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My brake wear alert just started lighting up so I know at least one of them will need to be replaced when I put in new pads. I'd like to order enough to replace all of them, just in case. I've searched but cant figure out whether we have 2 or 4. Can anyone help?
Side issue - anyone found a place with good prices on them?
Ref - Part Number 1715400617
Two, one front one back on the passenger side I believe. You will more than likely need two sensors every time you do a brake job front and back, because they rarely can be reused - they usually break when removing them from old pads. They function by basically just being a simple circuit that's broken when the sensor comes in contact with the rotor and is destroyed
This forum continues to be awesome. Thanks for the quick help, guys!
Do one thing before you change them. Pull them out, clean them with a dry rag and reseat them. Sometimes the work loose and lose contact and throw of an notificiation.
Two, one front one back on the passenger side I believe. You will more than likely need two sensors every time you do a brake job front and back, because they rarely can be reused - they usually break when removing them from old pads. They function by basically just being a simple circuit that's broken when the sensor comes in contact with the rotor and is destroyed
Yup there are two but I believe they function slighly different than you suggest. If they are calibrated to wear themselves to the point of no contact think about how golden a field that is for suppliers of pads.
My understanding is the circuit is formed between the system and the backing plate on the pad and as the pads wear they move increasingly away from the sensor. The wire loop in engineered to expand only so far calibrated to a point consistent with pad wear below a minimum and then it loses contact and that triggers the notice. There is no contact with the rotor as far as I know. That could lead to problems of a different kind
Last edited by Alex.currie44; 10-22-2017 at 12:03 AM.
2012 P31 C63 Coupe Trackrat, 2019 GLE63S Coupe Beast
Most of the pads I’ve been using don’t even have a cutout for the wear sensors. As mentioned above, you can’t really rely on them to work right, and they’re only checking the outside pads on 2 wheels anyway. I’ve got mine plugged in but ziptied out of the way. I’ve just always been in the habit of checking my pads every time I clean my wheels.
Yup there are two but I believe they function slighly different than you suggest. If they are calibrated to wear themselves to the point of no contact think about how golden a field that is for suppliers of pads.
My understanding is the circuit is formed between the system and the backing plate on the pad and as the pads wear they move increasingly away from the sensor. The wire loop in engineered to expand only so far calibrated to a point consistent with pad wear below a minimum and then it loses contact and that triggers the notice. There is no contact with the rotor as far as I know. That could lead to problems of a different kind
No, they actually wear into the rotor as the pads get thin. Once the rotor breaks the contact in the sensor, the light comes on. Very simple.
No, they actually wear into the rotor as the pads get thin. Once the rotor breaks the contact in the sensor, the light comes on. Very simple.
Interesting comment. Not what I understand as to how they function. I will have to ask my guys.
In the meantime from WikipediaDetailed description
Pads B are mounted on carriers G. These are pushed against the rotor A by the piston D which is pushed by the brake fluid E. This induces wear on the brake pads. The rotor A also experiences some wear, but to a lesser extent than the brake pads. The modules C are joined to the cylinder that houses the piston D and acts as counter-force to the piston D.
Sufficient wear to validate a change of brake pads is considered when one of the following cases are applicable:
The vXbox gap 1 is no longer visible or soon to be no longer visible.
The embedded sensor in the brake pad2 contacts the rotor and creates a connection to ground of the sensor.
The metal plate 3 contacts the rotor and creates a noise.
The distance between the cylinder for piston D and the carrier G becomes too large, causing the sensor F to send a signal outside the permitted range through the sensor wire 4, or ground the sensor wire 4 if F is a contact.
F can either be an analog sensor, with an electronic threshold value set to signal an alarm when it has been reached, or an on/off switch, activated at a certain distance.Under normal conditions, only one of the alternatives are used. Many front wheels on cars are equipped only with wear indicators.Electric wear indicator
Indication symbol for worn brake pads
Electrical wear indicator is the alternatives 2 and 4 as displayed above.
Last edited by Alex.currie44; 10-22-2017 at 03:59 AM.
