There are other threads about this but they refer to certain models etc.
I thought I would post a new one with the wording straight from the 2012 owners guide. I tlooks like Ford monitors this site and has made changes to keep up with the most popular mods from owners of earlier models.

Electronic locking differential (ELD)
The electronic locking differential (ELD) is a device housed in the rear
axle which allows both rear wheels to turn at the same speed. The ELD
can provide additional traction should the vehicle become stuck. The
ELD is electronically activated by the driver and can be shifted on the fly
within the ELD operating speed range. It is intended for use in mud,
rocks, sand, or any off-road condition where maximum traction is
needed. It is not intended for use on dry pavement.
The ELD is affected by the following conditions:
• The ELD will not engage if the vehicle speed is above 20 mph
(30 km/h) in 4x2, 4x4 Auto, or 4x4 High modes.
• The ELD will not engage if the vehicle speed is above or 56 mph
(90 km/h) in 4X4 Low.
• The ELD will not engage if the accelerator pedal is pressed beyond
50% during an engagement attempt
• In 4x2, 4x4 Auto, and 4x4 High modes, the ELD will automatically
disengage at speeds above 25 mph (40 km/h) and will automatically
reengage at speeds below 18 mph (30 km/h).
• In 4L (4X4 low), the ELD will automatically disengage at speeds above
62 mph (100 km/h) and will automatically reengage at speeds below
56 mph (90 km/h).
• The AdvanceTrac system has the ability to take over control of the
ELD and disable it during driving maneuvers when necessary.
Activating the electronic locking differential (ELD)
The ELD can be turned on by
pulling the 4WD control ****
toward you (4x4 vehicles) or by
turning the ELD control **** from
OFF to ON (4x2 vehicles).

 

Thank you. I had asked exactly this question in other threads and searched google repeatedly. This text above is by far the best & clearest answer yet for 2WD/RWD 2012 F150s with the electronic locker option installed.

I assume it goes without saying, but when the switch is OFF, the rear differential acts like an open (aka regular) axle? I think other offerings out there have managed to provide limited slip behaviour full time and then go to full-locking on a switch.

 

56mph in 4L? Boy I'd love to see that!

 

Tried mine yesterday in very wet ground. When I pulled in I was sliding all over. Pulled the **** and it was just like being in 4WD. My 2012 pulled straight through the wet stuff, up the slight incline and out on the main road. Wheels never spun.

 

vv

 

Well that is informative, I looked last night for an explanation on what Two-speed automatic 4WD (4x4) is. It is listed in the brochure for the 2012 F150 as standard equipment on a Lariat, but when you build your own on the Ford website there is no mention of this. I am guessing it is a form of all wheel drive but I am not certain.

Mark

 

Don't assume anything.
The rear axle has a limited slip type function performed by your traction control called "one wheel spin control". It takes care of the open diff symptoms while unlocked from 2011 on and is one reason the locker use is being expanded in 2012.

 

Hmmmm... I wonder if the 2012 TCCM will work in a 2011, similar to the 2011 Raptor TCCM... Probably does but without the off-road switch stuff.

 

I wouldn't pay to try crossing model years. Some have by accident got the wrong part number for their year and the whole truck was non-functional and the part is non-returnable.

 

Oh, I'm all set. I already did the mod with the correct model year TCCM and it works great! I was just curious.