For anyone that cares, here's how the coolant equalization tank level sensor works. (Or at least a partial explanation.)
I recently replaced my coolant tank, and since it's such a PITA to get at I replaced the level sensor at the same time. Unfortunately, as soon as I started the car up, the alarm light beside the temp gauge started flashing slowly. Grr.
I checked the connections to the sensor etc. No go.
I still had the old tank and sensor, so I cut the sensor area out of the tank:
The sensor sits in a tunnel in the tank that is sealed from the coolant itself. The float has a metal/magnetic ring that passes over the sensor. When the float is in it's lowest position (i.e. low coolant), the ring gets to this area on the sensor and makes a circuit:
The exact mechanism of how the sensor is activated I can't explain. Looking at the "blades" on it, intuitively it would make more sense that the ring pulled the them apart rather than together...
To test this I put the old sensor into its position in the old tank mount. I put a meter on the two little prongs coming out of the sensor to measure resistance. With the float in the "full" and "low" positions, I got 0 and 0.7 ohms respectively. Made sense; no circuit with tank full, and a warning circuit with the tank low.
When I put the meter on the new sensor in the car, I got around 0.7 ohms resistance, no matter where the float was. That meant there was a closed circuit all the time, and therefore the warning light.
Out comes that tank (again). Oh joy.