Steve.
I think what Redfish and Dana are saying is that the oil and temperture gage can be tested by connecting to a low voltage supply - 3 volts would be fine.
But the Fuel gage can't be tested that way. The stud connects to the IVR first, and if that doesn't see something close to 12 volts the fuel gage probably won't get anything from the IVR.
Wrong, Fuel gauge is tested same way but at posts A and S. If the gauge is on a workbench, the limiter isn't chassis grounded, back feed voltage into that limiter cannot go anywhere or have any effect at all.
That's why nobody ever really needed to open this fuel gauge and bend a little thingy. Just isolate that limiter from ground. Move the the 12 volt supply to post I from circuit board to external limiter or regulator. 5 volts in at post A.