Lack of bucking on the street tells me that it's not locking up all the time at all. There was a situation I read about where an Escape had the rear end locked in fully and it bucked like crazy. Turning would also make a lot of noise. We aren't seeing any real symptoms to point to a full lockup. Also if that were the case it would overheat and turn off the rear end on the highway which is not happening.

I wouldn't worry too much about a little bit of metal flakes in either unit. You might have damaged it a little from running it low but if it's not making noise it's usually ok. With a failure the gear oil generally turns into a solid gray paste full of metal, not sure random flecks.

If the fluid was from the rear end it's possible that when the 4wd engages it causes the metal to trick the sensor into not working, it may work ok now, hard to say but I don't see a hard failure, I see at worse an electrical failure or computer issue. Have you tried checking your grounds associated with that unit yet?

 

It won't buck or make noise because the transfer is blown therefore no binding. The drive shaft isn't being driven.

 

The transfer case is a 90 degree gear drive, if it was blown you'd know it lol. There would be huge chunks of metal inside of it and the car would still be turning the transmission output shaft into the input of the drive. To put it mildly YOU'D HEAR SATANS DOOR OPEN UP

 

If the clutch is in the rear end and the engine is OFF, why would rotating the wheels turn the drive shaft?

Your also mentioned a "sensor". The Ford manuals don't show any sensor and there is "only" one two wire cable connected to the rear end.

There is a 2009 just purchased used in another forum right now, where they are able to rotate the drive shaft freely with all four wheels on the ground, engine OFF, in Park, rear wheels aren't getting power and no dash alerts of any kind.

 

If there isn't a sensor then how in the world is it telling you it's hot? You can send multiple signals over the same wires too and there is kinda a computer for the 4wd, so I see it being very possible. They might not list a sensor since it probably comes with the computer for the 4wd or comes with the axle and isn't a separate component.

A lot of times there will be some movement when you turn the tires going into the driveshaft, it's very thick oil back there. Ever heard of viscous fluid limited slip? They use it in radio controlled vehicles all the time to turn an open diff into a limited slip by using varying degrees of oil weight. Some slight transfer of power is normal especially at low temperatures. If you couldn't turn both wheels at the same time at all with it up in the air then I would think that the clutch was engaged, which for all we know the clutch may be a normally closed clutch and open up electrically.

 

They actually refer to it as solenoid in the book which it isn't because it's just like your A/C clutch coil. Your guess is a good as mine on how they do it.

What your describing is exactly how a transmission torque converter works. The drive shaft being connected at the other end to the transfer case, it shouldn't be able to move anything.

There have some transfer cases that rotated the drive shaft with all four wheels off the ground powered up but stop as soon a load was put on it.

 

Observed the car from the side: when accelerating on snowy surface the front wheels slip & the rear wheels appear un-powered (eg no slip at all).
Then again lifted rear axle to jack stands.
Visually inspected connector to active torque coupling on front of rear axle: no corrosion or other indications of poor electrical contact.
Rotated propeller shaft: rear wheels turned as expected.
However, with front wheels on ground & transmission in park: should the propeller shaft have been free to rotate? Have a salvage transfer case & input shaft & output shaft are mechanically connected (as expected) and if the input shaft is locked (ie Transmission selector in Park) why would the transfer case allow rotation of the output shaft?
Other item: when on dry pavement the 4WD system seems to work just fine, it only disables 4WD on slick surfaces when wheel slip occurs. With Chains on front wheels almost no slip occurs unless under heavy throttle. 4WD appears operable until I put heavy throttle and cause front wheel slip. Then 4WD disables.

Conclusion: on dry pavement with the wheels rotating at appropriate speed the system senses normal operation and signals to the active torque coupling fool the system into thinking all is fine. But when on slick surface where wheel slip can occur the lack of power to the rear axle causes the system to sense differential wheel speeds and then disable the 4WD system. Therefore transfer case has worn through the gears and no power actually transfers to the rear axle. (Note: the flakes seen in the drained oil was not "a few flakes;" there were a lot of them present.)
Thoughts?
PS All fuses checked out fine.

 

Have followed troubleshooting guide in Shop manuals. Have concluded the wiring is intact & electrical components functioning as required. No shorts to ground & the controller & active torque coupling are functioning electrically as required.
So back to purely mechanical items failing. Again the leadback is to the only item without any kind of electrical sensor that would set a DTC is the transfer case.
Thoughts?

 

When you reference the propeller shaft do you mean the shaft coming out of the output side of the transfer case and going to the rear? So basically the rear driveshaft is spinning freely when the rear tires are off the ground and it isn't getting resistance to turning from the transfer case? If that's the case it sounds like a stripped out gear or something to that effect in the transfer case. I can move my truck later and put it up on a jack and see what happens with it but I'm really thinking that the rear driveshaft should be directly connected to the transfer case. I was under the impression that the disconnect happens at the ptu.

 

Yes, propeller shaft goes from the transfer case to the rear axle (the power transfer unit).
So when you do that put the front axle on the ground and car in park. Rear axle up off the ground on jackstands. See if you can rotate the propeller shaft. If you can the two rear wheels will rotate in the same direction. If you can't rotate it, then when you rotate one rear wheel the other will rotate opposite.
Yes, my conclusion is a stripped out set of gears in the transfer case. and yes the power transfer unit is where things are disconnected. apparently then it is deenergized (ie engine off-key off) it is connected to the rear wheels.