On a 2.5l engine look at the front top of engine, you will see the TWO temp units
ECT(engine coolant temp) sensor, 5volts only used by computer
ECT sender, 12volts only used by dash board temp gauge
So these two are NOT interchangeable
Picture here:
https://cimg5.ibsrv.net/gimg/www.ranger-forums.com-vbulletin/2000x1504/cbb5b012_f644_4eb6_b534_701e83534f6a_54c7b58714af8fcc11328766ff71b01143a73505.jpeg
Red/white wire is for the gauge, sender
Grey/red is for computer, sensor
Unplug the red/white wire connector and then GROUND red/white wire with a jumper wire to battery negative
Turn on the key
Temp gauge should go to HOT, all the way up, this means wire and gauge are OK
THEN use the same wire and connect red/white and black wire on same connector
Turn on key, should see same HOT from temp gauge, that means black ground wire on sender is OK
If all wires test OK then replace SENDER
You have to be careful when ordering these because ECT sensor is more popular and will not work for this, lol
So it must be TEMP SENDER not sensor
Fuel gauge stuck on full can be a few things
Corroded or disconnected wire at the tank connection
Bad anti-slosh module on cluster
Bad gauge in cluster
If you pull out the cluster you can check the tank wire
Its the only yellow/white wire on cluster connector
Test it with OHM meter
1989 and up Fords use 16ohms EMPTY, 160 ohms FULL
If wire is corroded or disconnected it will be higher than 160 ohms so will always show FULL
If wire and sender in tank are OK then you will see between 16 to 160 ohms and if you rock the truck, the gas in the tank will slosh around making float/sender change so ohms will change as well
If that happens then wire and sender are OK
On the back of the cluster is a separate module that pulls out, that's the anti-slosh module and a known issue on Fords
Point of the module is to prevent gas gauge needle from going up and down every time you go around a corner, as the gas "sloshes" around in the tank, hence the name
You can google: Ford anti slosh modules
Loads of info on fixing, replacing or by-passing these