My description pertains to a 49 3100, 47 may be similar.
Starting at the tank, the sender is a 0 - 30 ohm variable resistor. The sender is grounded at the tank with one "sender" wire running from the tank to the fuel gauge. The resistance varies depending upon the movement and position of the senders float inside the tank.
The fuel gauge is in effect an ohm meter. The gauges needle deflects from empty to full dependent upon the resistance it reads from the sender.
In order for the gauge to function it needs electrical power and a ground as well as the input from the sender. Generally the gauge grounds through the gauge cluster to the dash.
The gauge has 2 terminals. In my 49 the left terminal (when looking up at it from under the dash) is where the sender wire connects. Mine had a red warning tag on the terminal.
The right terminal is where the 6v electrical lead connects. Power is applied to this terminal when the key is turned on.
If your truck has been converted to 12v that doo-dad of which you speak of is probably a resistor that was wired in series with the power lead to the gauge. Generally you remove the power wire from the gauge and connect it to one side of the resistor then connect the other side of the resistor to the power terminal of the gauge. This knocks the power to the gauge down to around 6v - 8v to work the gauge.
If your 47 is 6v you don't need the resistor and shouldn't wire it in.
I just converted my 49 to 12V and just went through this exercise.
Pic of resistor hooked to fuel gaugeGood luck
Dave
