Is there any compelling reason not to leave a truck equipped with 4A mode, in that mode all the time?

 

Wear and tear and mileage.

 

I am not sure how it is for the Ford trucks, by my day job is at a GM Dealership. When "our" trucks are left in auto (same as Ford's 4A), there is a clutch that is engaged and is always spinning. That adds noise but also wears on that clutch over time.

 

Nope, none at all. You can keep it in 4A forever. I had mine in it for 6+ months on and off. Many AWD cars you don't even have the option including my late model Jaguar XJ. Always in AWD.

 

The milage impact will be minimal and I would bet not measurable.
As far as noise I've never had any out of mine when in 4a.
Could the clutch(es) wear? Maybe but it's not because it is engaged and spinning. Clutches wear while engaging and disengaging.

 

The fuel mileage loss is measurable on my truck. I can tell when it is in 4A versus 2H. There is slightly more noise and it is slightly more sluggish. It is not a lot, just a little.

 

How much? I haven't been tracking it or paying attention to any difference.

 

I can't tell any difference in noise. I'm thinking, when driving in 4A on dry pavement, there would be minimal if any clutch engagement, but I'm not very knowledgeable on how it works vs. 4H.

 

Realistically there is little chance of any meaningful wear. Hubs are engaged so the steering may feel a little tighter especially on lock to lock turns. Small mileage hit from parasitic drag.

Hell, it may be better for these parts to be turning rather than sitting idle for long periods of time.

 

The only major downside to keeping your truck in 4A all the time is what happens IF you want to "give it the beans" while in 4A and needing/wanting to turn left or right. The more power you command, the more the transfer case will command the clutch pack to engage more and more fully. The more that clutch pack engages, the more the driveline is going to behave like a "standard" 4WD system and be less and less tolerant of different speeds front vs rear output shafts. That is very stressful on the system. As long as you're not accelerating hard in a straight line (or nearly so), you'll just have a little more wear on the Transfer Center clutch pack.