Fuel sender or gauge?

[Doug Wilson]

I have a 1975 450 SL that the fuel gauges sometimes works and sometimes doesn't. I have taken the plug off the sender and checked voltage there and have 12 volts to one of the three terminals, the other two I have Ohm readings to ground. I have removed the sender and checked for Ohms from the terminal that would have power going into it to the other two terminals and get no reading. I do get an Ohms reading between the two terminals, however. Does anyone know if this is correct? I hate to spend $140.00 for a sender and then that not be the problem.
Thanks