Fuel Sending Unit problem?

I have Honda Accord Ex 1994. Since two months ago, my fuel gauge did not work properly. I diagnose my fuel sending unit using Digital Multimeter, it give an proper resistance reading. When I play with the float outside of the oil tank, it give an accurate reading too. But when I put it back inside the fuel tank, and start my engine, the fuel meter gauge immediately drop to below the symbol "E".

I don't think there are something wrong with the wiring, and the Sending Unit seem alright too if it was test outside the fuel tank.

What are the problem with my sending unit?