Pull the wing root fairing. On the -7, it is mounted in the center of the inspection plate. It is possible that the wire is grounded. This should cause a full scale deflection. On my 172, 50 ohms gives a full scale deflection, 240 ohms is zero fuel.
I am assuming that you have a float. It could be capacitance. If so, you will see a BNC connector and nothing coming out of the inspection plate.
If capacitance, measure the resistance between the center electrode of the BNC connector (tank side, cable disconnected) and tank ground. It should be pretty high if not infinite. If it is zero, something internal is shorted. if the resistance is high, I would suspect the converter or a ground.