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General E46 Forum
This is the place to get answers, opinions and everything you need related to your E46 (sedan, coupe, convertible and wagon) BMW!

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Old 07-30-2009, 11:12 AM   #1
SweTurbo
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Brake pad wear sensor

How does this thing work ?

First time i saw it it was just looped next to the brake caliper...

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Old 07-30-2009, 01:34 PM   #2
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It is supposed to be clipped into the inboard brake pad. If you look at your pad there will be a notch out of it where the sensor is supposed to be clipped into it.
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Old 07-30-2009, 01:56 PM   #3
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that thing is useless.
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Old 07-30-2009, 01:59 PM   #4
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that thing is useless.
agreed, very fickle when it comes to working accurately

edit: as for how it works, i have no idea sorry.
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Old 07-30-2009, 02:01 PM   #5
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agreed, very fickle when it comes to working accurately

edit: as for how it works, i have no idea sorry.
As the pad wears the sensor comes close and closer into contact with the rotor surface. Once wears to a specified thickness it causes the sensor to be tripped and burns out the circuit causing the brake light to illuminate.
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Old 07-30-2009, 02:03 PM   #6
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yarrr matey
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Old 07-30-2009, 02:34 PM   #7
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As the pad wears the sensor comes close and closer into contact with the rotor surface. Once wears to a specified thickness it causes the sensor to be tripped and burns out the circuit causing the brake light to illuminate.
supposedly yes, but that didn't happen to me and it's pretty dumb how there's on two sensors instead of 4 because one of my other pads were worn much more than the other side and it didn't sense it.
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Old 07-30-2009, 02:36 PM   #8
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supposedly yes, but that didn't happen to me and it's pretty dumb how there's on two sensors instead of 4 because one of my other pads were worn much more than the other side and it didn't sense it.
yeah, it's pretty hit or miss. my pads were really low when I had them changed earlier this month, probably a day or two away from having the light come on. glad I didn't have to replace the sacrificial sensors, even though they're cheap
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Old 07-30-2009, 02:41 PM   #9
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My fronts started going metal on metal a day after the sensor went. The rears went on for three weeks and when I changed the pads, it were not even close to metal.

It is recommended that you change the sensors when you change the pad (even if the sensors haven't worn through), but truth be told, if it's not 'tripped' there is nothing wrong with using the same sensor for the next set of pads.
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Old 08-03-2009, 02:02 PM   #10
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today i changed my front. the light came on. the right side still had a good amount of meat on the them but the left were almost metal to metal. Any signs of the left caliper going bad?
Also, i forgot how to remove the light on the dash? or is it suppose to go away by itself? I done the rear last yr but i forgot.
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Old 08-03-2009, 02:17 PM   #11
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today i changed my front. the light came on. the right side still had a good amount of meat on the them but the left were almost metal to metal. Any signs of the left caliper going bad?
Also, i forgot how to remove the light on the dash? or is it suppose to go away by itself? I done the rear last yr but i forgot.
Your left caliper might need to be rebuilt if it's coming into more contact than the right side.

To reset the sensor turn the key to position 2 and let it sit. The light will then go out.
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Old 08-03-2009, 02:23 PM   #12
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Key on or Acc?
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Old 08-03-2009, 02:28 PM   #13
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Key on or Acc?
Accessory
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Old 08-03-2009, 02:32 PM   #14
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Thanks, ill give it a try later
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Old 08-03-2009, 02:45 PM   #15
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How does this thing work ?

First time i saw it it was just looped next to the brake caliper...

Thanks // Pirate
I test drove a car where the sensor was disconnected. The idea was that the owner or car dealer was trying to avoid a brake light on the dash by avoiding having the sensor contact wear away. Until the sensor wears past the 3 mm point, the sensor is a closed electric circuit.
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Old 08-03-2009, 03:04 PM   #16
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How does this thing work ?

First time i saw it it was just looped next to the brake caliper...

Thanks // Pirate
Looped because someone didn't like having to deal with it so they completed the circuit by looping the wires.
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Old 08-03-2009, 03:43 PM   #17
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Originally Posted by ChrisTKHarris View Post
As the pad wears the sensor comes close and closer into contact with the rotor surface. Once wears to a specified thickness it causes the sensor to be tripped and burns out the circuit causing the brake light to illuminate.
I thought it was the other way around . . .that it BREAKS the circuit when tripped.

That is why you can just splice the two wires together to get rid of the dash light? (I've never tried this but supposedly someone on here says it works).
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Old 08-03-2009, 07:18 PM   #18
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I thought it was the other way around . . .that it BREAKS the circuit when tripped.

That is why you can just splice the two wires together to get rid of the dash light? (I've never tried this but supposedly someone on here says it works).
Yes it turns to an open circuit when the rotor comes into contact with the sensor and causes it to short out.
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Old 08-03-2009, 09:13 PM   #19
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Yes it turns to an open circuit when the rotor comes into contact with the sensor and causes it to short out.
Yes it turns to an open circuit when the rotor comes into contact with the sensor and causes its close loop circuit to open (not to short out).

The sensor mounted on the center of the brake pad will make contact with the rotor once the pad wears down to certain minimum thickness, then the rotor will grind away the continuing wire loop of the brake sensor resulting in open circuit or open loop, thus caused the brake pad warning light on the console to go off, (close loop = no warning light, open loop = warning light).

There are 2 brake sensors, one is on the front driver side and the other is on the rear passenger side. If the sensor has not yet tripped then it can be reused. I reused the rear sensor with no problem when I replaced the rear brake pads few months ago; however I had to replace the front one since it had tripped.
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Old 08-03-2009, 09:25 PM   #20
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The sensors worked perfectly on my car. All pads were worn off evenly when the the light came on. It can take about 1000 kms more from the light to get the pads changed. however this depends on why your pads wore off. Some of you seem to have had a caliper piston seizure which means that particular pads will wear off faster. Make sure you / mechanic checks the pistons when replacing pads. Clean all rust and use anti seize on all moving parts except the face of the pad and rotor obviously
Check regularly for uneven wear of pads
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