Quote:
Originally Posted by Chadm
How do you know if a replacement sensor is required?
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As I understand things, the sensor on these cars (E90) is pretty sweet. First, there is one sensor per axle. On the rear axle, the sensor is at the passenger wheel (OK Right Rear, in case that is how Right-Hand Drive versions of the car are made, too). The sensor is really two wires enclosed in a housing that clips to the inner brake pad (see 2nd pic below). The sensor is large enough to protrude into an open slot in the top of the pad. This is key because it allows the sensor housing to become "worn-down" as the pad wears.
The two wires inside the housing are situated such that one wire wears thru when the pads are about 50% gone. The other wire wears thru when the pads are (for all intents and purposes) 100% gone. When the first wire wears thru, the car computer notes the mileage since the last brake pad change and doubles it to figure out when to warn the owner about the brake pads needing replacement.
When the second wire wears thru, the car computer turns on the "BRAKE" light (the same one that indicates the parking brake is on) and it also turns on the red Service lamp that looks like a car on a lift at a service shop.
I think this system is quite ingenious, and I happen to like the way it works.
About two months ago, our 325xi 's computer began showing that our rear brakes would need service in 1600 miles, then 1300 miles, then 1000, maybe 600, 300, and finally zero miles. The zero mark occurred about two weeks ago, but I decided to let it ride until the second wire broke. That occurred about a week after the zero mile mark (maybe 300 miles of driving for us). Today, I replaced the rear brake pads and rotors.
I don't see any reason to replace the pads and not replace the sensor. Well, if one were to do that, it would render the computer portion of the brake check system useless. i.e. If the second wire in the sensor was not worn thru yet, and one put new brake pads on the car, then the sensor would only be good for telling when the pads were 100% worn (i.e. when the second wire did finally break). Of course, if one does not want the luxury of the computer and the function of both wires, replacing the sensor is not necessary. I have heard of people who say the old sensors break when transferring them to the new pads, but I did not experience that.
Here are a couple pictures of the sensor.
Old sensor on the right, new sensor on the left.
Old Sensor installed in the slot in the pad. Notice how the sensor is worn just like the brake pad.
One funny thing about all this is, I had my car in the shop late last year, and the BMW service guy says "the rear pads are getting thin". As if he's trying to sell me a brake job. But, as he should well know, this car has the Condition Based Service computer (which is the purpose of the sensor we are talking about), so there's no reason to mention the brake pads: the car will tell me when she's good and ready to tell me about an upcoming brake job. Also, for what it is worth, the computer thinks our front pads still have about another year's worth of driving. So, although I purchased all the parts for both axles, I won't do the fronts until the computer says the 2nd wire has broken. I look at the thickness when I rotate or change tires, just to be comfortable!