began. Since it was still under warranty, I swapped it for a brand new one before making my original post. Needless to say, it wasn't the battery. I have a new regulator coming so I am going to try that next.

if I follow this correctly everything now operates normally....

Are you certain you're not troubleshooting a non-existent problem?

Using a DVM, Digital Volt Meter, with the engine idling, measure the voltage on the battery posts vs the jumper point in the engine compartment. A difference of more than 0.5 volts might indicate a poor wiring or ground connection front to rear.

The alternator might be "generating" a proper battery charge voltage, ~13.5 volts, but only charging the battery to ~12 volts.

Edited by wwest

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if I follow this correctly everything now operates normally....

I'm pretty sure that at no time a car's charging system should drop to <11.7V. Nor should drop at a rate of 50mV/second. Which is what my charging system does every time after running for a few minutes. I would not agree "everything operates normally" now.

Using a DVM, Digital Volt Meter, with the engine idling, measure the voltage on the battery posts vs the jumper point in the engine compartment. A difference of more than 0.5 volts might indicate a poor wiring or ground connection front to rear.

This is a great idea. I hadn't thought to compare the difference from the battery posts to the engine posts. The only trouble i'll have with the experiment is moving fast enough from the front to the back. (I don't want to see a big difference in reading, just because of the time it takes to move.)

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"..operates normally now..."

Car starts, runs, radio operates normally, etc, etc........

In other words if you weren't now "watching" the charging rate/voltage you would have no idea of something being wrong....

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RESOLVED: It was the Voltage Regulator.

Replaced the regulator this morning. Cold start the charging system is stable at 14.4v. After driving a variety of roads, highway, city, and stop/go, the charging voltage is stable at 13.9v. :)

The regulator was about $50 shipped. I bought it from alternatorparts.com. Link to the one I ordered: http://store.alterna...regulator.aspx. The original part number was F 00M 145 225 (something I had trouble finding when searching for just the regulator.)

Thanks to Renntech for being such a great resource!

Edited by CMiYC

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RESOLVED: It was the Voltage Regulator.

Replaced the regulator this morning. Cold start the charging system is stable at 14.4v. After driving a variety of roads, highway, city, and stop/go, the charging voltage is stable at 13.9v. :)

The regulator was about $50 shipped. I bought it from alternatorparts.com. Link to the one I ordered: http://store.alterna...regulator.aspx. The original part number was F 00M 145 225 (something I had trouble finding when searching for just the regulator.)

Thanks to Renntech for being such a great resource!

Glad you got it all sorted out!

DC

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