Yes, it does impact MPG and wear and tear.

One of many posts on this subject: https://www.f150forum.com/f118/4a-mode-337230/

 

Excess strain on the drive system before it disengages can also cause damage over time. More of an issue on slick surfaces and icy patches. Example being that you're rears are spinning on a slippery patch so it engages the front wheels that have good dry grip and creates a shock to the drivetrain. Bad if you're trying to accelerate hard like pulling out of a parking lot for example.

This guy did just that pulling out of a parking lot in snow and broke his axle : https://www.f150forum.com/f118/1st-d...ot-tow-372238/

 

 

I'm no expert on this, but sounds like you're giving experiences from using 4H on dry surfaces. The OP asked about 4A which is more like AWD (not exactly), but is safe to use all the time and on dry surfaces. Of course, wear and tear, etc. I only use mine when it's raining hard or patchy snow and ice (rare in SC). If it's dry, it's off.

 

You didn't click the link... He was driving in 4A when he broke his axle. And that's what I'm talking about. 4H doesn't engage and disengage, it's always engaged. The issue is that it's NOT anything like AWD at all. The system needs to disengage when you're on a surface with grip and trying to make a turn. The problem comes with quick acceleration and surfaces with uneven grip because the system cannot respond quickly enough.

 

I'll admit I didn't click the link before I posted, but I just did. I don't see anywhere in the post where he says he was in 4A (didn't read all 17 pages if he clarified), He consistenly says "4x4" which to me means 4H/4L. I know it's a Limited and I know he has 4A, but he also has 4H and 4L and in snow many people feel 4H is a safe choice.

Let me know what I missed.

 

Post #82 in link. He was using 4A.

 

There's a little bit of confusion in the posts above.

Long story short: There may be some slight wear and tear and tiny MPG loss, and it's best to use 2Hi most of the time, but it won't hurt anything.

Keep in mind an F-150 with 4A has an entirely different transfer case than an F-150 without it. A truck with 4A has a Borg Warner Torque-on-Demand transfer case.

In a regular F-150 without 4A, the transfer case mechanically locks the rear drivetrain with the front when in 4Hi or 4Lo. It's either on or it's off. Engaged or Disengaged. You're either in 2WD (100% rear, 0% front) or 4WD (50 / 50 split front and rear).

In an F-150 with 4A, you have a different transfer case altogether, that instead of mechanically locking the rear with the front in a 50 / 50 only split, it has something of a variable clutch mechanism that allows the transfer case to split the torque in real-time any way it likes, as fast as it wants, between 100 rear / 0 front, and 50 / 50 rear front. For example 70% rear, 30% front, of 80% rear, 20% front, etc.

When you engage 4A, the front hubs lock via the IWE system, and so the front CV axles and drivetrain will technically be turning "freewheeling", maybe resulting in some slight MPG loss and some wear and tear. Under normal driving conditions, when in 4A, the transfer case will send most if not all power to the rear wheels so there's no concern about binding when turning or otherwise. If the sensors in the truck detect rear-wheel slip, then the transfer case instantly applies the clutch-mechanism to the front drivetrain to drive the front wheels for added traction, and applies only the torque split the sensors deem necessary. You can always monitor the front / rear split percentage in real-time on the dashboard that shows this split.

As for the guy who broke his axle, I'm guessing just a faulty part, not related to 4A.

 

That guy with the Limited is thrown up here every time someone asks about 4A. I don't know what happened to his axle, but the evidence is pretty far from conclusive that 4A broke his axle. I realize no one in this thread said that, but it was implied.
Since we're treating anecdotes as evidence, I drive in 4A All. The. Time. Thousands of miles, in and out of parking lots, in wet and dry and snow and dirt and pavement. I have looked for, but have not found, any detectable difference in mpg between 4A and 2H. 4A does not "engage" and disengage, like shifting a gear. It comes on slowly, like a clutch, because that's actually how it works. Is it possible that certain conditions could "shock" the drivetrain? I guess so, but I haven't seen it. I do know that when I pull away hard from a standing stop, the system shifts power to the front _before_ the rear wheels start to slip.
And I don't think Ford would tell you it can be left on all the time, in all conditions, if it was likely to lead to premature failure.
BTW, one of the reasons I leave it in 4A is because that way I don't have to worry about IWE failure. my check valve is good, but why take the chance?
I do not use 4A when plowing because for whatever reason, Ford deletes the 4A option from trucks that have the plow prep kit and would otherwise have 4A. I don't know why they do it, but not knowing, I figure I'll use the drivetrain settings they believe are appropriate for a truck that is plowing.