'01 F-350 SD w/PS. I've owned many Fords over the years. So I'm not unfamiliar with some of Ford's "better idea's".
Some with "auto" hubs and the like. (Swapped them for Manuals!)
This truck has what looks like manual hubs, says auto and lock. I understand from the owner's manual how to use them and the 2WD / 4WD high / 4-low switch on the dash.
My question is this (finally), is the front differential engaged all the time, or just when the "4WD" or "lock" hub turned?

 

The front axle is disengaged when the hubs are in Auto, and the switch is in 2wd. There are many posts about these hubs here, do a search and you will spend hours reading...

 

I usually do that but brain not working today.
Thanks

 

I have an 05 4x4, and the way i understand it is that the LOCK positon simply locks the wheels to the axle manually, then the switch in the dash only engages the t-case, giving 4x4. This is why you can 'LOCK' your hubs, and still drive around in 4x2 mode, its bad on gas and parts because your axle shafts AND your wheels are turning. But, if you use AUTO, then the switch in the dash egages the hubs to the axle and the t-case to the axles. This is better on fuel economy because only your wheels are turning in the front and not dragging the axle shafts with them. At least, thats how it is in mine, because when in LOCK, you do NOT have 4wd if your ESOF switch is not in 4wd.

Seems like if you had a vacuum leak, and your hubs didnt engage when you flipped the switch, then it wouldnt engage the t-case either, making the 'lock' function useless.

 

No The T-case Uses A Eletric Motor To Engage.

 

Your ESOF (Electronic Shift on the Fly) system relies on electronics and vacuum to operate. The switch on the dash engages both components (1) It sends a signal to the vacuum solenoid which pulls vacuum on the locking hubs (2) it sends a signal to the servomotor on the transfer case to engage either 4WD High or Low.
The positions of the hubs are: AUTO and LOCK. The LOCK feature is handy when any part of the vacuum system fails. The AUTO feature is the normal setting and allows the hubs to lock and unlock via the vacuum system.

There is no NEUTRAL position on your transfer case. It is in gear: Either – 2WD, 4WD high or 4WD low at all times.

In most cases the cause of malfunction is not following, or a complete misunderstanding of the operating requirements.
Operating requirements:
4WD Low:
(1) Bring truck to a complete stop. (if you have not engaged the system for an extended period of time you may want to get on fairly level ground)
(2) Depress the clutch/shift to neutral/shift to park
(3) Depress the brake
(4) Dial switch to 4WD Low

The ESOF system uses a high vacuum level generated by the vacuum solenoid, which is located on the fenderwell beside the battery to engage the locking hubs and a lower level to unlock them. In each case after the initial vacuum is pulled the hubs hold in the proper mode. A complete rundown of the locking hub system is: Vacuum signals are supplied to the locking hubs by system components, including GEM, wiring harness, solenoid, vacuum harness and vacuum seals.

As a first step in service, eliminate obvious mechanisms such as loose wiring and/or vacuum connections, and/or damaged vacuum lines. If one locking hub itself is defective it should be noted that the hubs work independent, i.e., if one hub is bad that does not mean that the other is defective – they are not connected in any way other than by vacuum supply lines. Along the same lines, if one is defective it can be replaced as an individual unit. The replacement of the hub will in no way affect axle, its balancing, or any other operational quality.

I am hoping to have a better and more expanded explanation by this fall so that Yeti can post it as a sticky. I know krewat has a few things to add. Maybe some other members. We all know the bombardment we’ll take about this come hunting, plowing, mudding season.

 

The only thing I have to disagree with you on is the vacuum pulses. There is a 20 second LOCK pulse, and a 5 second UNLOCK pulse. On my '01, the vacuum levels are both the same (whatever the vacuum reserve has in it), lock or unlock, just the duration is different, and nothing in my Ford manual mentions that during diagnostics, just the length of the pulse.

Two winters ago, the ESOF hubs started sticking and I got into the habit of locking them by hand. Nothing was wrong with them I could see, maybe they just needed a lube, but didn't care about it right away. I checked them out the way Ford says, found nothing wrong except they didn't want to lock or unlock when I wanted them to.

This year, one of my hubs got water in it, and took out the needle bearing the axle rides in. The other hub (that I always had problems with) looks and moves fine, just doesn't lock/unlock correctly. And believe me, everything looked fine. It's fun when the hubs lock in 4x2 but unlock in 4x4 Or one side does, the other doesn't and vice-versa.

So, I went to Warns anyway (part # 38826 for Premiums).

When it works, it works, when it don't, get Warns.