Fuel gauge still stuck after changing fuel sender unit in 96 e300

[charlie a.]

I just changed the fuel level sender unit in my 96 e300 but the fuel gauge is still stuck at full. The needle goes down when you turn the ignition key off and whenever i pull the connector out of the unit itself. However it goes back to full when you turn the ignition on or when I replace the sender unit connector back on. Any ideas how to check this problem?