I have 1999 Ranger 2WD 3.0L MFI FFV OHV 6cyl with a couple problems that runs great otherwise.

Out of those two problems, one is currently being fixed but the other has stumped me. My fuel gauge always reads at full, or slightly above while the truck is on, but when I shut it off the needle slowly descends to read 3/4 of a tank and holds it's spot there, right on the line.

Any tips or guidance would be appreciated, thanks in advance.

 

Reads like the wire from the sender(float in the gas tank) to the instrument cluster(gauge) is disconnected

When key is OFF the needle can go anywhere, not a "sign" of anything so not important

Gauge uses OHMS to see fuel level in tank
16 ohms is EMPTY
160 ohms is FULL

OHM is resistance in a wire or device
0 ohms is a dead short, direct connection
1,000,000 ohms is no connection at all

so if the sender wire or its ground was disconnected that would be above 160 ohms so gauge would read FULL with key ON

You can start at the gauge end of the wire or the gas tank end of the wire
I would start at the gauge end myself
Get an OHM Meter
Pull out the instrument cluster
There is a Yellow/white stripe wire on pin 12 of center connector
Unplug connector and test yellow wire OHMs to ground, should be between 16 and 160 ohms, if not then wire is disconnected or corroded somewhere else
while meter is connected, rock the truck side to side or forward and back a little, gas in the tank should start to slosh around causing the float to go up and down and OHMS will also go up and down, if so then sender and wire are good, gauge or cluster is bad

 

Very excellent advice l, thank you very much. I will take a crack at it tomorrow when I'm off work and let everyone know of the results.