Fuel gauge issues.
 

I just purchased a 1990 GMC 1500, extended cab , longbed , 5.7 ltr.
I'm having trouble with the gas gauge.
It only wants to show between empty and 1\4 tank.I have already tested the gauge in the dash and believe it to be good. When I ground the purple wire to the sending unit ,the gauge shows empty and open it shows full. When I hook it back up it only shows a quarter tank, with approximately 20 gallons in the tank. I replace the sending unit today with a brand new 0 to 90 ohm unit, and it still only shows a quarter tank.
The voltage on the purple wire to the sending unit only has 6.3 volts on it , and I was expecting 12. I don't know if 6 volts is correct or not
Whatever the problem is, the sweep on the gas gauge isn't matching the sweep on the sending unit. Like its been divided by two, pointing back to the 6 volts.

Any suggestions?

I had similar problem but replacing the sending unit solved it. I used a Spectra branded thing from rockauto. This is too late, of course, if you didn’t already do it but did you check the gage moved full range before putting sending unit into tank by manually moving float? I’m curious if you checked it and it worked before you put it back and but now it’s not working.
The other thing I noticed when I did mine was I had to rotate the filter on the pump pickup from the way I first put it together to keep the float from catching on it. Again, not something easy to check now...

Yeah the new sending unit I returned was a spectra brand. It did the same thing that the original was doing. Only showed a little over quarter tank at the full position. The original I check with an ohm meter and came up with 0- 110 ohm.

Not positive but it sounds like the float connecting to the fuel pump broke/came off. Those things are really fragile and I broke mine while pulling the fuel pump out. If your feeling ambitious drop the tank and pull it out, if the rod connecting the float to the pump is broken off of the pump, you might be able to place it back and wrap a thin piece of fence wire around the rod and the plastic it was connected to. Dont use tape or zipties the gas will eat through it. If it's the original pump Its on borrowed time anyway, might be better to replace it.