View Single Post
Old 08-02-2010, 10:22 AM   #6
Registered User
Join Date: May 2004
Location: Bethlehem, PA
Posts: 918
My Ride: 2004 330i
Since you are doing a 4wd to a 2wd conversion, I suggest getting a shop manual. Do a bit of reading and you'll see why pulling a fuse does not convert your car to 2wd.

It would be like if I said, "If you pull the ABS fuse, on a 330i, the car will have an open diff." Sounds stupid doesn't it? In reality, it's true. Go put the LR wheel on ice and the RR on dry pavement and try to take off. The ABS/DSC unit will apply the brake to the LR wheel which will transfer power to the RR wheel and the car will move. It's acting like a LSD. Pull the ABS fuse and the car will just sit there slowly turning the LR wheel.

As for a 4wd BMW, if you put the front wheels on ice and the rear wheels on dry pavement. Start to accelerate and the ABS/DSC unit applies the front brakes thus transferring the power to the rear wheels. The car will move. Take out the fuse and the car will just sit there.

On a 4wd system with open diffs, the power is applied to all 4 wheels. My old Audi 5000 Turbo Quattro would get stuck in the snow pretty easy. This was because it had 3 open diffs. If one wheel would get stuck in the snow, all power was transferred to that wheel. At the time Audi had a rear and center diff lock. If you engaged these, then you would have to get both rears and one front stuck to keep the car from moving.

With the advent of advanced electronics, things changed. My old Audi A6 Quattro had a torsen center diff but an open front and rear. When I was in the snow, I could just punch the throttle and the ABS precharge pump along with the ESP would go to work and start transferring power to wherever it was needed. Once I got going, then it drove like a regular car. The problem with my 5000 Quattro was that trying to drive the car around with a locked center and rear diff was a PIA. I had to switch it on and off plus it needed to essentially stop for the diffs to lock.

The XI pretty much operates like an Audi. Now exactly but close enough. The Bosch ABS/DSC system (the 4wd ABS/DSC system is made by Bosch and the 2wd system is made by Teves) just applies the brake to whatever wheel starts slipping. When you are driving the car around on the street, power is delivered to all 4 wheels if you like it or not.

According to the shop manual, 38% of the torque is applied to the front. I have a feeling when you get to worst case scenario (front wheels are off of the ground) that 38% turns to 0%. It's like having a torsen diff. Jack one wheel up and it acts like an open diff. As long as the other wheel has some sort of resistance, then it'll transfer much more power than an open diff.

I still suggest doing some reading and see how these systems work. With a bit of knowledge, you'll find out that just doing things like "pulling a fuse" does not do what you think it should do.
mrshelley is offline   Reply With Quote