B body gas gauge

When I fill the tank with gas it reads 3/4 and goes to E very fast and I still have gas in the tank. So I took the sending unit out and moved the float to full and the gauge goes to 3/4. I also grounded the wire to the gauge by using a battery and the gauge stayed on E but when I grounded it with a body ground the gauge when past F. So I dont know if I need a gauge or a sending unit? Let me know if you need for info.
Thanks for the help.

sorry, not sure I understood.

You grounded the sending unit to the battery? just ran a long wire back? And it stayed on E?

Then you grounded the sending unit to the body? or the wire to the sending unit to the body?

Getting F on the gauge means the gauge is good.

3/4 on it with the current sending unit could go different ways. Did it always do this? replacement sending unit? or original?

Yes it always does this. I put a battery under the car ran a wire to the - and it read E then I ran that wire to the body and it read F. I think it is a replacement sending unit.
Thanks

Check the ohms on the sending unit. At E, it should read around 80. At "F" (full travel of the float arm) - you ought to be at about 10 (IIRC). If you are getting 20 or 30 ohms at full travel, then its the sending unit.

You have to have the battery under the car hooked up to positive car battery also or the gauge wont move.How fast does the gauge go to empty when you fill it?Run the tank low and pull the sender and ohm it and see what the Ohms are.Rocky

How can I bench test my sending unit?
Thanks

use an ohm meter.
Positive to the stud for the sending unit wire.
negative to the body of the sending unit. Just touch the gas line tube.

Then move the swing arm through the arc.
12-73 ohms should be the top and bottom readings.