My 2004 4Runner has just exactly this button. It locks the center diff. (At the same time it disables the traction control and stability control). I find this damn useful in both deep snow and off-roading in soft conditions. It makes the car much more directionally stable in conditions where wheel slippage is likely. And traction control and stability control can deplete needed forward momentum in certain circumstances, and do more harm than good.

Lame. That said, the one and only time I used it, it was on a 300-400 yard-long, 20-25 degree gravel decline ................ with my 5,000lbs trailer in tow. No joke. I took a wrong turn while trying to get to WGI and wound up in someone's back woods (whoops!). While going down that decline, it actually felt like the axles were locking/locked up. Pretty impressive, but still lame.

This is exactly what I thought the HDC did. What the hell? I don't see the benefit to running the HDC via the ABS - I only see the negative (i.e. all what you mentioned). I drove a fully Manual Toyota HiLux in the Andes Mountains in Peru about 3 years ago - what a helluva truck! Those diff locks really got my jollies going. There's something about throwing around a fully manual HiLux (or something comparable) that makes you realize you haven't quite yet earned your man card. :bustingup

Well, my Toy also has HDC in addition to the locking TC. And you'd be surprised how useful it can be if you actually go off-road, (stupid to do in an X5!). I also own a fairly serious offroad Jeep, a lifted Grand Cherokee. I have not yet gotten around to changing the axle ratios, yet I am currently running 5" larger diameter tires. Going downhill can be quite scary at times; especially on slopes too steep to walk up. It's not that bad if it's steep and strait down, but I've been down trails that are ultra steep and have switchbacks. You must maintain a modest speed. The slightest wrong move and it would be pretty easy to swap ends and roll, or go too fast and hit a tree head on. The best approach is to just put it in the lowest gear and roll down. But if your ratios are not low enough, brakes are necessary if the hill is too steep. Lock a wheel and you could be screwed. My Jeep does have ABS, but no stability control. Descent control would be nice there; it would keep you from swapping ends. The idea is that the wheels do not all brake evenly; they brake as needed; yaw control and ABS help keep you going strait.

For the X5, the road near my house is not only steep, but curved as well. HDC might be useful if I ever get around to getting snow tires for it, and I want to go down the hill in the snow. With the current tires, it would be asking for an accident, HDC or not. But with 2 other more capable 4WD vehicles with off road tires, and 4 snows on my RWD DD, the current plan is to park the X5 when the roads are snow covered. Maybe in a couple of years I'll get snows for it.
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Good points. I like your collection of toys ;)