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Why is my voltage gauge showing low when battery is charging?

Here’s what I know:

2001 National Surf Side 3311 motorhome. It’s on a Ford chassis.
Ford V10 Engine, Ford gauges, etc.

Near as I can tell it has a 3G Ford alternator.

While driving, the voltage gauge on the dash shows ~8V and doesn’t move much up or down. Every other gauge functions as normal (oil pressure, engine temp, tach, speedo, etc).

Testing with a good Fluke multimeter: At the battery it’s showing 12.6V while sitting and showing ~14V while running (so I assume alternator is charging).
At the alternator, I have 4 wires:

4 gauge black/red that shows constant 12V

Then a 3 pin plug that has the following wires:

Red/lt blue with 12V constant

Lt green/red with 13+V when engine is running

Brown/black wire with 7.3V when engine is running (this wire comes out of the 3 pin plug and goes back into a single pin plug that’s also on the alternator, it doesn’t go anywhere else.)

My question is should the brown/black wire show a higher voltage? If the battery is charging then what would cause a low voltage on the gauge? I am assuming the voltage regulator attached to the alternator is bad but thought I’d check here first before just buying parts. Thanks in advance.

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Older Fords use a separate external voltage regulator, a squarish metal box approximately 4x4x3 inches, mounted on a fender in the engine bay. I'm not familiar with Ford alternator wiring but your initial measurements seeing 12.6v on standby and 14v with engine idling suggests the alternator is outputting correctly despite the voltage gauge being incorrect. My guess is it's wiring between gauge and power source. A Ford service manual would help pinpoint wiring to check where the 8 volts is measured. In addition, the gauge ground may be an issue if power or ground connections are corroded.

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I’ve checked over the grounds and batteries and all seem to be in order and looking good. Vehicle only has 43k miles on it and has been stored under cover for years. In some diagrams I saw online it looks as though there’s a voltage regulator that’s attached to the alternator and can be removed (the 3 pin plug is plugged into this). I’m wondering if the wire showing 7V that runs from the 3 pin plug directly back into the 1 pin plug on the alternator may be the problem. I’m not sure if it’s supposed to have a higher voltage showing on that wire. However, I will check for the external voltage regulator and see what I can come up with.

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