Fuel sending units read WRONG time after time!

I think I am making progress. But the last hour the multimeter started being nuts. So I realized the battery had not been changed in at least 10 years.
So between my embarrassment about the battery and the garage is 113 degrees, I decided to take a break.


I am finding an abnormally in the voltages on the Instrument cluster. Incoming voltages are over 12.5 vdc. But the input voltage of the regulator is a full 2 volts less. Plus the output voltage on the regulator is not regulated and not 5 volts
Being a semi conductor I understand that it may lose a few tenth of a volt turning on. But not 2 full volts. I need to go further into the electrical.