fuel gauge

HELP, Please.

 So I had my wiring harness rebuilt,(thank you midlife) everything works except my fuel gauge and it worked before so thinking it's not the sending unit,. I tried 4 diffrent things the 9v plugs into but the gauge goes to full then back to empty. 11 ohms on the gauge, 15 on the tank which seemes right, it's almost full. Yellow white wire out of main harness and same going to the gauge. What am I missing?

With the key in ACC, take the connector off of the tank and briefly ground the sending unit line. Does the gauge peg high? If so, everything short of the sending unit is working as it should.

Do not leave the sending unit line grounded for any length of time, as you'll burn out the gauge.