coolant level sensor

[robert g]

Is there a way that you can test the sensor to see if it is working... My radiator is full and sometimes it says that I need coolant and sometimes it doesnt say anything... I took it apart and there was some weird brown stuff inside. I cleaned it out and put it back together.

[DaveVoorhis]

The sensor is easily tested. It contains a magnetic reed switch actuated by a washer-shaped magnet on the float. Test for conductivity across the pins -- with the float in one position, the switch should be open; with the float in the other position, the switch should be closed. The weird brown stuff inside the sensor is probably rust and water, especially if (as sometimes happens) the sensor wiring has been rusting away internally.

[uscharalph]

These are notorious for acting up.

[brosher]

Thanks for the info Dave. Do you have to drain the coolant to test/replace the sensor? It's only a $20 part, might as well just throw a new one in if your getting messages.

[DaveVoorhis]

If the sensor is on the bottom of the radiator-mounted expansion tank (520i/525i) you need to drain the rad and it's much easier if you remove it and change the sensor on the bench.

With the firewall-mounted expansion tank (535i/540i), draining isn't necessary.