How does a coolant level sensor work?!

[Ziggy]

Mine seems to have stopped working, so I was looking it it the other day & I can't actually see how it works! Maybe it's just something that's broken on mine but it's bugging me all the same! Anyone?

[A1BMW325iSport]

its a float with contacts inside,

the terminals inside are prob corroded hence why yours isn`t working

[Ziggy]

its a float with contacts inside,

the terminals inside are prob corroded hence why yours isn`t working

Just couldn't see any way the float was actually connected to anything! I guess that may have been the problem...!

[A1BMW325iSport]

the connections are at the top inside you can`t see them without taking apart

[Ziggy]

Interesting! Didn't look all that carefully, but all I could see was the black plastic bit which appeared to have no way for the coolant to get inside, and a white floaty ring around it, that didn't appear to be connected to the black bit in any way!

[Brianmoooore]

There's a magnet built in to the float, and a small glass reed switch inside the middle column.
Water gets into the middle and corrodes the reed switch's leads.
There are two sorts of sensor available - one that fits plug upwards for inner wing mounted coolant resevoirs, and one that mounts plug downwards, which fits the resevoirs attached to the radiator. Get the wrong one and your coolant light will come on when the water is fine, and go off when it is empty.

[Ziggy]

Obvious; why didn't I think of that?!

Cheers Brian