Always meant to look this up when you mention about high failure rate of PTUs, tire scrubbing and/or driveline windup. The following is from the '09 FE/FEH Workshop Manual but describes a FE here. Nothing like this is shown for for the FEH.

Differential
The differential allows the halfshafts and wheels to rotate at different speeds during cornering and transfers power to the Power Transfer Unit (PTU) for All-Wheel Drive (AWD) vehicles.



The differential assembly consists of the following components:

• Differential case (part of the final drive carrier)
• Two pinion gears supported by a pinion shaft
• Two side gears supported by the differential case and halfshafts

When driving in a straight line, both front wheels rotate at relatively the same speed. This means both side gears are rotating at the same speed, as well, while both pinion gears revolve (but do not rotate) with the side gears. During cornering, the wheel on the outside of the turn is forced to rotate faster than the wheel on the inside of the turn. Since the side gears must now rotate at different speeds, the pinion gears rotate on the pinion shaft allowing the drive axles to rotate at different speeds while still transferring output torque.
Differential Cutaway View



Item Part Number Description
1 7F465 Differential housing
2 — Pinion shaft
3 — Pinion gears
4 — Side gears

 

This describes the function of the FRONT differential only. The front differential input (differential gearset case) is driven by the transaxle output (FE), or the CVT/PSD output for the FEH. The front differential, "open" differential", can supply torque to the left and right front wheels independently but always SOLIDLY to the PTO.

If F/awd were standard, not an option, the PTO ring gear would likely be bolted to the front differential case.

 

Look again. There's two diagrams there, FWD and AWD(4WD). The differential is between the front and PTU. The only problem is the the ATC in the rear is electrically controlled, so as long as torque doesn't exceed 50% there's no problem.

Ford's own function test is to command 100% rear wheel torque(create a bind or driveshaft windup) with their IDS and if the vehicle resists turning, it's working correctly.

Years ago I drove off a wind-blown snow covered slick highway onto a dry parking lot in full 4x4. As I backed into a parking spot, I heard a wheel hop. I couldn't shift out of 4WD into 2WD until I repeatedly drove forward/backward slowly turning the wheels. I've heard of people having to jack up one end and let the tension release.

 

I may have it wrong, but my feeling is that the fuse controls just the electronics that engage the clutches. the input shaft into the ptu is still turning. the drive shaft going into the differential will turn anyway as the car moves, as the differential does not dissengage the drive shaft (on some 4wd cars you can "unlock" the wheel hubs so that you are not turning the paliers that go into the differential (front ones for rear wheel drive vars). This is done to save gas.Some are manual (you need to step outside and turn a know on the wheel hub) some are automatic (once you dissengage the 4wd, you drive a bit back and that dissengages it) on some others, they do that by having a disconnect on one of the paliers, wich forces all the torque going into that axle to go out on the dissengaged side.

Our cars have no disconnects. Moving any of the wheels will cause torque or motion on the others. I had an old Land Rover that had the hand brake on the rear traction shaft, not on the wheels. If the car was moving and you applied the hand brake, locking the axle, as one wheel turned forward the other would turn backwards (that's what the differential does if one wheel is locked) Or lift the car up, and with the engine off (and the input shaft locked) turning a wheel one side turns the other one the other way. If both wheels are turning forward, the input shaft will turn too, with or without engine input to it. It turns, and everything inside the differential exactly as if pushed by the engine. Just less stress and friction.

On cars with free hubs where you could also dissengage the input shaft they recommend that the traction be engaged every once in a while, because since there is no connection to the wheels, and no power going into the shaft, the differential does not move and oil does not circulate on it. But since all is stopped, no damage is done either.

The shaft form the engine to the clutches turns always, moving the oil around. They just turn free from each other.

NOw pulling the fuse and having a manual free hub on the rear axle will amount to gas savings, as axle, shafts and differential are not being turned by the car motion... but that for our cars does not exist (and it will need to be for the rear axle)

 

Seems that color graphic picture in Post #61 doesn't show unless I have logged into the system and keep my browser open. Sorry if that confused anyone. In fact, it might be only viewable by me anyways.