Faulty New Fuel Sending Unit?

I recently bought a '79 cj5. The P.O. put in a new fuel tank and sending unit. He told me that the fuel gauge didn't work. I figured no big deal. I tested the gauge( I put the sending unit wire to ground, and the gauge read full), so the gauge is o.k. The sending unit has a good, clean ground. I did a resistance test between the sending unit-to-gauge stud, and the sending unit ground. No resistance with a quarter-full tank. Seems like a bad sending unit. I wonder if the P.O. broke the sending unit during installation. I guess I'll have to drop the tank and check it out(or cut an access hole in the floor above it)! Any other thoughts before I do this? Thanks.
Rich

 

fill the tank and reck. the restance may be out of callabration

 

I tested the fuel sending unit, and it was shot. I installed a nice access panel to get to the sending unit without dropping the tank, and replaced the sending unit. I tested the resistance, and the reading was perfect for the amount of fuel that there was in the tank. I turned the key on to check the reading on the gauge, and it went to full(there was less than half a tank of gas)so that was not good. There is a voltage regulating strap that connects to the temp gauge and the fuel gauge. It drops the voltage from 12 volts to about 5 volts to the gauges, because 12 volts will cook the gauges. That strap tested faulty. That is what probably did in the sending unit to begin with. So next I will replace the fuel and temp gauges along with the voltage regulator strap.I'll get to the bottom of this, yet!

 

I used after market gauges that all ran off 12v. so I cant help you there. good luck just one thougt I had to move the sender up the part thet sticks down and cut down the rod the float is on to get the sender to have the correct range of motion. Note my tank is under the drivers seat.