96 328i Sedan brake sensor?

[eurogeek]

Anybody know what type are the ABS wheel/brake sensors? Proximity?

[Manolito]

I don't really understand your question. I expect that they are standard wound-core sensors triggered by spinning toothed reluctors, just like the crank position sensor and the cam position sensor. I never cut one open to see.

The front ones are just inboard of the brake rotors. Oddly, I don't see rear sensors in my parts list for your car. I think that I have seen them though, just outboard of the differential.

The sensors for your car cost about $60 each on line

[eurogeek]

I could have been more specific, sorry.

I've heard the slotted, sensing ring around the wheel bearing/housing called an "echo" ring and that ties directly to the info I found on Answersdotcom:

"Acoustic proximity sensors are similar in principle to infrared models, but use sound instead of light. They use a transducer to transmit inaudible sound waves at various frequencies in a preset sequence, then measure the length of time the sound takes to hit a nearby object and return to a second transducer on the switch. Essentially, acoustic proximity sensors measure the time it takes for sound pulses to "echo" and use this measurement to calculate distance, just like sonar."

So, I believe they are Acoustic proximity sensors.

[Manolito]

Wow. I would not know where to go to get that level of technical detail on these sensors, other than to make friends with someone who works at BMW or at their supplier.

I am really surprised to hear that they are acoustic sensors. Not that acoustic sensors don't have many good uses, but I would not have expected to see them used beneath a car. Too many opportunities for harmonic interference, and with a wheel spinning at 120 mph, the movement of the teeth is fast enough to make processing the sound-based signal a dicey proposition.

According to my calculator, at 120 mph, the edge of that wheel is going past the sensor at about 75 ft/sec and the teeth are only about 0.5 ms apart.

Magnetic sensors just seem to me like a much more logical choice, though I am sure that whoever designed that system knew a whole lot more about the problem than I know.

[eurogeek]

Yea, me too! I thought that this might be good to know before even thinking about further investigating the ABS system. Don't currently have any ABS problems, but........

First, magnetics are really great and have provided invaluable service to the auto industry, but the signals are better for discreet use, i.e. 1 or 0. Any scaling signal for precise control requires a signal that is as clean as possible. Especially for brake control!

So it seems to me that a clearly defined (dB?) signal would not only be very clean (considering that the sensor is set at a defined (I don't know) distance from the echo ring) but you probably already know that when harmonics are present, it takes more than a momentary interruption to really "change" the sound wave. But I am surprised the processor was fast enough, in 1996, to monitor and react to an abnormal signal.

Very interesting!