# Are brake sensors going off early? 6mm left



## TomNJ (Jan 14, 2016)

I posted this in the f25 forum but didn't get any replies. This is more general question about the brake sensors not the x3.

Replaced my front brakes and rotors recently. The red brake light came on couple of weeks ago but I've been getting a mileage countdown for months indicating I should expect it last month. I drove about 200 miles with the light on. F25 series x3 xdrive28i

I thought I read the light doesn't come on until 2.4mm. I measured each pad and they still had about 6mm of thickness left. 

Hard to tell from the pics but top of wear sensor was about flush with pad surface.

The way these sensors worked I thought when the first section wore down it triggers the mileage countdown. After the next one wears down then the break light comes on. It seems very wasteful.

I wasn't the owner the last time front brakes were changed (not on Carfax) so can't say exactly when they were done. Pads were Textar, couldn't make out info on sensor. Is this normal? If I didn't already order parts and want to get the windshield washer pump replaced I probably would have waited a lot more.


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## marcozandrini (Feb 13, 2014)

Your pix didn’t come through on my iPhone. The pad wear sensors are nothing more than a piece of wire. The insulation wears off when the pad reaches a certain thickness. The wire is then grounded by the rotor, triggering the indicator. The sensor’s thickness is likely model and year specific. So to say at what friction material thickness the indicator is triggered is hard to say.


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## CliffT (Apr 20, 2019)

If my job was to sell parts and service opportunities for BMW, I would have the wear indicators come on at half life.


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## TomNJ (Jan 14, 2016)

marcozandrini said:


> Your pix didn***8217;t come through on my iPhone. The pad wear sensors are nothing more than a piece of wire. The insulation wears off when the pad reaches a certain thickness. The wire is then grounded by the rotor, triggering the indicator. The sensor***8217;s thickness is likely model and year specific. So to say at what friction material thickness the indicator is triggered is hard to say.


Let me try again.



CliffT said:


> If my job was to sell parts and service opportunities for BMW, I would have the wear indicators come on at half life.


That's why I'm asking.


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## heyheyitsmarc (Apr 3, 2017)

The sensors work with your computer to analyze breaking habits. When you install new pads, you install a new sensor and reset the service indicator using the instrument cluster. Your vehicle will keep track of how many miles you drive before breaking the first wire embedded in the sensor. Your computer then calculates what it believes to be the remaining life based on the distance it took you to break wire 1. Once you break wire 2, the computer further refines the estimate. The estimate is pretty close, usually. A problem that can come up when buying a used vehicle is the driving habits change. The vehicle doesn't "know it's been sold" and can't adjust the estimate accordingly. After you change the breaks and do a reset, it will become more accurate to your driving style. It's all guesswork by the computer and no actual thickness measurements are taken. My CPO X3 has been showing 4500 miles left on the front breaks since my last oil change, which is due again in 2 weeks....


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## imtjm (Oct 5, 2004)

heyheyitsmarc said:


> The sensors work with your computer to analyze breaking habits. When you install new pads, you install a new sensor and reset the service indicator using the instrument cluster. Your vehicle will keep track of how many miles you drive before breaking the first wire embedded in the sensor. Your computer then calculates what it believes to be the remaining life based on the distance it took you to break wire 1. Once you break wire 2, the computer further refines the estimate. The estimate is pretty close, usually. A problem that can come up when buying a used vehicle is the driving habits change. The vehicle doesn't "know it's been sold" and can't adjust the estimate accordingly. After you change the breaks and do a reset, it will become more accurate to your driving style. It's all guesswork by the computer and no actual thickness measurements are taken. My CPO X3 has been showing 4500 miles left on the front breaks since my last oil change, which is due again in 2 weeks....


um...no.


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## imtjm (Oct 5, 2004)

TomNJ said:


> Replaced my front brakes and rotors recently. The red brake light came on couple of weeks ago but I've been getting a mileage countdown for months indicating I should expect it last month. I drove about 200 miles with the light on. F25 series x3 xdrive28i
> 
> I thought I read the light doesn't come on until 2.4mm. I measured each pad and they still had about 6mm of thickness left.
> 
> ...


