Safety Comment: Do not measure Resistance (ohms) while a circuit is hot. Now, not sure what a 30-90 sending unit is, so hard to advise. Seems like you have a voltage reducer to reduce 12 volts to 6V for the gauge. They sell 30 Ohm senders and 90 Ohm senders. What is a 30-90 sender? Hopefully you bought a 30 Ohm sender for your 30 Ohm stock gauge. 90 Ohm was not used until 1967. If you have a voltage reducer hooked up properly and a 30 Ohm sender and a 30 Ohm gauge and they are both in operational condition. It's the ground. So isolate this little fuel gauge system from all of the truck wiring and test. Use a fuse inline while testing and, of course, hook it up right!
