For a while my 1990 gauge was stuck at empty then one day I filled it up and it registered full but now is stuck on full. Is the idiot fuel light sensed from a differant source then the gauge? When it was stuck on empty it did not come on.

 

I'm really not too sure because when this happened on my '95 the dumb dumb light came on even tho the gauge was not working, but I don't know if it still does cus I don't let it go that low anymore

 

try dumping some fuel cleaner in your tank maybe it is sticky with old gas

 

you may have to change the fuel level sensor on the submerged fuel pump assembly in the tank,
but before doing that with its cost,
try three bottles of
Chevron Techron Concentrate Plus (the Plus is important)
in your next 3 tank fulls of fuel

the Plus version of Techron has a special cleaner that sometimes can remove sulfur deposits of the silver alloy strip of the fuel level sensor

but when I changed out my 1995 fuel pump assembly
I found the ceramic backing behind the fuel level sensor strip
had cracked in two

 

You said it wasnt working before but said it was always on empty. Now it stays on full. That is a new one on me, but, based on everything I have read in this forum as well as my own gas gauge issues, I would suggest you run her out of fuel, drop the tank and clean everything out, then make damn sure you position the fuel pump correctly so as not to cause the sending unit to stick, and mount the tank most of the way up.

Put about 5 gallons of fuel in and check the gauge, if not working check your positioning again, mount up again, check again, if still not working you will need to replace the sending unit or the entire pump. My guss is though that if you get it positioned correctly all will be well.

The Dakota I bought had a fuel pump put in by a "professional" just before I bought it. Gauge was cockeyed and didnt work, I realigned it and all is well now.