I changed the coolant sensor and to my delight it was defective.
I hate to spend money for nothing.
The original was corroded inside and the reed relay was detached. It was an original part from 1992 by TRW. As you can see from the attached photo the corrosion entered the area where the reed relay is and ate the connections hence no circuit. If yours is bad change it....
I ordered mine through my local autoparts store, who buys from altrom.com
and they are OEM suppliers although they have their own part numbers.