Continuing the Fuel Sending Unit discussion

First check the sending unit in the tank like this post is talking about.
You would think this circuit is a bridge type, but I don’t think so.
The power goes thu the gage to the reostat (sending unit) and then to earth.
Your gage operates on a heated up piece of metal which moves the needle.
In my experience, most inop fuel gages are bad because something else failed first.
I’m not sure what it’s called but on the back of the instrument cluster there is a little relay looking plug in thingy. (We could call it a voltage limiter)
It functions sort of like the old school flasher. It allows a certain amount of current to flow. It heats up and opens the circuit to the fuel gage. Then it cools down and closes allowing more current to flow. In other words, it cycles power to the gage.
If it fails, the gage gets full 12volts uninterrupted and soon fails.
You can tell this is happening because the gage goes coo-coo (pegs) till it burns up and quits working.
There are other problems but this is what I have seen.
Other people may be able to help you more or correct anything my memory is wrong about on this. I assume 71 cars had this system like my later ones do.