I have been testing this breadboard of a coolant level alarm and it works great. The sensor is from a VW. As you can see, when in contact with the coolant, it is silent. When not in contact, the piezo buzzer sounds.
I believe the VW type sensor will be much more reliable than the float/reed switch level sensor on the W126 coolant expansion tanks. Mine has a buzzer which you can't miss and will nag you till you correct the coolant level.
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