It's difficult to understand what you are really talking about. There are two parts to this:

first part: the brake service warning, when reset indicates certain miles before you should get your brakes serviced. This is a mileage estimate and has zero input from the brake pad wear sensor, driving habits, etc. It's just a count down clock that suggest you should get your brakes checked out based off an estimated number of miles BMW thinks the brakes should last. However, that isn't hard and fast, since, for example, we reset my wife's brake service indicator three times before it actually needed new pads and rotors. It has nothing to do with the brake pad wear sensor. You could run down the brake service mileage before before the brake wear sensor is even close to being tripped. You or whoever changed the pads and rotors should have reset the brake service indicator corresponding to the brakes you replaced (e.g. front, rear or both).

second part, is the brake pad sensor, which trips a brake pad sensor warning light (circle with dash lines on left and ride side of the circle). The brake pad sensor is simply a circuit: when the sensor wears down with the pad, the circuit breaks and the brake pad sensor warning light comes on. The mileage countdown is independent of the wear sensor. The brake pad sensor can also trip if there is any break in the sensor wire. For example, I used an OE pad sensor when I replaced my pads and rotors, and the wire connector flared out a little more than the BMW OEM version. The sensor wire got cut from rubbing against the wheel (easy fix, new sensor just zip tied back). If the brake wear sensor light is coming on, it could simply be a bad sensor. Maybe the previous owner was a cheap ass and didn't replace the sensor when replacing the brakes or something. They are too cheap to not replace, so who knows.


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## heyheyitsmarc (Apr 3, 2017)

imtjm said:


> It's difficult to understand what you are really talking about. There are two parts to this:
> 
> first part: the brake service warning, when reset indicates certain miles before you should get your brakes serviced. This is a mileage estimate and has zero input from the brake pad wear sensor, driving habits, etc. It's just a count down clock that suggest you should get your brakes checked out based off an estimated number of miles BMW thinks the brakes should last. However, that isn't hard and fast, since, for example, we reset my wife's brake service indicator three times before it actually needed new pads and rotors. It has nothing to do with the brake pad wear sensor. You could run down the brake service mileage before before the brake wear sensor is even close to being tripped. You or whoever changed the pads and rotors should have reset the brake service indicator corresponding to the brakes you replaced (e.g. front, rear or both).
> 
> second part, is the brake pad sensor, which trips a brake pad sensor warning light (circle with dash lines on left and ride side of the circle). The brake pad sensor is simply a circuit: when the sensor wears down with the pad, the circuit breaks and the brake pad sensor warning light comes on. The mileage countdown is independent of the wear sensor. The brake pad sensor can also trip if there is any break in the sensor wire. For example, I used an OE pad sensor when I replaced my pads and rotors, and the wire connector flared out a little more than the BMW OEM version. The sensor wire got cut from rubbing against the wheel (easy fix, new sensor just zip tied back). If the brake wear sensor light is coming on, it could simply be a bad sensor. Maybe the previous owner was a cheap ass and didn't replace the sensor when replacing the brakes or something. They are too cheap to not replace, so who knows.


To quote you: "Um, no"

There are two wires inside the break pad wear sensor located at different depths. When the first one wears through, the resistance measurement being monitored by the computer changes. The computer then knows how thick your pads are and makes an educated guess based on how many miles it took to get there how long it thinks it will take to get to the next wire (also a known thickness). When the second one wears through, it becomes an open circuit and triggers the final countdown / warning. If you had a warning come on before the sensor was even contacted, you've probably got a bad sensor.

As a side note, the resistance measurement of the sensor is why if you don't replace one that's been triggered, the computer won't let you reset the service indicator. (It says something weird like "Ineligible for Reset" IIRC)

My X3 has been at 4,500 miles for expected brake life for the last 15k miles now because it hasn't broken the second wire yet. If the countdown was independent of the wear sensor like you claim, it would have triggered the service light a LONG time ago...


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## dpritchett (Sep 6, 2006)

Those sensors are notoriously unreliable in my opinion - the only way to know for sure is to inspect your brakes and measure thickness. 

On my F10, I experienced the '3100 miles and holding' (i.e, the sensor would show 3100 miles remaining and not move for thousand of more miles after that, while the thickness was fine) several times, and when I redid the brakes at~65k and replaced the sensors, the rear mileage dropped from 31k to about 18k in the course of 5000 miles, while the front increased (?) from 31k to 45k in the same period.


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## imtjm (Oct 5, 2004)

heyheyitsmarc said:


> To quote you: "Um, no"
> 
> There are two wires inside the break pad wear sensor located at different depths. When the first one wears through, the resistance measurement being monitored by the computer changes. The computer then knows how thick your pads are and makes an educated guess based on how many miles it took to get there how long it thinks it will take to get to the next wire (also a known thickness). When the second one wears through, it becomes an open circuit and triggers the final countdown / warning. If you had a warning come on before the sensor was even contacted, you've probably got a bad sensor.
> 
> ...


um no...but depends on if you have a dual stage sensor or say a 2005 which has a single stage.


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## heyheyitsmarc (Apr 3, 2017)

imtjm said:


> um no...but depends on if you have a dual stage sensor or say a 2005 which has a single stage.


OP says F25. A 2005 is not an F25; it's an E83


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