BMW 325xi 2003 automatic sedan

[catj5]

I recently took my automatic 2003 325xi BMW sedan to the dealer for an inspection about a whistling/squeeking sound I hear when I start the car.
When he hoisted the car up the machine and pressed on the gas, I was
VERY surprised to see that only the back wheels spun. The front wheels
were not moving at all. I forgot to ask him this issue because I was
so focused on the squeeking/whistling noise.

However, I thought all wheel drive meant all four wheels are spinning all the time
when the gas is pressed. Can anybody help me on this issue?

THANK YOU.

[resqguy]

The only 4 wheel drive systems where all 4 wheels spin all the time are those that "lock" all 4 wheels and the power/torque is applied equally to all wheels. The BMW system is biased where the back wheels get most of the power and apply power to the front as needed.

With all 4 wheels off the ground I'm not sure what to expect. Did the dealer disable the traction control system?

[CiscoKid]

usually rear 70% front 30%

[gizzy]

This is due to how the differential works. Unless it is a locking differential, then power is only applied to one wheel at a time. On 4WD/AWD, the power usually will go to diagonal wheels (ie LR & RF or LR & RF) when their is no resistance. The wheels that spin are the ones with the least amout of resistence. The reason for all of this is when you go around a corner, the outside tires will spin faster than the inside tires. If the differential was locked, one of the wheels would be skidding across the surface.