My advice, for what it’s worth, is to divide and conquer. One of the two devices is causing your problem. I use a variable resistor 0-100 ohms to check the gauge. Hook it up as you would the sender. Key on. Slowly move ‘pot’ or variable resistor till gauge reads empty. Key off, disconnect pot and measure the ohm value. That will be what your gauge will want to read empty.
Repeat test for gauge for full reading. Now you know if it is truly a 0-30 ohm gauge. Does your sender send the correct ohms at full and empty. If everything is correct, then the only thing left is grounds (which you checked) and the wire from sender to gauge. Possible tohave a high resistance (meaning only a few ohms) connection. Good luck.
