Permanent Check Coolant Level message?

[flaviu]

Hi everyone,
It seems this is a long overdue explanation for how the coolant level sensor works on BMWs...
Everyone seems to agree the sensor is a reed relay that gets triggered by the magnet on the floater,the magnet on the floater causing the reed relay to close, right?!Only partially correct- But then, if the sensor is out of the dry-well or if the connector is disconnected, the "check coolant message" becomes permanent, also if the connector pins are shorted ( with a staple or something else), the message goes away!!! This seems to be "backward " !! So the explanation is simple:
The sensor should be "normal closed"- no check coolant message ! They achieve this, by having a SECOND magnet on the dry well !!!! This causes the reed relay( coolant level sensor) to close AS SOON inserted on the dry well! ( tested by me on an OEM tank). Now when the coolant level drops and the floater gets close to the reed relay, the magnetic field from the magnet on the floater will counter-effect the one on the dry well( sensor enclosure) to open up the relay= check coolant message !
This is how it works ! and this is why many after market expansion tank will throw this "check coolant level" message !- they don't have the second magnet on the dry well to keep the relay closed in normal operation !
OEM expansion tanks can fail also ( probably by this second magnet getting either loose or weak with the time- not being able to keep the relay closed in normal operation= coolant level OK, or by floater magnet falling on the bottom of the tank and counter effect the magnet keeping the sensor closed)
I hope this helps everyone understand once for all how this wicked system works!