I don't plan to do any work on it, I'd just like to have a good idea of how the 4x4 system in our trucks works. For example, which parts are spinning at all times? Which parts only spin when in 4x4 mode, and how does actuation of the 4wd work, exactly?

 

If you have an open rear diff, you have 2 wd. 3 wheel drive if you can lock the rear.

 

I was really thinking more in terms of which shafts are turning when in 2WD vs. 4WD Hi or 4WD Lo.

 

For an XL/XLT, when you turn the **** to 4 high an actuator in the transfer case moves to engage the front output and spin the front driveshaft. At the same time a vacuum solenoid under the hood engages both front hubs (integrated wheel ends in Ford speek). Power is split evenly front and rear with no differential action, so use only on loose surfaces. When you select 4 low a shift fork moves a set of gears in the transfer case to enable more gear reduction. You have to be going slow and in neutral to let it engage smoothly.

On lariat or higher trucks they use a set of clutches on the front output of the transfer case to send power to the front diff. In 4 auto the computer uses sensors to decide when to engage and when to disengage, allowing use on pavement without binding or other wear. In 4 high lock the clutches engage and stay engaged, for all intents and purposes acting the same as the lower level trucks. Same with 4 low. These trucks have the same vacuum actuated IWE setup as the XL/XLT.

All can be combined with an electronic locking rear diff that will send power equally to both rear wheels at lower speeds, automatically disengaging at higher speeds.

The rear transfer case output is always physically connected unless you select the neutral position on the tc, and this is only used for towing behind an RV.

 

So, when I'm in 2WD (XL truck), the front forward drift shaft and the front half-shafts are NOT spinning at all? I'm just ascertaining what are the parasitic losses of the 4x4 system when highway driving.

 

Correct. Unlike other systems that have the front axle always spinning and use a disconnect elsewhere, this minimizes power loss and improves fuel economy.

 

Weight and friction inside the transfer case.

 

This is such a terrible response, I hope no one takes it to heart.

Zero TX posted a great summary.

 

His response is technically correct, but not he explanation OP was looking for. Open diffs and 4wd are just not great at all.

As stowaway said: The short answer was already covered, but fortunately by design when in 2WD, the front driveshaft and half shafts are not spinning. The front hubs disconnect, and the transfer case disconnects.

I too am surprised the spinning mass in the transfer case is good for a 1-2mpg loss.

 

I didn't say it was a bad answer, I said it was a bad response. It conveys all of the wrong information.

Yes, you may not always get power to all 4 wheels, but that's not the end of it.