A4WD vs. 4WH ?

I know it just works and works great. When I have A4WD on in the rain I can tell it's more surefooted

I have seen a lot of posts on the "Brown Wire Mod", and I don't really want to start messing with "stock"...
However, question is this: how much "drag" is caused by the transfer case / front diff when the A4WD is not "engaged" (i.e. computer not sensing the rear wheels spinning)? I ask for this reason - on a hill by my house which I travel everyday in either my truck (99 Expy 4x4 with 5.4 and 4r100) or a 2008 Suburban 4x4 (with 2wd, Auto, 4hi, 4 low) in "2wd only" and the Suburban picks up considerably more speed (i.e. crest of hill at 45 mph in both yields 48 - 50 mph in my truck, and over 50 mph consistently in the Suburban).

Any ideas?

A 1999 Expedition will not be as efficient as a much-newer 2008 Suburban or even a 2008 Expedition for that matter. Your Suburban may be producing more torque, have different gearing, tire size, etc. I do know throttle response will be improved on the newer Suburban over a 1999 Expedition. All those things must be taken into account. There is never a "silver bullet answer".


The drivetrain/front axle for Auto mode, when in good working order, has little resistance – virtually none. This "back-driving resistance" of the front axle when torque is not supplied was reduced even further with the 2003 model Expedition redesign.

Axle resistance also translates into mpg. To give you an idea – as far as axle resistance – the Two High mode offers only a half-a-mile per gallon of gasoline improvement over Auto mode. That’s how little resistance there is, and shows there is almost no improvement in mpg concerning Two High vs. Auto.

This mpg improvement figure is solely based on axle resistance, not how much Auto mode calls the center multi-disc differential into action. Naturally when the center multi-disc differential is transferring torque front and rear, mpg will increase a little more.

Excellent - thank you.
One note I neglected to clarify in my original post - when I said "...picks up considerably more speed...", I meant to convey those speeds were COASTING DOWN from the crest of a hill, not getting to the top of it...
That being said, I think your response still applies, and I greatly appreciate your thoughts.

Hammer

Thanks so much for this explanation. My Dad and I were just having a "heated" discussion about this, this morning, and this post clears it up for me!

What are your thoughts on this scenario:
When driving on snowy/icy roads in A4WD I notice the truck is harder to control through acceleration, coasting, and, especially, braking. However, when I switch to 4H, control is much better and everything feels more "stable". Is that an appropriate use of 4H? How much would tire type & condition affect the ControlTrac system's ability to appropriately compensate?

What about when the snow & ice is patchy? Based on the comments here, it sounds like it's just best to leave it in A4WD? For example, I was on a road yesterday, turned a corner, where there was a large patch of ice, and started to lose control. If I'm in an area where I know there's going to be a lot of patches of ice and snow, am I better off leaving it in A4WD, or should I temporarily switch it to 4H until I'm out of that area?

Thanks!

The issue with running 4WD "just in case" is that if the wheels cannot slip, you risk doing damage to the differential / transfer case. In your example, had you turned the corner in 4WD, and there had not been ice, you may have some binding occur, and thus damage.
Last week we had some ice here in ATL, and I was in A4WD almost all the time. When I got to places I knew were icy, then I moved it into 4WD and the truck was a champ.

The first week of January we got about 12" of snow. The roads were unplowed and I ran in 4WD Low because 1. I wasn't going over 40 MPH 2. I have air suspension and I wanted the clearance (air suspension raises a couple of inches in 4WDL). After the roads were plowed and they couldn't get down to pavement, I ran in 4WD High to get better grip until the snow started to melt and we started to see pavement a couple of days later, then I switched back to AWD. Once we were back to dry pavement, (a couple of days after that) I switched back to 2WD and the truck worked like a champ. I'm totally happy with it.

winter time I use A4WD, works fairly well. summer time 2wd unless im crossing a shallow river out in the country ill use 4wd

One of the things I've read is using 4x4 Low on ice and snow isn't a good thing because the extra torque to the wheels can cause them to lose traction. The possible exception being a deep snow where losing traction wouldn't be as big a deal. With my '11 EL, I usually drive around in 2H. I'll usually switch to AAWD during wet conditions (just to periodically use that mode) and patchy snow/ice conditions. I'll use 4H and 4L as conditions merit. Usually only going to 4L in deep sand where I didn't air down enough and get stuck. Or, if I haven't run it in a while and am on gravel/sand/grass surfaces.

Here's a site with rich information on 4x4 systems and using 4x4 in various conditions. Note that AWD systems he is referring to are not the same as Auto 4x4 mode for our pat time 4x4 vehicles:

all 4WD/AWD on one page - a simple, easy to understand overview - all available 4WD, all wheel drive and 4x4 systems and their differences - 4WD concepts, their function, their layout, their purpose, their advantages

As an added bonus, here's a great reference to the Ford IWE system that engages and disengages the front wheel hubs. This is important to know.

Everything You Wanted To Know About The IWE System....And Then Some! - F150online Forums

Enjoy!