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Old 02-08-2019, 05:49 PM
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Default X5 transfer case failure

I have a 2001 X5 4.4i. Was at an intersection, hit the gas, heard a bang and then that grinding sound from stripped splines. I found out that the front driveshaft splines were worn down at the transfer case, which is causing the noise. What I don't understand is why this would cause the vehicle to not move at all.

From what I understand about the xDrive transfer case, the rear driveshaft has a "permanent" connection. Could a disconnected front driveshaft cause 100% power loss in the drivetrain?

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Old 02-09-2019, 04:47 AM
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I finally figured this out on my own thanks to a YouTube video of a disassembly of an older xDrive transfer case. Just posting in case anyone else cares(probably not).

My original assertions about a permanent connection to the rear driveshaft only hold true in the newer transfer case. The newer vehicles are essentially "full time" RWD and uses a dynamically controlled clutch to modulate power to the front driveshaft. In this case, if the front driveshaft was disconnected, power would still make it to the rear.

In the original xDrive case(such as mine), this is completely mechanical, with no clutch. It uses a planetary gear system. The input shaft only drives the planetary carrier. The rear driveshaft is attached to this system by being the ring gear, and the front driveshaft is connected to an elongated sun gear(via a chain to offset it).

This means that the system can only work if the planet gears are relatively stationary, which means that there is tension on both the sun(front) and ring(rear) gears. Since the front driveshaft was mechanically detached, there was no tension on the planet gears from the sun gear(front), however there was a lot of tension on the ring gear(rear). Because of this, the ring gear became essentially stationary while the planet gears were easily driving the sun gear around. So the carrier easily spun around the ring(rear) gear with no tension to transfer any power to the back. Hence the vehicle could not move anywhere.

Park would also not engage for the same reason. The planetary carrier would remain stationary from the output shaft of the transmission being locked. When the ring gear(rear wheels) would turn, from gravity pushing the vehicle, the planet gears would be able to freely spin because the sun gear(front wheels) is completely loose.

All around nasty. I can see why they redesigned this.

Last edited by bmwnewb; 02-09-2019 at 04:53 AM.
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