100710-1904 EST
bill:
First, I will describe how the Aerostar works. This is a full time all wheel drive with a differential in the transfer case. The gearing is such that under normal conditions about 60 to 70 % of the drive torque goes to the rear end, and the balance to the front end. The rear end has a limited slip differential, and the front a standard differential.
There is a front and rear tooth wheel in the differential. Associated with each is a Hall sensor magnetic pickup. If the difference in the differential count between these two sensors exceeds a few counts in a given time, then the electronic control energizes a clutch that locks-up across the differential. This lasts for about 6 seconds. The assumption is that slip is occurring between front and rear when this difference is larger than the threshold. What this system lacks that would make it better is the electric clutches in the differentials such as used in the Jeep Rubicon.
Following is my guess on what is done in the Escape. This is essentially a front wheel drive car with the addition of a full time solid transfer case to supply some torque to the rear. The clutch at the rear just in front of the differential is similar in size and power capability to that in the Aerostar.
From our previous discussing I believe the maximum current to this clutch is in the range of 10 A. This would mean maximum power would be in the range of 120 to 140 W. This is very similar to the power dissipated in the large electric clutches we use in the pinion preload adjust machines. These clutches are good for upwards of 1000 #-FT before the air gap is modified for quicker dropout.
You have also indicated that under normal driving conditions that substantially less current is supplied to the Escape clutch than the 10 A level. This means less torque, but some torque.
There will always be some difference in front drive shaft RPM vs rear, and this changes with tire pressure, stopping, accelerating, and turning. Most of the time this is a very small amount. At 2000 RPM for the drive shaft the difference might be more than 0.1% and less than 1%. If as high as 1%, then it is 20 RPM. Suppose under slip conditions the torque slip point of the clutch is 200/4 = 50 #-FT. 20 RPM * 50 #-FT / 5252 = 0.2 HP = 142 W. Not a lot of power to be dissipated in the clutch. It will produce clutch wear. However, it is providing all wheel drive that improves stability. When real solid four wheel drive is required the clutch would lock-in with full current.
Can you instrument to monitor the front and rear pulse rates to determine how much difference there is? If it seems large, then you could see if changing either front or rear tire pressure to reduce the difference would reduce the noise.
Does my guess at all seem like it is what they are doing?
.