i got a 1990 ford f150 and the gas guage goes from 3/4 to a full tank I unplugged it at the tank and turned the key on and it still stays were the guage is at any thoughts on the problem

 

Single or dual tanks? Which one is doing this? For how long? Do you know the truck's history?

At the tank connector, on the side that goes to the truck harness, jumper the Y/W wire to ground, then turn the key on momentarily and check the gauge.

Then test resistance with a multimeter from the tank side of the connector to ground, fill the tank, & test again.

 

Test tank side and origanly it said 44ohms then put 5 gallons in and it red 87 ohms. Put the the other side to ground and the gas gauge slowly real slowly started to drop I tested the wires from the back to the cab for a complete sircuit before this and to check if there were any high resistences and there was a complete circuit and no high resitances It has a single tank and has been doing this for 3 months or so, don't know the history bought it at an auction and the gas guage worked for 3 months or so then the fuel pump went so I changed it and the gas guarge worked for another couple months then it reads between 3/4 and full. I took the instrument cluster out and the printed circuitboard on the back looks like it has been wet it some places? It is kinda blochy

 

Your test results are all normal. I'd clean the tank connector pins (both sides) and apply silicone grease, and maybe try to tweak them slightly so they make better contact. Then I'd do the same to the tank switch jumper in the dash. If the problem persists, I'd drop the tank & look inside for something interfering with the float arm.