1991 Yami LCD Multi function gauge- Fuel reading

[Fishhawk'r]

I have a 1991 boat with Yamaha LCD gauges.
The fuel sender unit has a mechanical to electrical conversion tab with a needle gauge on the top of the sending unit to which the wires to the dash mounted fuel gauge are attached.
The dash gauge was reads 1/4 tank when I have a full tank, so I replaced the mechanical to electrical conversion tab which gave me a correct reading at the sending unit gauge, but not at the dash mounted gauge.
The wires FROM the dash gauge are Gr/Rd/Bl and the black lead has a dual female connector from which there is another short lead with a circlre connector.When I connect this short Bl lead to the battery Neg term I get a more accurate reading on the dash, but still not correct.
What should that short black lead be connected to and why aren't I getting matching readings at the tank and on the dash ?

This is driving me nuts!!

Thanks for any help

[boats.net]

All black(ground wire) wires on gauge need to connected to black wire in gauge harness going to key switch, also the fuel tank sender needs to have ground wire connected to it.