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Originally Posted by 007X3
What is this have to do with it. In my post I said there are accelerometers in the transmission among other places that always know vehicle velocity, pitch, etc... I never made mention of a rotating shaft. The transmission knows actual vehicle speed at all times. Now when the tires are rotating faster then what is programmed at given velocity due to slightly smaller tire diameter the X-Drive unit goes into default, Meaning all wheel lock. This is a soft Trouble code and does not let itself be known without a detailed scan. No dash warning lights will go off.
The point you were trying to make is that the car has no way of knowing how fast it is going other than wheel speed sensors. Thus when I stated that is has accelerometers to tell is exact speed you stated that accelerometers measure acceleration not speed. Back to my point Velocity (speed) = acceleration/time. Meaning as soon as you start moving the accelerometers report to the ECU and the ECU then computes Vehicle velocity based on acceleration/time formula. I'm not sure how intimately familiar you are to a modern automatic transmission but I can tell you with great degree of certainty that the transmission always knows exact vehicle speed, independent of wheel speed.
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I tried to walk away, really I did. Sorry this is so long. I suspect that ol' MD will stick with it.
The transmission does not have accelerometers that know what vehicle speed is. I tried to explain that nicely. If you continue to believe that such parts exist, please provide a parts reference. You have now introduced the concepts of vehicle pitch but that is a different issue, that is related to the DSC control over brakes, and to a lesser extent, throttle.
The rotating shaft I refer to is the output shaft of the transmission. It is how the transmission calculates the vehicle speed. The counter calculates an expected vehicle speed from an assumed final drive ratio. It isn't complicated, it has been working that way since the forties at least (that is the oldest car I have worked on with a speedometer cable driven from the transmission output shaft). In that sense, yes, there is a different speed calculation than the ABS wheel sensors. My point that there isn't a different measurement is that the output shaft of the transmission is directly related to the wheel speed sensor (by the final drive ratio). They don't vary, they are directly proportional. Apart from the output shaft of the transmission, and the related wheel speed sensor, there is no built in speed sensor on the car.
Let's continue with your theoretical approach to vehicle speed calculation. You are saying that every time the car speeds up or slows down, the computer inputs that acceleration, calculates it over time, and comes up with a calculated speed over the ground. The car must also then filter out the forward acceleration from the three axes of acceleration it is measuring, and compute the resulting forward vector. Incredible. Just think of the error that must be introduced after a several hour drive (since the car has no way of self-correcting this acceleration/time calculation). The whole concept is mind-boggling.
We could do an easy test here. You could drive your car off a cliff. As you plummetted towards the ground, take your foot off the gas, and check your speedometer. If I am right, the speedometer will be zero. If you are right your speedometer will function as an airspeed indicator, indicating your velocity up to terminal velocity. If you want to reduce the damage potential then you can skip the cliff-jumping part. Just spin your tires in the snow. You won't go forward. Using accelerometers, the speedometer would show zero. I suspect you will find that spinning tires results in a rising speedometer reading. Let's just assume that if the transmission knows vehicle speed then that is what it is indicating on the speedometer.
I may not have worked on a modern BMW/ZF/GM automatic transmission, but I do know the inside of C3, C4 and C6 Ford transmissions, Chrysler Torqueflite 727s, and various GM models. I also know that ABS systems use an algorithm that takes inputs from four wheel speed sensors and computes a resultant average that is then compared to the delta at each wheel. It does not use a calculated wheel speed from the transmission output shaft because that is less accurate.
If you want to get back to the issue of when the transmission goes into lockup, then yes, it happens at a certain speed. That speed comes from the rotational velocity of the output shaft. We have now come full circle. Lockup doesn't relate to vehicle speed over the ground (true distance covered) as much as it relates to fluid pressure in a circuit inside the transmission valve body.
This combination of auto repair and physics discussion is lots of fun, but I don't think you are any closer to figuring out why you have a noise under your X3. Hopefully, though, you now know how the vehicle systems interact a little more.
Cheers
JCL