Quote:
Originally Posted by ovekvam
I did actually google how these brake pad sensors work, and found a lot of unreliable information. A lot of people seem to think the sensors have two circuits, are magnetic and other strange ideas. I think all that is wrong.
If you take a look at the connector, it has only two pins in a plastic housing. That means is has ONE circuit. The resistance between the two pins is 0.3 ohm, which is pretty much a short circuit. The sensor does not stick to metal, so it is not magnetic.
Two things can happen to it as the disc starts grinding down the plastic.
1) The circuit can be grounded to the disc.
2) The circuit can be broken.
If it was an analog proximity sensor of some sort, it would have been more expensive, and would have more pins in the connector.
Since the sensor is buried rather deep into the brake pad, there will be no damage to it at all until the material is almost gone. At this point, the pads should definitely be replaced.
Before this happens, the car can only guess based on you driving pattern how much pad life there is left, and it does a very bad job at this, at least with my driving style.
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I have changed brake pads and sensors twice on my E87. In both instances, the pads were worn down enough to trigger the sensor. When its triggered, you should indeed replace it(which I did twice). If its still intact with plastic and all, there is no reason at all to replace it.
I think you are correct in saying that the sensor detects either a short circuit or a broken cirquit. This happens when the pads are worn enough to start waring out the sensor.
As I recall, there was still sufficient brake pad material left after the sensor had triggered the alarm, at least for a couple of 1000 kms normal driving. So I would never replace brake pads earlier, but ofcourse you should know better according to your own driving style.
All in all, I think the sensors are a good way to know when to replace brakepads.
As for the Idrive status of your brake pads, its probably indeed just an estimate based on your mileage and driving style. It goes down gradually on my car, so maybe something isn't 100% working if yours keeps saying 120000km..
By the way, how often do you replace brake disks? Every second time you replace the pads? (Disks should always be replaced together with new pads!)