Wires going to the built in gas tank

[DCRthe3rd]

Hi, gas gauge has not been working on the big boat , on top of the gas tank is what I BELIEVE to be the sending unit , it has 3 wires.
Green wire goes to the center of this unit and a red wire and a blue wire go the outer edge of this unit.
I suspect a bad ground.
Can anyone tell or guess what these wires are for?
I am hoping this is a standard enough set up and I can figure it out without going to a boat shop cause I am sure it's more $$$$ than I'd like to spend.
The motor for the boat is a johnson

[Longknife 76]

The center wire is the gauge wire and the outer two wires are ground wires. run another jumper wire to a GOOD ground and to the body or mounting screw of the unit. Turn on ign key, motor does not have to be running. If it works  then you need to clean the ground wires at both ends. If it does not work, get a 12 volt test light or a voltmeter and test the center wire, you should get a dim light, or a low voltage reading, or possibly a pulsating voltage. If you get a reading  I would suspect that the sending unit in the tank is bad. If you get no reading then either the gauge is faulty or a there may be a bad connection in the wireing. If you can get to the back of the gauge you should get a low or pulsating voltage on both terminals, if only one has voltage then the gauge in bad. Hope this isn't too confusing, it is identical to the 12 volt systems in older automobiles.....Ed

[DCRthe3rd]

Thank you , tomorrow I will look into this , a little more background , the gauge stopped working all at once , it worked before I winterized the boat and was not working the next spring when I took it out , the tach also stopped working at the same time "no big loss there" , I was looking for loose wires and on the back of the gas gauge there are alot of wires , I assume some are jumper wires to the speed odometer , etc The speedo works by the way.
Also the gas gauge is pegged full all the time , the tach is pegged below zero