Brake wear sensor

[Gaz mr-s]

Not MR2 related, but some of you will have dealt with these. A 3-series BM, one front & rear.
Firstly I don't understand how they work, one contact at the pad end, & two pins at the other.??

I have two new ones & can't measure a dc path to either of the pins. Should I be able to?

 

[mr2garageswindon]

OSR by any chance? I had an E46 brake pad wear sensor issue with new sensor fitted it still brought the light on.
The wiring had become rotten on its way into the little box that houses the sensor connector and the pins in the "Car" side of the multiplug.
Ended up hard wiring sensor to good wire further up the loom.

[Bossworld]

I think it's a closed circuit until the pad wears down enough for it to make contact and therefore render it open and trigger the warning.

[Gaz mr-s]

OSR by any chance? I had an E46 brake pad wear sensor issue with new sensor fitted it still brought the light on.
The wiring had become rotten on its way into the little box that houses the sensor connector and the pins in the "Car" side of the multiplug.
Ended up hard wiring sensor to good wire further up the loom.

Does OSR mean offside rear? 

I'm not testing a used one....this is 2 new ones.  Why are there two pins in the cylinder & should one of them at least have continuity to the pad end?

[Gaz mr-s]

I think it's a closed circuit until the pad wears down enough for it to make contact and therefore render it open and trigger the warning.

I think you mean open circuit.

[thetyrant]

BMW pad wear setup and the sensors are way more complicated than they need to be!   many other makers just use a wire inside the plastic sensor on pad that once broken as it wears through due to low pads puts light on dash, other have a single wire sensor that earths onto disc when sensor wears and does same, both nice and simple

BMW setup has both pad sensors (1 front 1 rear typically) and the CBS (condition based service) system which is an computer calculated servicing depending how you drive, the new sensors have a resistance that the ecu monitors and as it starts to wear on the disc when pads get low it alerts on dashboard, usually a yellow warning, if you carry on driving and wear the sensor right down and break the wire totally you get a red warning iirc.

  The CBS system monitors how car is driven and takes a guess at the pad life, this should always be well on side of caution and will give an advance warning before pad sensor touches discs, often around 20-25% of pad life still remains with CBS warning system but its a good time to properly check brakes which nobody seems to do these days even when servicing car at dealers!. Lots of people moan that CBS is too cautious and looks loads of pads left but i think its a good guide for those that dont look after there cars and often the inside pad you cant see is more worn as well so time to inspect things at least, if pads have plenty of meat you can reset the obc for low pads and carry on for more mile, also these people often think pads are worn out when they are pad backplate onto disc!... and not 3-4mm of material left which is the correct time to replace them

If you cut the pad sensors off and join the 2 wires together making a closed circuit ecu doesnt seem to mind and treats it same as new sensor, hence you can reset the pad wear via obc and just monitor pad life old fashioned way which is what i usually do on my bmw's, the CBS still counts down of course to give you and idea on pad life but oyou shouldnt hit the red low pad warning, one thing to watch is if you try and reset low pads on obc with a tripped/worn sensor without replacing or bypassing it and you create a whole world of pain to sort it as car gets very confused on some models.

HTH

[Bossworld]

I think it's a closed circuit until the pad wears down enough for it to make contact and therefore render it open and trigger the warning.

I think you mean open circuit.

Don't think so mate, check thetyrant's post - states you can lop the sensor off and join the wires together to bypass it. Would surely mean it's closed by default?

[Gaz mr-s]

Thanks for that huge explanation Ian....must be a 'morning' person.   
The car won't get looked at 'til next week.

If the brake pad connector can be cut off & the wires shorted stops any alert, does that not mean there should be a dc path through the cable? 

[Bossworld]

Thanks for that huge explanation Ian....must be a 'morning' person.   
The car won't get looked at 'til next week.

If the brake pad connector can be cut off & the wires shorted stops any alert, does that not mean there should be a dc path through the cable? 

There are two possible things IMO (and I could well be missing something) that might affect the reading.

1) if it is that simple a design, do you get a reading if you hook the multimeter up to the two pins in the connector itself?

2) this PDF suggests there are two resistors in the BMW circuit, that the CBS uses as per the poster above. I changed my pads on my R60 before they ever made contact so I've only ever seen the pad mileage countdown on the display rather than any warnings.

https://www.brakepartsinc.com/dam/jcr:24db1e41-6fd5-4661-9508-0ec1604bb88d/Brake%20Wear%20Sensor%20Tech%20Bulletin.pdf

[thetyrant]

Thanks for that huge explanation Ian....must be a 'morning' person.   
The car won't get looked at 'til next week.

If the brake pad connector can be cut off & the wires shorted stops any alert, does that not mean there should be a dc path through the cable? 

The Ecu doesnt seem to mind if its a direct path through the circuit (cutting off sensor and joining wires) or it has the correct amount of resistance that a new sensors, its the wrong resistance (cheap ebay sensors apparently sometimes cause issue because of this not allowing obc to be reset) or open circuit (sensors destroyed) that upset it.
 Ive never measured the resistance on sensors to see what it is but if you get your multimeter on it you will soon find out 

[Dev]

  I replaced the brake sensors on a old BMW I owned. Easy but I think the whole system is worse considering people these days disregard warnings on their dash.
 
  The mechanical sensors built in to the older pads like what is found on our car is much better. Once the pad wears down it starts to scrape the rotor making a noise letting you and everyone around you know that something is wrong with your car as it gets progressively louder. Its embarrassing like fingernails though a chalkboard. Most people will take the car to get new pads as soon as possible but in the mean time you still have enough pad material to keep you safe.
 
 All of these electronics may seem sophisticated and state of the art but they fail and are costly to fix when they go wrong. Cars today have too many fragile modules and that is why I will not buy a modern car that doesn't have a dip sick. 

[Petrus]

I will not buy a modern car that doesn't have a dip sick. 

You mean a car designed for dip stick drivers ;-)

Although electronis cán and thus wíll occasionally fail, user failure is só much more common.

You´d be surprised by how many drivers would not know where to look/ how to use the dip stick.
And yes, surprisingly many drivers will be careless about warning lights too.
It all illustrates that there is no such thing as idiot proof; only more idiot resistant.

[Dev]

I will not buy a modern car that doesn't have a dip sick. 

You mean a car designed for dip stick drivers ;-)

Although electronis cán and thus wíll occasionally fail, user failure is só much more common.

You´d be surprised by how many drivers would not know where to look/ how to use the dip stick.
And yes, surprisingly many drivers will be careless about warning lights too.
It all illustrates that there is no such thing as idiot proof; only more idiot resistant.

 The problem with these electronic dipsticks is failure and inaccurate readings. They will eventually have to be replaced and don't last forever driving up ownership costs. Another stupid technology is the BSI sensor that monitors the battery and makes adjustments. These sensors fail and cause all kinds of problems but a good tried and true voltage regulator works. Another dumb innovation is the electric water pump that leaves you stranded once it fails and if you disregard the light it will overheat the engine and blow the head gaskets.  Modern mechanical water pumps are much better.   
 I have owned and worked on a BMW, the car is garbage. As I was replacing parts which was often I come to realize how bad the engineering is. Lexus is many times better and it has a dipstick. 
 
Getting back to brakes the BMW would go though pads quickly with brake dust everywhere. Im still on the original brakes with the Lexus at 60k miles and this is a heavy car. Not one single problem and the quality of the materials are better also.