Are there any letters beside the post on the gauge? You need to pay more attention to them that the color wires. Someone may have just used what ever color wire they had at the time. What brand gauge is it? Teleflex, Faria?
One of them should be marked with an I. That is the one you want to put on a switch. It is your the ignition terminal or wire. If wired correctly with the color code it should be purple if I recall correctly. That doesn't mean the purple is 100% the wire you need to put on a switch. Like I said someone may have goofed the color on the wires. It happens.
I hope this helps.
The pink goes to the sending unit and it should be marked on the gage. The black is ground You can break it at the switch. The purple wire is the power on wire when the switch is on. I have a schematic some where I'll see if I can locate it and post it for yea.
so if i get this right power dosnt go to the sending unit ,purple to the switch,then to 12v+ black to neg- ,then pink straight from sending unit to gauge ,where does the black go off the sending unit to neg-on the battery?
The purple is hot off the switch. The black on the gage goes to the sending unit ground or you can ground it good under the consol and be sure to ground the sending unit to the neg on the battery. Just hold off today I have that drawing somewhere and I'll post it for you.
Maybe this will clear things up. http://www.faria-instruments.com/downloads.php
Scroll down on the link to fuel level gauge.
Put your switch on the ignition wire terminal.
I would only recommend any boat ground be tot he battery. You can get battery ground under the dash if available.
I hope this helps,