Questions about gas gauge issues

[SCOTI]

Hey guys, I'm trying to sort through my gas gauge issue & I need some info. RECAP: My gauge used to read slightly past full when the tank was filled, but it went down as the tank was emptied. Now, the gauge needle sits @ about the 3-o'clock position all the time (full or empty).

I dropped the tank to check the wiring to the sending unit. Both wires appear solid & had good connections. What is the function of each wire (1-blk & 1-tan)?

For testing, I'm running a hot/12v wire from the batt to my multi-meter. Grounding the meter to the frame shows 12.43v. Thinking the tan wire was the pwr source, I ran the 12v power directly to the terminal the tan wire was connected to. I went to verifiy the voltage on the meter, & it shows zero volts even w/the hot/12v wire connected.

Since I was pretty sure the tan wire was for pwr, I didn't know what to think about the zero volts so I decided to verify the grounds. Grounding the meter to the frame shows 12.43v. When I ground the meter to the black wire of the sending unit, it shows 12+v. I then tried grounding the meter to the terminal the tan wire connects to & it shows 12+v.

One wire appears to be the ground (blk), what is the other (tan) wire? Is it the switched 12v power source or should it be grounded as well?

Please advise quickly because it's getting dark & I need to put this thing back together to drive to work @ 2:30am. Thanks.... Scot