Replace them.
I checked the sending units with an ohmmeter, looking for changing resistance as I moved the float arm. If it went open, the sender was bad. I was able to fix one of them with a needle nose pliers to bend the brass wiper that had worn off so it rubbed on the resistance coil again. Both were with the sender removed from the tank. And with the sender removed from the tank is when you find out if there still is a float attached. I've used a large cork coated with SealAll to replace the float a couple times. SealAll stands up to fuels including ethanol. When testing the sender with the dash gauge and the sender out of the tank you need to add a ground wire to the sender flange or you will get no where. One quick check for another problem I've seen is to run a wire from the sender flange to the dash ground because sometimes with gaskets at the sender its ground has gone away and that's a way to make it not work.
Its hard to suggest a simple test procedure because there are two schemes in use. One has low resistance from the sender for a full tank, the other has low resistance for an empty tank and sometimes the gauge is intolerant of shorts on the sender circuit, has to have a minimum resistance (often 33 ohms). But different makers of gauges use different resistance ranges.
There's only one safe test with a test light and that is to see if there's power on the gauge.
Some gauges need a ground at the gauge also, some don't. In cars and trucks used to be that gas gauges were supplied from a voltage regulator in the dash so the gauge reading didn't change as the generator or alternater charged or didn't charge.
If the gauge rests at E without power, its sender probably is low resistance at full, and if the gauge rests at F without power, its sender probably is high resistance at full.
Gerald J.
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