There are two types of senders: those that drive a gauge, and those that drive an alarm. tommays is correct about the behavior of a sender that drives a gauge. In many cases, a sender that drives an alarm is mounted on the ground side of the alarm circuit; in case of an overheating situation, the switch inside the sender closes to complete the circuit to ground.
I have tested the alarm on my boat by touching the wire to ground, as you suggested.
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