Originally Posted by
MCDavis
AWD is computer controlled. It's a rear drive biased system that engages the front drive when the system senses slip in the rear. The AWD system is also safe for use on dry pavement due to the nature of the transfer case and front end not being "locked" like a typical 4wd. IIRC the Harley also has a 4HI that allows the driver to lock in 4wd if the conditions warrant more traction.
The other models with selectable 4HI 4LOW are just that, a 2wd truck until you engage the 4wd yourself. 4wd should only be used in conditions where traction is limited (i.e. snow, mud, grassy inclines, etc). There is not enough "slip" in a traditional 4wd system to warrant use on dry or even wet pavement as the system components may bind and fail.
Thanks for the excellent explanation! If the HD's (and Lariat Limiteds) are available in 4x2 and 4x4 (which I assume from this thread is technically "AWD" + 4-Hi only), and the AWD only engages the front axle if slip is detected, then why is the HD/Limited mileage so much worse than the 4x2 model? The 4x2 does 14 city/18 hwy but the 4x4 does 13 city/16 hwy. That seems like a dramatic drop in fuel economy for a system that should not be engaged all that often. What am I missing here (other than additional weight from the 4x4 system)?