90 fuel gauge fault

My 1986 V8 90 fuel gauge stuck at 3/4, i filled the tank it went to full but stuck there,i replaced the sender unit and when the float is at the top the gauge reads empty and when the float is low it reads 3/4 but thats it even when you move the float by hand its either one or the other, it will not stop anywhere else on the gauge,no matter where you stop the float any ideas.

Probably a faulty gauge

If you remove and connect to 12v, does the gauge show full then?

A dirty connector somewhere in the gauge wiring or as SH says a faulty gauge.

Thanks for that i will get checking these points.

Get a multimeter onto the guage and check which one is the variable, the - and the + and make sure it's wired up right. I had this problem and that's what solved it.