Voltmeter Gauge / Electrical Short??

The voltmeter gauge in my 99 F-250 SD 5.4L was going from 8 volts to 18 volts and the battery light would flash occasionally. I had the battery checked and the alternator by two different parts stores and both said the alternator was bad and both said the battery was good. The battery is the original battery from when I purchased the truck new. Now the gauge seems relatively stable at around 13 volts but the battery light while flash sometimes and the dash lights will flicker. Is it possible that the battery has a short even though it tested okay? Any other possibilities??
Thanks Bill
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Based on what information you presented, the alternator needs to be replaced. It sounds like the voltage regulator (internal) is bad. There isn't a cheaper alternative, if that's what you're looking for.
Spdloader
The voltmeter gauge in my 99 F-250 SD 5.4L was going from 8 volts to 18 volts and the battery light would flash occasionally. I had the battery checked and the alternator by two different parts stores and both said the alternator was bad and both said the battery was good. The battery is the original battery from when I purchased the truck new. Now the gauge seems relatively stable at around 13 volts but the battery light while flash sometimes and the dash lights will flicker. Is it possible that the battery has a short even though it tested okay? Any other possibilities??
Thanks Bill
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Thanks for the reply. Actually I just replaced the alternator and I am still having the same issue.
Bill D

battery
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Okay, I didn't see that in your original post.
It used to be recommended that the plug be changed as well when you replace the alternator, sometimes they will be faulty.
Incidentally, my '99 is on the original batteries, six years old now, that is the exception on stock batteries. Time for a change for sure.
Spdloader

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Most of the time, when an old battery fails me, it's because it shorted out on one cell (thus a 10 volt battery). I've never had one of those shorts be intermittent, so I really don't know if that's what you're seeing. But it sure seem plausible. If I had a 6 year old battery in my truck, I'd stick my volt meter on it. If it tested good, but something was acting up, I might buy a new battery anyway. Because it wouldn't last much longer if I had it, and batteries don't cost much.
If everything works okay but your battery light comes on intermittently, I'm afraid there's a question that needs to be answered. Just what is that light connected to? If you can understand that, then the problem will be very clear. If that was my truck, I'd look at the wiring diagram (I'd have one) and see just what that light really does.
I would never in a million years trust some auto parts store to test my alternator or my battery. It doesn't make any sense. Buy a volt meter. About half or two-thirds of life's problems require a volt meter to solve. You can load test your alternator or battery a lot easier in the truck.
Is it possible that the

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