85 gt fuel guage problem

Did a stroker build on the ol.85 gt..swapped in a1990 gas tank and a 1990 sending unit with a quantum fuel pump cause the 85 gt did not have the fuel pump in tank..anyways the fuel guage will not read right.. the float arm read backwards from stock, empty was full and full was empty..anyways took the one yellow wire off and sodered to other end it started readed right by hand up and down with float..manually push foat up and down it works right now but does not read right at all when it's in gas floating in tank..my electrical dude did an ohms test on it and put an adjustable ohm thing on it and got ohms to match pretty good but still doesn't read right..any answers on the fix for this would be appreciated..gas guage worked fine stock until 1990 sending unit and tank

AeroCoupe said:

You will need a 83-86 sending unit for a CFI/EFI tank which will fit the tank you have. The sending units are different than the 87-93 and as you experienced read backwards.

From LMR’s website:

Ohm Reading Notes

• 1979-1986 Mustang Fuel Level Sending Unit Ohm Readings are approximately 73 ohms empty and approximately 10 ohms full.
• 1987-1997 Mustang Fuel Level Sending Unit Ohm Readings are approximately 22 ohms empty and approximately 145 ohms full.

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Thank you ....