Alternator overcharging, but only under load?

1995 FORD THUNDERBIRD
160,000 MILES • 4.6L • V8 • 2WD • AUTOMATIC
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CARTER MINIX
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When the car is started and running in park/neutral the alternator output is 14.5-14.7 volts. However, after driving the car the output changes to 18+ volts. During this the battery begins boiling over. I am looking to resolve this issue. I have a DMM and other tools to help resolve the problem. Before this I had an issue with the charging system not operating properly resulting in the car dying/draining the battery. Doing some research, I found the mega fuse was blown. Before then the alternator was recently replaced. I then did research about the mega fuse. Upon replacing the mega fuse, the car began charging again, however I now have this issue of overcharging but it seems to only be under load or when driving.
Mar 14, 2023 at 4:34 PM
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JACOBANDNICKOLAS
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Hi,

First, if it is putting out over 18v, I wouldn't run it. It will end up damaging other electrical components.

The voltage regulator is integral to the alternator. Disconnect the neg battery terminal. Remove the alternator and have it bench tested at a parts store. Something has shorted internally, or the regulator is faulty.

I attached the schematic and a few other pics below so that you have a reference.

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below. Note: Pic 1 is the diagnostics for an overcharging alternator. Basically, an auto parts store does this when they test it.
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Mar 14, 2023 at 9:50 PM
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CARTER MINIX
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Is there any chance of the harness being the issue, this is not a road use vehicle, and a few sections of harness were chewed through by mice. I’m not 100% sure if any of it was to the regulator but it’s a possibility.
Mar 15, 2023 at 6:20 AM
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JACOBANDNICKOLAS
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Hi,

I have a feeling that if there is wiring damage, it led to the regulator failing. So, yes, the harness is a big issue and concern.

Carter, keep in mind that these types of issues can lead to a fire. Here is what I suggest. Inspect the wiring harness damage. Anywhere there is damage needs to be repaired and insulated.

Next, remove the alternator and have it bench tested. If the regulator has failed, the alternator needs to be replaced.

Here is a link that explains in general how one is replaced:

https://www.2carpros.com/articles/how-to-replace-an-alternator

Let me know if you have questions and how things turn out for you.

Take care,

Joe
Mar 15, 2023 at 5:40 PM
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CARTER MINIX
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Would you be able to get me all the ground locations mentioned in the troubleshooting guide from pro demand?
Apr 6, 2023 at 2:26 PM
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JACOBANDNICKOLAS
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Hi,

As far as the grounds are concerned, there is really only one you need to worry about. Between the battery negative terminal and the left front of the engine. If you follow the negative battery cable, it will lead you to where it is bolted to the engine. The alternator is grounded through its mount via the engine ground. There is one other. It's a smaller gauge ground from the battery negative and is attached to the radiator support on the driver's side.

Is that what you needed?

Let me know.

Joe
Apr 6, 2023 at 10:36 PM