MTM086715A0007 Coolant Level Sensor

I installed the mini tek sensor 086715A0007 to my 400 D aquahot , after splicing in the pigtail and plugging it in the aquahot worked with the low coolant level light out , however when i unplugged the sensor the light did not come on. is this correct . Should the low coolant level light not come on when the sensor is unplugged ? If i take a wire off the pigtail with it unplugged from the sensor the light comes on please advise , i am concerned that the unit will continue to run with the level below sensor Thanks

There's more questions to be asked here to understand this better. This switch is commonly used for a high coolant/water alarm. Not as a low coolant/water alarm