What would be the cause of the fuel gauges not reading correctly on a 86' Ford F150?

I need to know why my 86 f150 fuel gauge is not reading correctly, I need to know what I need to replace? I know there is a lot of fuel in each tank, and I have already taken out the sending units in both tanks and replaced the strainer filters and pumps.....the fuel is being sent, but just not registering on the gauge? Place let me know how to fix this problem
 
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