All-wheel drive is a type of four-wheel-drive system or mode that automatically shifts power between the front and rear wheels to maintain traction and is usually adequate for lighter-duty winter driving and light off-pavement use. Four-wheel drive can reasonably be used as an umbrella term for any system that powers all four wheels, but in the context of this-or-that, the term is often used to describe such a system or mode that excludes the properties of AWD and is heavier duty, meant for serious off-roading.
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The above description is the best we can do at this point in history, because the definition of AWD and any distinction between it and 4WD has become less ironclad as technology has evolved and people (and automakers) have begun using terms however they see fit. AWD is so different from how it started out decades ago — and there are now so many exceptions even to my good-faith definition above — that I suspect one or the other designation isn’t long for this world, though it won’t go without a fight, like the long-obsolete term “crossover.” The best approach for consumers is to focus on what a vehicle and its features can do, not merely what they’re called.
Look around the internet (if you must), and you’ll find explanations different from the one you see here — some from smart people who insist that, if a system has a particular piece of hardware, it’s 4WD, and if it doesn’t, it’s AWD. There was a time when it was that simple, when AWD and non-AWD were opposites in most ways, but those days are long gone. To grasp all of this better, it might help to understand what “all-wheel drive” originally meant, and how it’s changed.
No Intervention From the Driver?
When early AWD came along, most 4WD systems had to be turned on when needed and then off again when decent traction was restored, so one of AWD’s big plusses was that it did the work for you — so much so that there were no controls at all. That changed with the increasingly common AWD lock button that divides torque 50/50 between the front and rear axles, ostensibly for slippery surfaces. Wait, doesn’t the AWD system already automatically compensate and shift power? Yes. Most AWD systems lack this button, and its necessity is indeed questionable. In what’s usually considered an AWD system (the light-duty type), this button doesn’t lock the front and rear axles together as solidly as an old-school 4WD system does; it merely pre-activates the same clutches that would otherwise react once underway and wheels start slipping — but because it’s done beforehand, the lock is more reliable and it can help prevent unnecessary overheating of the center differential that might result as its clutches engage and release repeatedly while attempting to achieve the same ends: steady forward progress.