Yes there are two speed sensors one on the front drivers side (US) and one on the rear passenger side. it could be either sensor.

 

Fighting the trifecta lights now on my R53 with 105k miles.

Replaced both front sensors with cheap aftermarket (ebay). Still had a problem. Replaced each new front sensor with another cheap aftermarket (amazon). Still had the problem. Dissected the wiring harness for front left sensor back to the fuse box and re-armored it, couldn't find a problem. I was perseverating on that area because one of the front driving lights (same harness) wasn't getting 12v anymore, so it smelled like a problem in that harness. It wasn't.

So I bought that C110 scanner ($70, about what i spent on sensors so far flailing), which seems to have worked great:



Oh. Front left. Ah. I suspect the issue is deeper than that, I'll probably dive into it tonight, but I'll start there and with that wiring harness.

The scan tool is pretty slick. Half the price of just one scan from a BMW dealer.

For fun, I may throw a scope on the sensor and test it live and see what the signal is supposed to be. There is a nice thread starting down that direction here:
https://www.northamericanmotoring.co...or-is-bad.html

That would give people a simple way to test the sensors without the special tool (assuming they already have a $79 DSO nano and can use it).

 

Pulled out the ABS (DSC?) module so I could get at the wiring harness and look for chafe points and look at the connector for moisture / corrosion.

It was clean and good. The wiring harness looked like it had a little wear (110k miles), but no damage either external to the wire (rubbed through insulation) or internal to the wire (good insulation but conductors broken or about to break). To test for internal breaks, I take the wire and run it over the tip of the same surgical forceps I use to dissect the cable wrap, putting moderate tension and a light bend on the wire. A good internal wire will have a smooth arc as it goes over. A sketchy wire will have an observable (by both feel and looking) kink. A broken wire will be fairly obvious, it will go to a sharp angle, and you will often see the insulation stretch.

To get that module out, I had to remove bolts for the power steering pump and coolant reservoir, and move them forward. That let me find a leak from the seam of the coolant resevior, I tried a patch for that (because it was out anyway) that may or may not work. If it holds up I'll post it in another thread.

Then to remove the plastic cover of the ABS module I had to pop out a vent between the hood and the firewall, which gave me easy access to one or two hidden bolts up at the top. Then the plastic cover wrestles out (a lot of wrestling) and gives you access to the module. One or two bolts, and the module electrical connector (pull out the locking slide piece to release).

I also pulled all the brake line connectors as well of course, which drools brake fluid everywhere, and will probably make for interesting bleeding exercises. That might be a show stopper for somebody going down this road, but this car is overdue for a good flush and bleed anyway. Remember brake fluid can dissolve paint pretty quickly.

Then dissect the electrical tape to get access to the individual wires. When I put these things back together, I use plastic split loom cable armor (at most auto parts stores) and wrap that with tape. It makes the bundle bigger, but gives a lot more protection from rubbing. It also makes it easier later to "detape" without risking cutting a wire, and leaves less of a sticky mess on the wires. I also cable tie the "loose ends" of the tape down so they can't unravel if the adhesive stops being adhesivey.

Once the module is out, you can get easy access to that harness. That's nice because you can now test the four wheel speed sensors right there with no more dissasembly (leave the wheels on). Is it worse to pull this out, or is it worse to pull all four wheels and get to all four connectors? It is probably pretty close, and perhaps depends on if I end up needing a brake bleed or not. I'll let you know.

Testing is then easy. I have the wire colors at home (I'll post later) that were pulled from a post that pulled them from the Bently manual. They are pretty obvious anyway, as they are the only four (or maybe four of the five) twisted pair connections to that connector. I stripped off a 16 gauge piece of solid core wire to insert into the female spade connector (the other end clipped to my multi meter).

I measured between pins, which should test both the sensor and the associated wiring for each sensor, so this is a really good test. The problem is that you aren't really testing the semiconductor in the sensor live, you are just testing resistance.

When I tested from that connector, I would see something like 10 Megaohms resistance when I forward biased the sensor, and 6.4 Megaohms resistance when I reverse biased the sensor. I would take those absolute values with a grain of salt (as it is likely very highly dependent on how your multimeter works). But however your meter works, all four pairs should act the same, and all four of your sensors probably did not die at once. So measure and document, and sing the "one of these things is not like the other" sesame street song from your youth.