That picture is not actually where the sensor is though, it's clipped into the pad's backing plate itself. I even changed two sets yesterday, on a Sprinter and a Bmw m3, I'm probably a little more familiar with how they work than you are.
If you cut the sensor open or look at a worn sensor, you will see the two wires embedded in the plastic part that contacts the rotor. You can see where they meet the rotor, the difference between a worn and unworn sensor is quite obvious.
That picture is not actually where the sensor is though, it's clipped into the pad's backing plate itself. I even changed two sets yesterday, on a Sprinter and a Bmw m3, I'm probably a little more familiar with how they work than you are.
If you cut the sensor open or look at a worn sensor, you will see the two wires embedded in the plastic part that contacts the rotor. You can see where they meet the rotor, the difference between a worn and unworn sensor is quite obvious.
I have sent my SM an email because I stiil am puzzled. Are you sure you are not confusing this with the wheel speed sensor?
I know some guys try to re-use theirs (cheap *******s lol) but my understanding is that if the sensor is tripped and
the warning light has gone off then they are permanently cooked.
I'm at about 30,000 miles and I started getting brake pad warnings a couple weeks ago. It lights up the first time you apply the brakes after starting the car. If you acknowledge it, it disappears until the next time you start the car.
I've reused my sensors every time I've changed back and forth between track pads and street pads. Frankly, it doesn't look like there is much that can go wrong with them. However, if they wear far enough to trip the warning, they're toast. I didn't know whether I had one bad or two so I purchased a pair. Turns out only the rear was worn enough to trip the alert, but I replaced both since I had them.
I just put new the new pads and sensors on today. The Mercedes sensor is similar to item 2 in Alex's schematic. There's one on the outside pad, passenger front and one on the outside pad, passenger rear. It clips to the backing plate and sticks out about 1/8" toward the rotor. When the pads wear down to about 1/8", the rotor starts to make contact with the sensor and begins wearing the protective coating off. I presume that causes a short to ground when it makes contact with the rotor, and that this is what trips the dash alert.
My front pads were thin but the sensor was not yet touching the rotor. The back pads were a little bit more worn and you could see that the surface of the sensor had been in contact with the rotor:
it hasn't always been like this. The W124 had 4 sensors. When I first did brakes on it, I was able to re-use 2 out of 4. It's not because the contacts become exposed, but because they have on the side a plastic catch and once that snaps when you try to remove them, it's hard to secure them back in place.
For now, I bought new ones front and rear, I bought two for the front, cause the parts diagram seemed to show two.
So I have one available.
Timely thread - yesterday when checking the air pressure on all the cars I noticed the C63's rear pad sensor mount screw evidently decided to back out and the sensor bracket was pretty much just hanging on for the ride and scraping up against the inside of the rim. Not having the time to really mess with it I just zip tied it to the caliper. Now the check brake pad sensor is on. Maybe I should have just left it alone! Plenty of pad left on the brakes. Anyone know the p/n for the screw that holds the sensor to the caliper?
On another note related to sensors - my daughter has a 06 BMW E90 3 series. Replaced the front pads a few months back and followed the directions on reseting the warning light but the f^&k^&( light will not reset. Not normally a big deal BUT she was rear ended in a traffic accident recently - long story short is car was totaled and we kept it as it was a pretty easy fix. But since I can't get that stupid warning light to go out it may not pass the salvage retitle inspection. All warning lights have to be out according to the rules. Jeez. I'd rather have the metal warning tab like on my GM truck than this electronic BS......
Timely thread - yesterday when checking the air pressure on all the cars I noticed the C63's rear pad sensor mount screw evidently decided to back out and the sensor bracket was pretty much just hanging on for the ride and scraping up against the inside of the rim. Not having the time to really mess with it I just zip tied it to the caliper. Now the check brake pad sensor is on. Maybe I should have just left it alone! Plenty of pad left on the brakes. Anyone know the p/n for the screw that holds the sensor to the caliper?
On another note related to sensors - my daughter has a 06 BMW E90 3 series. Replaced the front pads a few months back and followed the directions on reseting the warning light but the f^&k^&( light will not reset. Not normally a big deal BUT she was rear ended in a traffic accident recently - long story short is car was totaled and we kept it as it was a pretty easy fix. But since I can't get that stupid warning light to go out it may not pass the salvage retitle inspection. All warning lights have to be out according to the rules. Jeez. I'd rather have the metal warning tab like on my GM truck than this electronic BS......
