"Senders for gages or for alar
"Senders for gages or for alarms? The only thing NC or NO could refer to is senders that are switches for alarm or warning light circuits. Senders that drive gages are resistive circuits to ground either internally or externally grounded, i.e. one or two contacts on the sender. The hot goes to the gage and depending on temperature/pressure etc. a better or worse connection to ground (the engine block) is made and that drives the gage. An alarm is normally a NO circuit unless the circuit is changed with a relay.
Warning! I have no particular knowlege of Mercruiser gages, i am just explaining how gages and senders work in general.
Ted"