This behavior is consistent with a hall effect sensor, but I'm not sure how they did that with just two wires. Perhaps it is really CANBUS all the way to the sensor. Pretty cool if so, but geesh, talk about making things proprietary. Are the CAN chips really that cheap and that easy to just slot in without registration or anything? Time to go learn about CANBUS. If they are, that's really cool.

Perhaps it is a (clever actually) AC carrier on the DC power source. It would be a little complex, but it would work fine, and save money and weight of four fairly long wires. It makes me want to test one of these things live and understand how exactly it is working. If it is canbus, it must also use a digital AC carrier on top of the DC power source. Which is really cool if it works, it saves a conductor per sensor, which saves a lot of wire, weight, and cost over a whole vehicle.

Sure enough, three of my four sensors acted the same, one was different, but was only different when biased backwards. That sensor was actually new (either cheap from Amazon or cheap from Ebay, I got both and swapped at least once and didn't bother to mark each, they were only $12 or something). I have four new ones on the way from Amazon (two front, two back, $48 total). I figure at least one of each will work, and maybe I'll win the "cheap crap lotto" and get 4 good ones and keep some on the shelf, or hack one to understand how it really works.

This "one broken" matches what my $70 scanner said (screenshot above) said also. So maybe that sensor was just bad out of the box, it is causing death spiral on the canbus, and the steering angle sensor is just fallout from a screwed up device on the same bus, or maybe just an unrelated failure that has been bad for a long time but never discovered until I got the good scanner.

New parts may go in Friday, I'll post more updates and details.

 

Still chasing, but a few more updates.

First, removing the DSC/ABS module is not the best way to start. It would be close compared to removing all four wheels, but removing it requires a brake bleed, which requires removing all four wheels. Derp. But you will have to do that anyway if you are looking for chafe points on that wire bundle to the connector (I checked, mine was fine at 107k miles).

I'm still getting the same codes, with a new sensor that passed some minimal bench tests (more sanity checks than actual functional tests). It has some other problems as well, so I need to start learning about canbus to see how likely it is that something like a bad sensor can crash a whole bus and make other controllers or sensors look bad.

I really need a detailed wiring diagram of this car, which is proving difficult to chase down.

 

I think the C110 might not be showing everything as far as stored codes , you cleared all of them and then choose read codes again ? and testing for the line power, did you do this with the c110?

PM me i might have some links to help you .

 

Thanks for the help ECSTuning.

Found some information on some BMW forums, and it looks like the wheel speed sensors are hall effect sensors, possibly analog ones, and they are indeed an AC carrier on top of a DC signal.

I'll break out the oscilloscope and make a test wiring harness and measure some spinning wheels and post the results. The DSO Nano is only about $80 and is a decent little storage scope with a good enough bandwidth for most automotive work. Fantastic for checking these kinds of things right at the source.

 

Welcome.

 

More info...

Kudos to the C110 scanner. I spent a little more time with it, and found the live plot screens. I can drive the mini around and actually plot the data from each individual wheel speed sensor in real time. Wow. Sure enough, that front right sensor is working pretty well most of the time, but it drops to zero at odd intervals (like once a minute or so). You can see it dip in the plot, and the lights immediately come back on.

So I played with the stack of cheap ebay and amazon sensors on my bench now. The hookup for these is +12v to a 10k resistor, other side of the resistor to the pin at 3 oclock, and ground to the pin at 9 oclock. This is all oriented looking down into the connector oriented so the plastic slot guides are at 12 and 3 oclock.

I'll try and post pictures of all this later when I am not at the shop laptop.

Anyway, hooking the scope to it, and waving some ferrous metal in front of the sensor, I get.... NOTHING. On any sensors. Not even the ones that mostly work in the car.

Allow a slight divergence into the topic of hall effect sensors. They work by sensing a magnetic field. Above a certain magnetic flux, they turn on. Below it, they turn off. Or maybe vice versa, whatever it is when you put a magnet on a shaft and a hall effect sensor pointed the right way and close enough, every time the magnet passes by, you will get a square wave out of the sensor. Which is how they measure RPM. More magnets on the shaft will give you more granularity.

