Rear window operation
The rear window demister is not working on my wife’s 2000 BMW 320i. There is power to the main feed wire to the relay, power to the blue wire and power to the blue/red wire (about 6V), but I do not think power should be in the last of these as I believe it to be ground. The black wire has no power to feed the grid on the window.
I put in a brand new BMW relay to no effect. I wonder if you can shed any more light on where the fault could lie? Chris Barrett I am assuming that this is a saloon and not the Tourer version. If you do have the Tourer with an opening rear glass, then the problem is often a failure of the wiring within the hinge.
According to the wiring diagram I have, your HRW is connected to an antenna module via four wires, which appear from my data to all be black. The connections to the antenna do suffer from corrosion and this normally results in poor radio reception. I would start by checking the wiring from the antenna unit, which is located in the rear quarter panel behind the trim.
Another common failure on the 320 is the element within the glass, which would involve fitting a new window. In addition to this, the system is operated via a control unit and problems with the screen can be the result of a failed control unit or a faulty alternator, among other things. If there are no physical signs of electrical failure, it may pay to get the system checked by a dedicated BMW system to pinpoint the problem – this could save hours of diagnostic time.