fuel sending unit / fuel gauge mystery

[airbornewally]

OK, so I've recently inherited a 1972 C10. My father-in-law has used a stick for as long as I can remember to measure the amount of fuel in the tank. I have read many threads regarding this problem (not necessarily using a stick, lol), but haven't really been able to find a solution yet. With the sending unit installed in the gas tank, and the wire attached to the sending unit the gas gauge reads 'empty'. If I remove the wire, the gas gauge spikes past 'full'. However, if I remove the sending unit from the gas tank, attach the wire and ground the unit, the gas gauge works! I have ohmed out the sending unit. I know it's supposed to read between 0-90 ohms, respectively representing empty thru full. The reading I got on the sending unit was closer to 15-105 ohms. I have made sure that there are no cracks or pinholes in the float. It just seems to not work when the darn thing is installed in the gas tank! I know I must be missing something. If anyone has any ideas, I would be greatly appreciative.