71 dart instrument cluster

Usually, the sending units either work or they don't. When I first diagnose a non-working gauge, I start with removing the wire from the sender and attach a jumper wire from the wire terminal to a good ground. I then turn the ignition switch to the ON position. If the gauge starts to move, I immediately turn the switch back off. You don't want to let it stay pegged all the way over and overheat. That tells me the gauge at least works and that the sender is bad. How accurate the gauge is, is another matter. If nothing happens, check the voltage limiter and replace as necessary. You can do this test with the temp, fuel, and oil pressure gauges. When it comes to the fuel sender, make sure it's properly grounded when making the test. There's a ground strap that jumpers across the hose section at the sender. Make sure that's making a good connection each end. If you narrow it down to the fuel sender, hopefully your gas tank is near empty, or, at least below the level of the sender hole. Pull the lock ring, remove the sender, and check the float for flooding. It's pretty common to have a cracked float which causes it to flood and sink, giving you an empty reading on the gauge. New floats are easily found. When installing them, put some grease in the groove where it attaches to the sender arm. Then roll the float into place. Those brass floats are very thin and it's easy to put a hole in one of you just try to snap it into place dry.