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Originally Posted by mkaresh
Makes me wonder why the fluid was in bad shape. Does bad fluid cause the transmission to fail, or does a problematic transmission burn the fluid?
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Both.
The fluid will get contaminated by a failing gear or bearing, or by clutch pack material. This is the usual failure mode. Then, people see contaminated fluid and believe that the fluid caused the failure when in fact it is the other way around.
If the fluid is burnt smelling, and brown, then it has been subjected to very high heat and the friction properties may not still be within spec. The different fluids specified for different transmission models have different frictional characteristics, made to work with the wet clutch packs. Fluid that is out of spec causes the clutches to slip, generating more heat, and you are heading for a failure.
With BMW transmissions, whether they are made by ZF or GM, I believe that the BMW electronics are often the cause of a transmission failure. A sensor fails, shifting controls don't work properly, and then a failure results. Given the history of other electrical problems on modern BMWs the control package is more likely to be a cause than the transmission itself. Automatic transmissions are actually very reliable devices, using the same technology for decades. What has changed are the control packages. The controls are often specific to the manufacturer, so a ZF transmission in another car may be reliable, but not so in an X5. Some newer controls have helped substantially; old transmissions had very perceptible shifts between gears. Modern transmissions have more gears, less shock when the shift happens, but also have an algorithm that reduces engine power (by changing timing usually) when the shift happens. That makes for a very smooth shift, and the result is less wear in the transmission.
I find it interesting that people talk so much about miles to failure in an automatic transmission, and comment on the variability. In my business we use heavy duty industrial transmissions in mobile equipment. Our predictors for planned component change-out times are just as often the number of cummulative shifts as they are the miles or the hours. Overloading is a factor, but shift count is a good indicator of wear-out for us. The transmission controllers keep that info; I don't know if BMW has any similar technology.
03-24-2008, 02:47 PM
Hybrid Mode
