I like FIMudCJ's idea of testing the sender wire both hot and cold and would do that first.
I have learned that typically if the gauge shows function the culprit is usually the sender wire and most likely the sender it's self.
A good test for the a CJ gauge is to momentarily touch the sender wire to ground.
If you get a good clean sweep on the needle, it is probably OK.
Just don't hold contact long or you will cook the gauge.
The backwards YJ gauge may be different in that one might need to alter this test to include a heavy resistor to the sender and then touch to ground.
I am leery of suggesting you try the CJ test to your gauge.
In the CJ's case, the LESS the resistance the more towards full the needle will read.
In the YJ's case with MORE the resistance the needle will more full.
I don't know if a YJ's fuel gauge needle rests at full or empty when the ignition is off but if you can interpret these facts you should still be able to apply this "shorting" test to see if you can get a good, clean sweep of the needle which should eliminate the gauge as the faulty component.
And yes, the CJ and YJ's sender resistances are backward.
In fact, I was able to sucessfully work with a guy on a YJ body and gauges / CJ frame and sender setup. The resistance paramiters were similar in both - just reversed. He switched the leads on the CJ sender's wiper arm and Bingo! it worked!
The original version of my "CJ Gauge & Sender Diagnostics" article in the ORC Tech section is inaccurate in the jumper strap/regulator section and has been corrected and updated at it's original and primary location at ...
The regulator is actually inside the CJ's fuel gauge and the jumper is just a plain jumper strap. Confusion exhisted among most as to the locaton of the 'bi-metal "strap" or "strip". Only after I later disected a gauge was I able to confirm it's location and discredit my (and many other's) original assumption that the regulator was in the strap!
Getting changes and correctons made at the ORC location is like pulling hen's teeth. Bookmark the URL above for the most current version.