"Sending unit is a variable re
"Sending unit is a variable resistor. ~ 80 - 240 ohms.
Do NOT hook the battery + lead to the S terminal.
The S termnal is the connection to the sending unit.
Battery power is only to power the guage.
Connect a ohm meter, one lead to the center terminal on the sending unit and the other lead to the threads.
What ohm reading do you get?
Next put a small pot of water n the stove and submerg the sending unit ( just the end portion of the sending unit that makes contact with water) and turn the heat on the pot.
watch the ohm meter. the reading should change as the temp increases. Waych carefully to see if it is gradual. if it jumps a great deal from say 180 ohms to 100 ohms with a very small change in water temp. the sender is bad. If the ohms change steady and gradually then the gauge is bad.
Of course you have checked the wiring in the boat first????"