Are you talking about the wires at the sender? Only the middle two are used. From there the signal goes to the PCM where it is finally sent to the Gauge cluster in your dash. The PCM reads the resistance and changes it into a data signal for the gauge to use. If you tested your sender in your hand it could still be bad. It would be real easy to hold it in such a way as to let the sliding resistor make contact and then not make contact once installed.
Hope this helps.

 

then how can you make the gauge swing from full to empty? if in fact the gauge and or wiring was bad you should NOT be able to control the gauge at all from the tanks sending unit plug.

 

My gauge goes the full range empty to full when you turn key on and off. The gauge hardley drops down and does not show much past 3/4 full when the tank is almost empty. When you use the high volume pumps, does that do a number on the sending unit?

 

Not sure about this but it could possibly be your pcm needs to be reflashed. It controls the signals going to the instrument cluster.

 

it will be fun getting a reflash at a dealer...with the pump wire that was tapped before i bought it, and the blakc smoke it puffs...and the straight 4" exhaust, and intake...they will never think the truck has been tampered with