When I use them for my Arduino projects, I just glue a magnet to a shaft.

On my Buell XB12X, the crank position sensor uses a magnet *behind* the sensor, built into the sensor assembly. Then it rotates a steel cup with teeth cut into it (at uneven intervals). As the cup moves by, the sensor magnetic field turns on and off because it is reshaped by the steel cup teeth. The speedo sensor on the Buell is the same way, it is near the 5th gear pair, and it actually senses when each gear tooth goes by. You can tell these, because they are magnetic. And you can test them by waving a screwdriver in front of them (so long as it isn't stainless or aluminum ).

However these mini wheel speed sensors did not appear to be magnetic. And waving a screw driver in front of them did nothing.

However when I waved a magnet in front of them, they behaved exactly as I expected. The voltage between the pins on the sensor dropped from 12 to about 8v (I think, going from memory). That's why you have to have the resistor in there, so you can see the voltage drop, but don't have to run 3 wires to the sensor. Clever, but a little complicated.

So my question... is there a ring gear down in that hub with magents on it? Or just a magnetic ring gear? I'm at the point now where I either have to buy the $45 pelican sensor instead of the $12 ebay sensor (if it's the sensor), or tear into that hub (if it's a magnetic ring gear). On the Buell, that speedo sensor can fail if too much metal sticks to it, and you can recover it by just pulling it and removing it's ferrous furball. If there are strong magnets on a ring gear, maybe they just need cleaned.

Anyway, I'll keep digging as I get the time and continue to update. But if anyone knows already they can save me some time.

 

Here is a screen shot from the C110 that shows the sensor drop out readings... that should be a smooth acceleration curve. The "v's" are where the sensor must have read zero.

Now I have to figure out if it is the cheap sensors I am using (more than one has done this), a problem with the thing the sensor senses (that bearing in there?), a problem with the controller reading the sensor, or a short in the wiring somewhere.

 

Here is another shot that shows better plots, but isn't focused. Sorry, hard to drive, hold the scanner, and shoot a picture. Probably illegal too, so of course i didn't do any of that. Because that would be illegal and stuff.

 

I have the trifecta... (first step is admitting your problem). A nicely used 2005 S w/ 150K.


I pulled the wheels and both front sensors had been cut and taped together. I think the previous owner was trying to probe the problem close to the source. Also, the sensors almost fell out of the hole so I'm guessing it was a recent investigation.


I tried soldering/shrink/tape but a new light came on from our friends at "service engine soon". I am certain I got the wires correct but then read that these things are temperamental so...


I replaced all four speed sensors cause I didn't have anymore time to invest if this didn't fix the issue. I started the car, hit the tire pressure reset and the tire pressure light went out. I moved down the driveway about 20 feet and the other two lights shut off. Glorious!!


100 yards later they are all back on again. &^$@*&#$!!.


If I shut the car off, restart and hit the reset, the whole thing is repeated again. Distance for lights to come back on varies only slightly. Doesn't seem to matter if driving straight or turning.


F33 is good.


Stink.

 

Finally got my issues resolved, and I thought I would summarize what I learned.

The wheel speed sensor is a simple hall effect sensor that simply detects the presence of a magnetic field of a particular polarity. Basically, it is a transitor that turns on or off as the result of the presence of a magnet pointing the right way.

This is wired into a voltage divider circuit, which lets it be a two wire system instead of a three wire system. So basically, you feed the sensor assembly power through a resistor, and the sensor will either go lower or higher voltage depending on if the magnetic field (of the right direction) is or isn't there.

That's the sensor part. And that is what people have been fussing with. The Ebay and Amazon cheap parts are, well, really cheap. And some work and some don't. The mid tier parts from places like Pelican seem solid and reliable (the one I got was Delphi I think).

But there is also a "transmitter" part, which is a ring of magnetic material that is built into the wheel bearing assembly. It is basically many little magnets pointing alternate directions built as a single unit into a thin ring shaped piece of tape. You can see what I believe is the alternating magnets in the tape, so in theory you could probably fabricate a functional replacement with some tedious cutting and gluing of vinyl magnetic sign material. But to fix it, you would have to pull the wheel bearing assembly, and given you can buy whole new assemblies (with the sensor) for $125, by the time you are that far in you might as well just replace the unit.

