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      11-09-2013, 08:03 AM   #1
ovekvam
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Replaced brake pads

Our 116iM has been driven quite a bit on race tracks, so the brake pads were ready for replacement up front at around 24000 km. At the back, more than 50 percent pad material is still left.

The iDrive says I have 2 200 km left in the rear, and 120 000 (!) km up front, so the car really doesn't have a clue.

It was rather easy to swap the pads. On my previous BMWs I have used a 7 mm allen bit, but BMW have changed this to 8mm. Luckily my 8 mm allen key was enough to do the job. I didn't need the ratchet.

It was possible to take the caliper spring on and off with just my fingers, and I could also push back the piston without tools, so the whole job was done very quickly. If I had larger wheels (18"), I think I could have done it with the car standing on the wheels.

After I reset the service counter for the front brakes, it still said 120 000 km left. My dealer said it was abolutely necessary to replace the brake pad sensor, but I doubt this advice, so I just swapped over the old one to the new pad.
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      11-09-2013, 03:05 PM   #2
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I have heard from several sources that the sensors should indeed be replaced with the pads. The fact that the service prediction hasn't changed would suggest the sensors have failed already.

But if you are comfortable enough to know what you can expect from the pads anyway as I suspect you are then maybe the sensors can be ignored quite easily.
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      11-09-2013, 03:10 PM   #3
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Quote:
Originally Posted by Bob UK View Post
I have heard from several sources that the sensors should indeed be replaced with the pads. The fact that the service prediction hasn't changed would suggest the sensors have failed already.

But if you are comfortable enough to know what you can expect from the pads anyway as I suspect you are then maybe the sensors can be ignored quite easily.
I guess the only thing these sensors can do, is measure if the pads are totally finished, which they aren't. I think the front pads have been listed at 120 000 km since the car was new, so these indications are rather useless anyway. I only trust regular visual inspections.
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      11-09-2013, 03:22 PM   #4
Bob UK
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Originally Posted by ovekvam View Post
I guess the only thing these sensors can do, is measure if the pads are totally finished, which they aren't. I think the front pads have been listed at 120 000 km since the car was new, so these indications are rather useless anyway. I only trust regular visual inspections.
Good policy imo, I'd have to do some Googling to find out how the sensors actually work tbh but if the counter talks to the ecu and reduces there may be a time component as well. I can't see them measuring the pad depth periodically with something like an I/R pulse but I suppose it is a possibility.
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      11-09-2013, 03:52 PM   #5
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The pad sensor is a piece of plastic with a metal strip in it. All it can do, is measure a short circuit (to the brake disc) or a broken strip (by the brake disc). For this to happen, the pad must be worn down almost to the back plate. As long as the plastic housing is not damaged, I think it is safe to reuse it. But I don't know for sure.

In some previous cars, I have replaced the pad sensors with a short circuit link directly in the connector, just to avoid warning lights.
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      11-09-2013, 04:59 PM   #6
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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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      11-10-2013, 03:56 AM   #7
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Seems we can deduce a top tip from your investigations then :-

If BMW says the pads need changing, have a look before putting your hand in your pocket.
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      11-10-2013, 12:42 PM   #8
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Quote:
Originally Posted by ovekvam View Post
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.
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!)

Last edited by Math; 11-10-2013 at 12:48 PM..
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      11-10-2013, 03:49 PM   #9
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Quote:
Originally Posted by Math View Post
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.
But you have 8 brake pads, and only two sensors. If a caliper gets slightly stuck, a pad could be down to the metal before the one with the sensor is triggered. I also noticed that a half worn pad will fade faster than a new one. I think the heat capacity is simply too low due to reduced mass and thickness. That is why I don't like to wear the pads 100 percent down.

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Originally Posted by Math View Post
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!)
I don't know yet for the F20, but on earlier cars I have had 2-3 sets of pads per disc.
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      11-11-2013, 06:14 AM   #10
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Quote:
Originally Posted by ovekvam View Post
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.
BMW data for typical BMW OEM wear pad sensors as used for CBS braking, illustrates them as having a dual circuit (two conductor loops) within the sensor. Therefore three values available across the pad wear. A 'new pad' resistance, reading another value at the 1st stage, and a another value for the 2nd stage.

The example given is:

Quote:
As a result, the electrical resistance at the connections of the sensor changes with the degree of wear:
• < 5 Ohms for new brake pads
• 470 Ohms on reaching wear stage 1
• >100 k Ohms on reaching wear stage 2
The resistance is measured by the relevant control unit and communicated to the corresponding driving dynamics system via the bus system. Here the wear stage is evaluated for the purpose of activating warning indicators and for the "condition based service" function.
Think about it, how else can we have CBS for pad wear? There has to be a baseline and some kind of wear 'map'.

The simple single stage wear sensors are only able to give a worn-out signal.

Both single and dual stage wear sensors need replacing at pad change, if you want 100% communication to the ECU.

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      11-11-2013, 09:28 AM   #11
ovekvam
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Quote:
Originally Posted by HighlandPete View Post
Think about it, how else can we have CBS for pad wear? There has to be a baseline and some kind of wear 'map'.

Both single and dual stage wear sensors need replacing at pad change, if you want 100% communication to the ECU.
Ah, I understand how it is wired internally, then. A short circuit on top, and a resister underneath. The whole sensor is however rather flat, so the pads will have to be worn far down to trigger the top wire. I will usually replace the pads before it comes to that point.

The service indicator counts down before the sensor is triggered, so it must be based on how hard and frequently we are braking.
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      11-11-2013, 10:40 AM   #12
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Quote:
Originally Posted by ovekvam View Post
Ah, I understand how it is wired internally, then. A short circuit on top, and a resister underneath. The whole sensor is however rather flat, so the pads will have to be worn far down to trigger the top wire. I will usually replace the pads before it comes to that point.

The service indicator counts down before the sensor is triggered, so it must be based on how hard and frequently we are braking.
I've tried to sort this one out in my mind, how it can countdown without contact for a good few miles. I come to a similar conclusion, there must be a countdown (which can get well out of phase in reality on some cars) and only when we trigger and break the sensor circuits do we really get to an accurate (if it is that) wear rate. I suppose it is only the last part of use and countdown that really needs to be accurate, but can cause panics along the way, it appears, with false countdowns.

I know for my car the rear pad rate is now reversing, I've gained 2k miles in the last 5k miles, gone up from 14k to 16k miles. I'll be maeasuring my pad thickness when I swap over to winter wheels in the next few days. I wonder if my rear pads have now worn to the first break in the sensor circuit and it has recalculated the mileage.

So you are changing pads well before the first contact with the brake disc. What have you got left, over 6mm pad thickness?

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      11-11-2013, 10:49 AM   #13
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I would estimate that my front pads were about 70 percent worn down, and the sensor was still not touched (getting close, though). Maybe 3-4 mm left of the pads. I didn't actually measure them, so take my figures with a grain of salt.

The sensor sits very deep. A part of it is actually on the other side of the back plate.
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