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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Mechanical Maintenance: Break-in / Oil & Fluids / Servicing / Warranty > How accurate are the brake sensors?



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      09-23-2012, 08:54 PM   #1
nycbr
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How accurate are the brake sensors?

The computer is telling me I have 3K left on the rear brakes. Do I need to change them now or can I wait for a warning light? I don't want to take a chance on scoring the rotors.

Thx

PS - why do the rear brakes wear out first? And any recommendations on pads with stock rotors? Thx again
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      09-24-2012, 08:55 AM   #2
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Originally Posted by nycbr View Post
The computer is telling me I have 3K left on the rear brakes. Do I need to change them now or can I wait for a warning light? I don't want to take a chance on scoring the rotors.

Thx

PS - why do the rear brakes wear out first? And any recommendations on pads with stock rotors? Thx again
The 3K left is just an estimate based on how long it took you to to trip the first stage of the sensor. Since you have no warning light, I'm guessing you haven't tripped the 2nd stage yet. The only way to really answer that is to have the pads inspected (for remaining mm)

Rears go first because DSC uses them actively to keep you safely on the road.
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      09-27-2012, 08:35 PM   #3
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It is pretty accurate. I would wait until 1k to change unless you want to change it now while the weather is not bad instead of changing them in the dead of winter.
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      09-27-2012, 08:40 PM   #4
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they arent very accurate actually...
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      09-27-2012, 08:59 PM   #5
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Relying on the sensor reading, I always had plenty of meat left on my pads, but it looks like people had different experiences. YMMV
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      09-27-2012, 11:03 PM   #6
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Mine had plenty left with both the CBS and wear sensor tripped. Once you get to that stage you'll want to replace them weather they are actually worn out or not due to how annoying all the warnings are. If I were you I'd replace them when it gets down to a few hundred miles. Any sooner is probably a waste and any later and the car will go nuts complaining.
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      09-29-2012, 08:38 AM   #7
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Okay, I'll explain how the system works. First off the wear sensors are pretty accurate near the end of the pad life if the car is driven in a consistant manner (i.e. driven to work the same route each day). The brake pad wear system works in two stages. When the pads are new and the system is reset, the CBS computer uses a standard calculation to estimate the remaining pad life. The wear sensors have two stages (two loops of wire) that trigger the system to adjust the pad life estimate once the "first stage" of wear is determined, meaning the first loop of wire breaks in the sensor.

The computer assumes the car is being operated in the same manner as it was during the first amount of miles it took to break the stage one wire loop and adjusts the pad life estimate accordingly. Once the second stage loop of wire breaks, the brake pad replacement light is triggered in the gauge cluster and remains on until such time the pads and sensor are replaced. Once the pad wear replacement light illuminates, you have a few thousand miles to replace the pads. They have a few millimeters of pad material remaining.

You can safely wait until the brake pad replacement indication shows and then get the pads replaced within the next thousand miles. With BMW's system all you really need to do is wait for the replacement indicator to light and then get the pads changed. Don't sweat it.
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      10-02-2012, 10:57 PM   #8
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Okay, I'll explain how the system works. First off the wear sensors are pretty accurate near the end of the pad life if the car is driven in a consistant manner (i.e. driven to work the same route each day). The brake pad wear system works in two stages. When the pads are new and the system is reset, the CBS computer uses a standard calculation to estimate the remaining pad life. The wear sensors have two stages (two loops of wire) that trigger the system to adjust the pad life estimate once the "first stage" of wear is determined, meaning the first loop of wire breaks in the sensor.

The computer assumes the car is being operated in the same manner as it was during the first amount of miles it took to break the stage one wire loop and adjusts the pad life estimate accordingly. Once the second stage loop of wire breaks, the brake pad replacement light is triggered in the gauge cluster and remains on until such time the pads and sensor are replaced. Once the pad wear replacement light illuminates, you have a few thousand miles to replace the pads. They have a few millimeters of pad material remaining.

You can safely wait until the brake pad replacement indication shows and then get the pads replaced within the next thousand miles. With BMW's system all you really need to do is wait for the replacement indicator to light and then get the pads changed. Don't sweat it.
Thank you for explaining this
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