So here is what happened with my 2005 R53. I had a sensor fail, and the trifecta light went on. I did not have the good scan tool yet, and I got two sensors for $30 or something from ebay, so I figured by the time I did the work to remove the wheel I might as well just replace the sensor (as opposed to testing it). I knew it was one of the two front sensors, so 50/50 chance of getting it right the first time, and if I was wrong, I had another sensor on hand anyway. $30 total, and a time investment of removing one (or maybe two if unlucky) wheels and sensors. Less time than trying to debug a possibly intermittent sensor.

It was a good plan, but like all plans....

So I do the first wheel. The sensor, naturally, is stuck in there tight and isn't coming out for love or money. So I have to drill it out. Fine, it's plastic, not a big deal, I've done a lot worse with sheared exhaust studs in Buell heads. Put new sensor in, problem remains, so do the other side (which is also effectively glued in there and has to be drilled out).

Problem remains. Do the flailing I describe above, including engineering the sensor and getting the fancy shmancy scan tool. Scan tool lets me see that it is an intermittant failure, and narrows it down to one wheel, so I flail a bit longer using other cheap (and not as cheap) sensors. Still a problem.

Finally, order a new NSK wheel bearing assembly and replace it, and the problem is now obvious:



R53 wheel bearing assembly with damage resulting from drilling out failed (and stuck) wheel speed sensor.

You can see where I drilled slightly into the bearing and magnetic ring when I was removing the old stuck sensor. That was causing my intermittent failures.

So, lessons learned:

1) When you go to pull your sensors, there is a VERY good chance they will be stuck in there.
2) When you go to drill your stuck sensors out, if you are going to get a little sloppy with the drill, get sloppy in the direction AWAY from the wheel hub, not near it.
3) You have to buy a $150 wheel bearing assembly if what is likely a $1 magnetic sticker fails or is damaged (it is soft plastic). You could likely build a new sticker in under an hour, but you would have to remove the wheel bearing assembly to replace it.
4) If your axle nut is stuck, you will explode a long loyal 18" breaker bar that served you well for about a decade, and have to replace it with a 24" breaker bar, which also won't get the nut off, and you will then have to cut it off with tungsten carbide burr heads on a dremel tool, that will put little slivers of mini nut into your palms for 18 days (and counting).

 

Oh, and at 110k miles, the factory bearing (with the now damaged sensor ring) still felt perfect.

 

Yep, those sensor can get stuck and be a pain to get out. I had one stick and snap, so I pop out the wheel bearing and the pushed upwards on the senor head till it popped out. Cleaned the hole and applied some very light anti seize around the cleaned hole and new one back in.

 

You could probably rig up a depth stop, so you never drill far enough down to risk hurting the soft magnetic ring, but I don't know if "the rest" of the sensor would then have room to fall free past the axle (there is a gap there, but not a big one).

Removing the bearing should be a pretty easy job, but mine wasn't. That axle nut was NOT going to move, not with heat, impact gun, or massive torque on a breaker bar (over 300 foot pounds). So I had to cut that nut off, and that made removing the bearing a PITA also. Especially because I wanted to be careful to not hurt the axle. And of course the torx bolt that held the rotor on was self welded also, so that had to be drilled out after ruining my torx bit.

If you can get that hub bearing assembly off though, the that would make it easy to safely get out that old sensor. .

 

Whyme, if I were you, I would borrow or buy the C110 scanner (or something like it). For under $100, you can then trace it to a particular wheel. Then I would take that hub out and inspect the abs magnet ring. I bet yours is chewed up, perhaps when you pulled out the old sensors, or perhaps when the previous owner was flailing with the same problem.

Reading the forums, I think people get the false sense that it's always the sensor, and that replacing the sensor is a trivial task. It isn't always the sensor, and while the replacement of the sensors is theoretically easy, if you have to drill it out (and there is a good chance you will) it is actually pretty easy to damage a $125 part (the bearing hub with the integrated magnetic ring).

