How does the coolant level sensor *actually* work?
 

Just building up the waterpump housing for my motor, and was fitting the coolant level sensor. Was having a think about how it actually works... It doesn't look like anything is actually going on in the 'sensor' itself. Measured the resistance from the connector terminal to the tip, and its essentially zero... Dunked it in water, still zero. Dunked it in really cold water, still zero, dunked it in freshly boiled water from the jug, still zero... Tested the brand new sensor I have too, same result.

It's got to just be a way for whatever its wired to (ECU? CPU #2? Dash?) to measure the resistance between this sensor and the aluminium housing, via the coolant, right? This assumes the coolant will be conductive to some degree (probably a pretty safe bet, even if you put distilled water in there, there will be contaminants that will make it somewhat conductive). When whatever is reading this sensor sees it go open circuit, because the coolant is down below the level of the sensor tip, coolant alarm goes off?

Would explain why its plastic too I suppose, needs the tip to be insulated from the housing. Wouldn't be too hard to make a replacement though, something with a metal thread maybe, gives me the heeby jeebies screwing that thing in.

Anyone cut one apart and had a look inside?

https://cimg1.ibsrv.net/gimg/www.rx7...8769483030.jpg

Electrical resistance.

Yes, as said, measures resistance ( or conductance if you prefer to look at it that way). It’s processed at the dash on the speedometer board by a comparator that triggers the warning. There are other functions that share the buzzer.

Aces, figured that must be the case, means when the wire breaks off I'll be able to fab up a nicer replacement pretty easily :-).

When the car is filled with coolant it creates a path to ground from the tip of the sensor to the water and to the engine block. Coolant goes down, the ground is interrupted, circuit is broken and warning goes off.

It's a VERY simple sensor, just grounds to the engine through the coolant, that's it. It's generally reliable save for damaged wiring.

Dale