[66AC]

I would get some very hot water (previously boiling) in a small pot, and immerse the sending unit into the hot water with the ignition of the car ON but not running to test the sending unit. This of course assumes all your electrical connections are correct and good.

Also, there probably is a way to read the impedance coming from the sending unit with a multi-meter. Perhaps you can find information on that from Smiths or the web? I have SW gauges and there is published data for SW's water temp gauge as I recall.