From a web page on diagnostics for gauges you might try the following;
With an ohmmeter check the resistance between the round sender post (pink wire) and ground.
It should be:
*73 ohms empty
*23 ohms at 1/2 tank
*10 ohms full
If the resistance falls in this ballpark (depending on how much gas you have in the tank), then the sending unit is fine.
If it shows infinitely HIGH resistance, then the sending unit could be bad OR the wire from the tank to the gauge could have an open.
Best of luck, these gauge issues can be maddening