How does the superduty 4 wheel drive actually work?
 

The 4x4 systems I'm familiar with are Land Rovers. They're very simple to understand...full time 4wd and you lock the differentials when you need to.

I recently got a super duty 4x4 and am trying to figure out how the heck these things work.

I know I have a limited slip rear..so that doesn't lock.

At first I thought that 4x4 switch was open diffs and the hub locks were to lock the diff (the front one only of course.) That would make sense right? But, when I flip into 4x4 the front wheels don't actually spin unless I also lock the hubs. What's up with that?

So unless the center diff locks when you engage the 4x4 (which I'm guessing it doesn't?), the only thing I've got is the limited slip rear? Is that about right, or am I missing something?

If you have the electronic shift on the fly (commonly referred to as ESOF), then you have 2 settings on your hubs, Auto & Lock.

Auto - When selected, the hubs will (and I'm using this term loosly as they're finicky) engage/disengage the hubs based on your switch selection in the cab (2wd = hubs disengaged, 4wd hi/lo= hubs engaged).

Lock - When your hubs fail from a vacuum issue, etc (and this will happen, sounds like it already has with your hubs), you can get out and lock the hubs, flip the switch in the cab, and have 4wd. My hubs on my 2000 failed to automatically engage/disengage over time so I capped the vacuum lines and operated the hubs as if they were a "Free/Lock" setup. When in 2wd, left 'em in Auto so they were disengaged, then when I needed 4wd, move them to lock. In the winter, they stayed locked all the time, no harm in leaving them locked when in 2wd.

It sounds like your hubs are either shot or there's a vacuum leak since they're not engaging with the cab switch anymore. You have some options:
*If the hubs are shot, you can replace with the overpriced unserviceable factory ones, or get a set of aftermarkets (Warn, Mile Marker, etc)
*Chase down the vacuum leak, replace lines/T's as necessary
*Cap the lines, use the hubs in a Free/Lock manner as mentioned above

There's a TON of information on troubleshooting the ESOF system in this forum... The manual has this outlined too if you have one.

if ESOF is in 2x4 it powers only the rear drive shaft(in high)
if ESOF is in 4x4 high it powers the front and rear driveshaft(in high)
if ESOF is in 4x4 low it powers the front and rear driveshaft(in low)
if your hubs are original, the 2 settings will be auto and lock.
if the hubs are in auto, when you select either of the 4x4 settings a vacuum pulse is sent to the hubs to lock them to the front axle(if your defrost kicks on in 4x4 you've got a leak) in lock they are always locked to the axle(slightly more wear and tear on the front components)
front is open diff

The switch in your dash controls the transfer case. Your only choices are 2Hi, 4Hi, and 4Lo, meaning rear output shaft only, front and rear output shafts, and front and rear output shafts with an addition ~2.7:1 gearing reduction. There is no center differential, the transfer case outputs identical front and rear shaft speeds. There is no All-Wheel Drive option, and no way to only lock in the front axle without the rear axle.

The front hubs will mechanically disconnect the stub shafts from the wheels. Unlocked hubs means no power to the wheels, locked hubs means the wheels are powered. The hubs do not affect the differential at all. With your ESOF (Electronic Shift on the Fly) 4WD system, the switch in the dash will activate a vacuum pulse that will lock the front hubs automatically when the hubs are in the "Auto" position. The "Lock" position overrides the automatic function and ensures the hubs are locked. This is a failure point in the system, the vac lines often fail and the hubs won't automatically lock. Simple solution; lock the hubs by turning the dials. Better solution: fix the vac system.

In stock form, the front differential is open, with no means of locking it available.

The rear differential is a clutch based limited slip. They are prone to burning out, and an incorrect amount of friction modifier in the gear oil can prevent them from locking up. There are ways to know exactly what's going on in them, but replacement with a better unit is typically better than rebuilding the stock unit.

Aftermarket limited slip, auto locker, and selectable lockers are available for the front D50 or 60 front axle (MY dependent), and for the Sterling 10.5" rear axle. Warn makes true manual hubs that will delete the problematic ESOF vac lock hubs.

Silverbullet, thanks for the quick reply. I do indeed have the esof and I'm sure I have a vacuum leak as the ac flips to only the defrost vents when I flip to 4x4.

I have read up a bit on the auto vs manual hub stuff and consider myself at least somewhat informed.

But nowhere can I find anything on why we have hubs in the first place. I mean, like I guessed before, they don't have anything to do with the differential locking. And they really don't engage the front axle as that's done at the transfer case. So what's the purpose of having the wheels spin separately from the unpowered front axle?

They just lock the wheels to the axle shafts. They can unlock to prevent wear on the front end end, and in the case of aftermarket lockers, you can always unlock an individual wheel to allow for differential wheel speeds. It's mainly just a nice touch by Ford to give us easily replaceable hubs that aren't a part of the wheel bearings, and there's no reason to use drive slugs on these trucks since the front axle is only used in 4wd and not as part of an AWD system. If you'd prefer, there's really no problem running with locked hubs all the time, many people do for months at a time, but some people will say there is a MPG reduction associated with needless parasitic losses in the front axle. Personally I can't tell a difference when they're locked or unlocked.

The ability to unlock the hubs saves wear and tear on the drivetrain. If the hubs were locked all the time, the front driveshaft and axle shafts would be constantly turning (because the front wheels would be turning them), even if you were in 2wd.

Wow you guys are fast on the replies. Thanks so much.

Ok, I get it now. The front end wear makes sense to have hubs, especially on a big truck like this. No need to rebuild/replace the giant front diff any sooner than need be.

I like that the center is effectively locked when you need it. But I'm not a huge fan of not having any control over my axles. Guess it's not that common to have 2 wheels off the ground for these big trucks.

I probably will go for some manual hubs as my current ones are getting a bit hard to turn. I'd also rather have a simple system of on/off than have to worry about vacuum leaks and room to move for them to engage and whatnot.

Thanks again everyone for the helpful replies.

Warn Premiums are a good swap for your front hubs, but be sure to delete the vac lines off the pump, and plug the holes in the knuckles.

If you really want to upgrade your diffs....}> there's are e-lockers and air-lockers that would give you all the selectable control you want.

This may or may not help in the meantime (Guzzle's hub maint.). I did this on mine and they worked again for another 6-12mo or so before finally no longer being "auto's"...

Welcome to guzzle's 4x4 Autolock Hub Lubrication Maintenance Web Page

I wish the SD did have an all wheel drive option; how come it doesn't?

Because adding another option would mess up the assembly line. When the assembly line gets messed up, that pisses Henry off!

Those workers must be awfully PO'd, considering the number of options I got to choose from on the 2012!:-drink

Before giving up on the hubs, buy the $40 kit from Ford and take the hubs off and clean them up. Some fresh lube and new O rings and you should be fine for a while, unless you have actual vacuum line problems. Nice to keep the ESOF if you can. My hubs were almost impossible to turn until I cleaned them up, now they are fine (for another winter).