I bet a lot of the "bad ebay sensors" aren't actually bad sensors, but rather they are damaged sensor rings.

 

I have seen rust build up on the back of the wheel bearing that had 150K+ miles ( in salt prone areas) and rub into the head of the sensor and then it goes bad. Mine did this.

 

My other sensor finally failed when that wheels bearing finally failed at 137k miles. I only replaced one bearing in the exercise above.

The first symptom was growling on turns, and it was on my "to-do" list to look at, and I ordered a new bearing knowing I'd need it soon enough regardless anyway. I waited a little too long, and the back of the wheel bearing popped off a shield, and that shield chewed up that hall effect sensor head.

I still had a pile of the "stupid cheap" sketchy ones I got on Ebay when I was chasing the problem before, and used one of those with the new bearing, and it's working fine so far.

So, if you hear a growl on turns for a while, and you see your ABS light suddenly go on, your bearing just progressed from "failing" to "falling apart", and things are getting really serious, and now you need an ABS sensor in addition to a new bearing and new axle nut.

For what that's worth....

 

Your many long and detail posts pique my interest. I have never experienced speed sensor failure thanks to the mild PNW. I am just curious on what is the speed sensor made of. For the low price, only two conductors, and the simple construction of the tone ring that is the integral part of the wheel hub bearing I believe they are just plain simple inductive pickups, like moving magnet turntable cartridge. As there is no magnet involved but just gear-like tone ring passing the pickup in close proximity the sensor probably has gazillion turns of winding, and hence the high DC resistance.

I am quite sure it is not on a CAN bus which would make the sensor much more costly. The BMW service manual shows each with a coil symbol and the two conductors go straight to the ABS module.

 

OK. A few minutes of Binging I found what looks to be very credible answer that the wheel speed sensors are indeed Hall effect despite there is only two conductors. That make sense for why the resistance is so high measured with a DMM (as reported by those who tried). Of course I am making a reasonable assumption that Mini share the same or similar wheel speed sensor design with the Bimmers of the same era.

Here is a long and detail post over at Bimmerfest.


AC signal over a signal wiring pair that also carry the DC power needed by the Hall effect sensor

 

Yup! And the hall effect sensor itself is far cheaper than a traditional inductive pickup. I bought some for a cool little Arduino project I was doing, and I think I paid $1.75 each for them retail, meaning in bulk they are probably like 25 cents.

I think this mini system is different than the diagram though. In the mini, there is a ring with a bunch of magnets in it, and the sensor just picks that up when the magnet is close. The downside of this is the need for the magnet ring, which is built into the bearing, as opposed to just a metal ring with a bunch of slots. But you can probably make the magnetic ring (basically the same material used to make magnetic signs) for the same or less than you can make the metal ring with a bunch of slots anyway. And it's a flat flush piece, so it would be incredibly durable (me and my drillbit not withstanding ). Not a slotted spinny ring that can rust / crack / collect ice / etc.

In the diagram, they have he magnet behind the hall effect sensor, so they can read a spinning ring that isn't magnetic. This is similar to the hall effect speedo sensor in my Buell motorcycle, which is the same setup, and it mounts in the transmission and is positioned very close to the spinning gears on the final output shaft. It actually picks up on each tooth as it passes. Works fine, but seems like something that probably took a lot more precision in manufacture of the sensor to make work. But it makes sense there, as it uses the gear that is already there (one of Buell's design philosophies was to always try and get one part do two or more things).

The part that I hadn't thought of was how to make it work with two wires instead of three. Very clever.

 

And on a funny note, I forgot I exploded my breaker bar replacing that first hub, and got a replacement.

I broke that replacement doing this hub (putting the nut back on this time). So it's a torque to yield nut used by a torque to shatter wrench apparently....

(In defense of the breaker bar, I am not shy about hitting it with a 5 pound dead blow hammer when necessary to address a stubborn frame bolt in an old rusty Saab).

 

I have a 2004 R53 Mini Cooper S, and I'm having some problems with the "Trifecta". I have replaced all wheel speed sensors and it seems to work until I put it into the second gear. The light stays off when I start it and begin driving, but as soon as I put it into second gear the ABS and Traction Control lights come on and not the tire pressure light. Anyone get any ideas?