On the BMW it sounds like you may have made a mistake on the reset process. It should simply reset after replacing the sensors and doing the reset procedure via the garage cluster. Does it show the “- - - -“ lines when you pull up the brake pad maintenance screen? If so, you will need to get the system reset via ISTA software at the dealer. I used to work there during college and saw this often.
When I did the rears and followed the reset procedure all went well. For the fronts I did the same procedure and light stayed on. Even tho going thru idrive it shows 30K miles to next change. I've had some people tell me that
1) Aftermarket sensors don't work (I used a non-bmw sensor) as BMW dealer was closed on the Sunday I did the repair.
2) You have to replace both the front and rear sensors as the car is looking for a change in the resistance values to confirm the sensor was actually replaced (urban legend?)
I plan on replacing both front and rear sensors with BMW parts and then doing a reset and seeing if that works. If not luckily the guy the sublets space from me at my business is a retired BMW indy and he has all the more advanced scan tools.
Still like the squealing metal solution better ;-) Hell even on the E36 you just had to replace the sensor - didn't have to reset thru the computer. German engineer's motto - "Why make is simple when you can make it complex?"
Hell even on the E36 you just had to replace the sensor - didn't have to reset thru the computer. German engineer's motto - "Why make is simple when you can make it complex?"
The E36 kept me busy with the stupid sensor for several weeks.
Timely thread - yesterday when checking the air pressure on all the cars I noticed the C63's rear pad sensor mount screw evidently decided to back out and the sensor bracket was pretty much just hanging on for the ride and scraping up against the inside of the rim. Not having the time to really mess with it I just zip tied it to the caliper. Now the check brake pad sensor is on. Maybe I should have just left it alone! Plenty of pad left on the brakes. Anyone know the p/n for the screw that holds the sensor to the caliper?
There is no screw or bracket. The sensor slips into a groove in the backing plate of the pad. It's just a very snug fit. There's about a 3" wire with a plug on the end that gets attached to the wiring harness. If you've got screws and brackets loose, it isn't the pad sensor.
If your 'check brake pad' alert is on, it may be because the sensor wire got tugged enough to disconnect the wiring harness plug when you zip tied it to the caliper.
I have sent my SM an email because I stiil am puzzled. Are you sure you are not confusing this with the wheel speed sensor?
Dropped in to see my SM today.
We pulled things up in the system using my VIN which is a 2013.
We confirmed two sensors. One front one rear passenger side.
He had a sensor. The sensor is built around what is basically a twisted pair that forms a core. The ends connect to the circuit so there is continuity.
it goes into the back of the pad and contacts the rotor in the same plane as the pad surface. It is a cylindrical pin about 1.2 mm dia.
As the pad wears, the sensor wears as well until the twisted pair are worn away and the circuit opens and continuity is lost, the open circuit is detected and the warning light illuminates.
They are not reuseable at this point. If you get them out before they trip and they have not worn to the fail point they will still function with new pads because the tip will be below the plane of the new pads but with pad wear they will eventually come back into play.
He suggests using old sensors with new pad can be problematic because if the fragile nature of the wires. He showed me a new part and the wiring is not impressive. $17 CDN seems cheap in the cost of doing a brake job on these things.
The supply wire for the sensor does have a screw that attaches it to the caliper. That is what the sensor wire plugs into.
Ahh, understood. Thanks for clarifying. Wish I could help with screw size . . .
As Alex mentioned above, breaking the circuit triggers the alert to come on. Inadvertently loosening the connection where the sensor wire plugs into the harness could easily do that.
Ahh, understood. Thanks for clarifying. Wish I could help with screw size . . .
As Alex mentioned above, breaking the circuit triggers the alert to come on. Inadvertently loosening the connection where the sensor wire plugs into the harness could easily do that.
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Been there, Done that, ABS sensors too back in the day. I swear they were designed to fall out on the W124s.
10-21-2017, 10:29 AM
