2001 pre-Xdrive No power to front wheels

[alberniken]

Hi all

I did search and couldn't find an answer to the subject. I was hauling a trailer over mucky ground, and only the rear wheels were spinning.

As I understand things, the 2001 would have a center differential rather than t-case, which would be open with no locking features. This means that in situations of low traction to rear, the rears will spin and no torque will be sent to front.

I assume that normally, if my ABS/Traction control was working, the system would tweak the brakes to shuttle power to the front? My ABS system is off-line.

On a side note, the trenches I dug in getting out were equally deep for both rear wheels. AFAIK there wasn't an LSD option for first gen x53....


Is there a way to lock up the center diff, short of welding the spiders?


TIA
Ken

[jbfrancis3]

You wouldn't want to permanently lock the center diff if you plan to ever drive on pavement. You're correct that the brake system is used to transfer power to non-slipping wheels under the circumstances you experienced. Might be worth tackling whatever is plaguing your ABS/stability control.

[alberniken]

Originally Posted by jbfrancis3

You wouldn't want to permanently lock the center diff if you plan to ever drive on pavement. You're correct that the brake system is used to transfer power to non-slipping wheels under the circumstances you experienced. Might be worth tackling whatever is plaguing your ABS/stability control.

Kinda figured that ; used to have an Eagle Talon TSi that someone had put an LSD centre diff into. That was a beast for gravel rally. Was kind of hoping similar might be available for my X5.

Thanks
Ken

[oldskewel]

I think your understanding is exactly right.

With the transfer case acting like an open differential, it will spin the rears unless you get that ABS fixed. (or some other mod like welding)

Here is some more info on how that NV125 TC works, copied from the attached PDF article:

x5 transfer case tech article.pdf

"How does 4WD work?

The first transfer case used in 4WD BMWs was the New Process NV125 unit. The main component of the NV125 is the planetary gear set. This is what divides the torque to the front and rear differentials.

Here’s how it works: The transmission output shaft drives the entire planetary carrier, which provides a 68% rear/32% front torque split between the drive shafts. An annulus gear driven by the carrier directs power to the rear drive shaft. Sun gears in the planetary gearset transmit torque to a drive chain that then drives the front axle drive shaft.

This is a very strong design that is usually trouble-free. A key benefit is the ability to split torque under varying conditions thanks to the ADB (Automatic Differential Brake). When the DSC control unit senses a loss of traction based on input from the wheel speed sensors, the brakes are pulsed on the wheel that is spinning. This pulsed braking directs more torque to the other wheel on the same axle. If both wheels on the same axle indicate a loss of traction, the DSC module applies hydraulic pressure to both wheels so that torque flows only to the other axle. This original system is simple and sturdy."

BTW, that has a common typo - the actual split is 62:38, not 68:32. That split ratio is a direct result of the geometry of the planetary gearset in there, i.e., the relative sizes of the gears used.

And hey, I just had an idea that may have helped you in that situation - parking brake

[alberniken]

Thanks for the Doc; looks like I will have to get my ABS/traction control sorted.

Where I grew up, using the parking brake was called "farmers' posi-trac"
cheers
Ken

[jbfrancis3]

Originally Posted by oldskewel

Here is some more info on how that NV125 TC works, copied from the attached PDF article:

Attachment 79327

Great article, thanks. Yes, it is dead simple inside. There's also a YT video of an an NV125 teardown/replacement.

[alberniken]

Originally Posted by jbfrancis3

Great article, thanks. Yes, it is dead simple inside. There's also a YT video of an an NV125 teardown/replacement.

I dove down the rabbit hole last night trying to see if there was a way to add friction clutches or a lockup device. No joy.


K

[jbfrancis3]

Originally Posted by alberniken

I dove down the rabbit hole last night trying to see if there was a way to add friction clutches or a lockup device. No joy.

How about the ABS as a lockup device?
Couldn't resist..

[alberniken]

Originally Posted by jbfrancis3

How about the ABS as a lockup device?
Couldn't resist..

That would mean fixing ABS and losing my Trifecta Christmas lights