1999 Ford Ranger Fuel Sender Not Working (Ford Ranger 1999)

1999 Ford Ranger Flex Fuel -

Fuel Gauge stays on empty unless I unplug the sender unit then it goes beyond full. If I try and ground the yellow wire from the gauge to vehicle ground nothing happens (stays on full). What doesn't make sense is that if it were the guage then why would it move when I plug it back into the sender unit but not move when I ground it to the vehicle. I also noticed that a +4.38 volts is measured between the the yellow wire from the gauge and vehicle ground. I pulled the sender from the vehicle and the float resistance seems to be with in specs.

Another wierd twist is that Ford has a (what I will call a black box) integrated into the wiring that is coming from the sender unit. The yellow wire passes through the black box along with a black-orange wire and a +12 volts. This black box has nothing to do with the Fuel Pump or the Fuel tank pressure senser it seems to only be affecting the sender unit and this black-orange wire.

Ford doesn't even know what the black box is and I can't find it in any schematics I've looked at. I can't even find the black-orange wire in any schematics.

A new fuel pump/sender for this truck is over $600.00 at at this point I'm not sure replacing it will correct the problem.

I'm tired of using mileage calculations to determine how much fuel I have, I'm not a pilot.

Can anyone help, does anyone have a schematic thats accurate. Can anyone identify the black-orange wire.

Thanks in advance.

-ROI