I think I can shed some light into this. I've worked on these systems before. Some things may have changed over the years, so feel free to insert a correction should I be in error ... I write this only to take what others have said and put it in one full description. I am speaking about the SuperDuty hubs from OEM Ford, not other designs. This also would not apply if you've "upgraded" to an aftermarket hub revision that eschews the auto-hub system.

The OEM systems are vacuum operated, and electronically controlled. That applies to both the SOF (shift-on-fly) ***** on the dash, as well as the manual t-case lever systems on the floorboard.

Here's how they work:
The hubs can either be manually or "automatically" locked in. You can either get out and turn the hub lock by hand, or leave it in auto mode. When done manually, you are using a cam ramp to over-ride the unlock spring, forcing the dogs together. When in auto mode, the vacuum draws on the chamber, which overrides the spring. Two roads to the same destination. Manual overrides the spring and connects. Auto overrides the spring and connects. When in "auto" at the hubs, once you disengage the 4x4 system, the spring should disengage the hub lock. If you take it out of 4x4 but leave the hubs manually engaged, they stay engaged even though the t-case is disengaged, until you manually unlock them again.

As for the t-case itself, the SOF system uses an electronic encoder motor to engage it, versus a manual lever. However, BOTH systems send an electronic signal to the vacuum circuit to draw a vacuum on the wheel hub. Hence, both use the same system to engage the "auto" portion of the hubs.

What we really have is two t-case systems, multiplied by two hub mode choices. The t-cases (auto or manual) give us the range choices of high and low. And they manifest into drive in this manner:
SOF t-case x auto hub
SOF t-case x manual hub over-ride
Manual lever case x auto hub
Manual lever case x manual hub over-ride

As for the t-case itself: we have 4x2H, 4x4H, 4x4L. What is not present (at the factory level) is 4x2L


As for the OPs question, there are times when using a 4x4 system in low, without 4x4 engagement at the wheels, might be preferable.
Example: perhaps your trying to park a large RV into a tight spot in the RV park and you want very precise low rpm control and very low speed due to the tight fit and perhaps even an incline in the parking area. The advantage to "low" range in the t-case allows for about a 2.5:1 reduction in any equivalent throttle input. You get more control via the speed reduction, as well as a torque gain. But doing this in 4x4 can be a PITA if you're on pavement. Hence, the desire to have 4x2L is born. You want "low" t-case range, but don't want 4x4 drive.

There are a few ways to gain this in the SuperDuty trucks. If it's a very rare thing, just go unplug the vacuum signal line out of the lub locks; do it prior to starting the maneuvers. On most trucks it's up near the firewall; I think it varies from year-to-year so you'll have to find it. If you want this option to be available often, I suggest installing a switch that will "open" the circuit electronically, blocking the signal to the vacuum control. Either way, once you interrupt the vacuum signal, then just put the t-case in 4L and have at it; you'll have 4x2L.


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From the factory, the trucks with the manual transfer cases have the manual locking hubs. The trucks with the switch on the dash that controls the transfer case have the auto or vacuum controlled hubs. Two systems, not four.

 

The manual case is also shift on stop, not shift on fly, so with a manual case the truck has to be stopped to engage 4wd, at least this is what I was told by a dealer so I could be wrong about it.

 

Edit: Shift in and out of 4hi as you would with esof. 4 lo is a different ball game, both t-cases need the tranny to be in neutral I believe.

 

Hi Steve, welcome to the forum I never thought about getting a 2X4 low before but sounds like a good ideal and I could see a use for it. Have you come up with a way to wire it up to your upfitter switch? If so please share.

Thanks Allen

 

While it may not be true of all years of SD, I do know that for a time the manual t-case models did come with auto/manual hubs from the factory. I've worked on them personally.

 

I was in four low today