Originally Posted by nomo
As you've seen, the '97 & '98 Expeditions had a 2WD selection for the transfer case. This 2WD selection was eliminated in the later models and did not re-appear until the 2003s came out.
The A4WD selection is "automatic" 4-wheel drive. In this mode, the vehicle detects rear wheel spin and automatically engages the front wheels until the slippage is no longer detected.
For the '97/'98 models, A4WD uses vacuum to "engage" the front axle then, via viscous coupling, continuously sends ~20% of the power to the front wheels. When rear-wheel slippage is detected, the driveline sends up to 50% of the power to the front.
When Ford eliminated the 2WD option from the transfer case, they also changed the way that the A4WD system works. We never had one of those later models, so I didn't care how to find out how those systems worked. I'm sure you could find out with some aggressive searches.
Half-right... The A4wd description is right on. Another way to describe it is 'rear-wheel-drive-until-the-rear-wheels-slip.' The main difference between the 97/98 and the 99-02s was the center axle disconnect. In theory, the vacuum disconnect motor would break the passenger side axle shaft, allowing the diff carrier and pinion (and, therefore the front driveshaft) to stop turning. In reality, it still moved due to the design of the guts of the t-case.
The system does not, however, use a viscous coupling. It's an electromechanical unit controlled jointly by the GEM and/or the Torque On Demand Relay (which itself is controlled by the GEM). As far as I know, the only true all wheel drive transfer cases Ford ever used were the 44-06 cases used in the 2nd-gen Explorers equipped with a V-8. (and those did, in fact, utilize a viscous coupling, but were not selectable and had no low range) The A4WD system used the TOD relay to vary the duty cycle
of the transfer case, effectively varying the torque split, but the ramp-up from the minimum 13% duty cycle to the full 100% duty cycle was almost instantaneous, so for all intents and purposes, the system was either engaged or disengaged.
The t-case on the 97/98 works exactly the same as the t-case for the 99-02s. It's an automatically engaged system. The 99s ditched the center-axle disconnect front axle for a live front axle, and ditched the 2wd position since the front drivetrain was turning anyways. Yeah, you lose some of the control to a mediocre (at best) program, but it did increase reliability by eliminating the trouble-prone electro-vacuum-mechanical CAD system.
FWIW, you can wrestle control back from the system on the 99-02s by performing the 'brown wire mod'. The entire system is virtually identical (albeit super-sized) to that found in an Explorer. One snip of a wire, a simple toggle switch, and you get true 2wd, as wella s 2wdlow when you want it, and full auto (i.e. stock) function when you don't with the flip of a switch.