There seems to be some confusion among some of our readers as to the differences between all-wheel drive, full-time four-wheel drive, and part-time four-wheel drive, so maybe this is a good time to share with you the differences. Keep in mind that all four-wheel drive vehicles, no matter what the drive system is, generally have to have a transfer case. In reality, this is nothing more than a gearbox of one type or another that allows power to be diverted to both the front and rear driving axles. Do keep in mind that there are vehicles that do not have a low-range gear and were designed more for highway driving, as well as performance vehicles that have variation of some of these designs.
Part-time four wheel drive: Let's start out with this system as it's the one that more or less started the whole thing. If one wanted to dig deep enough, perhaps its use could be traced back to the 1890s, but we will deal with what the American public most likely had as its first encounter with four-wheel-drive vehicles, World War II, and the surplus Jeeps and trucks that came out of it.
This system uses a transfer case that allows for several different combination of driving modes. Two-wheel drive (2-Hi) is the normal driving configuration with power only going to the rear wheels. When additional traction is needed, the front axle can be engaged with a floor-mounted lever; however, there are other newer methods such as electric- or vacuum-shift motors that are actuated by a dial or a pushbutton on the dash. This position is generally referred to as four-wheel drive high-range (4-Hi). The transfer case generally, but not always, has a low-range four-wheel drive (4-Lo) position that in effect lowers the overall gearing by anywhere from 1.96:1 to a 4:1 ratio, depending on application. Some specialized transfer cases have even lower gearing. Some aftermarket modifications or specialized transfer cases offer the ability to drive the front or rear axle independently in either low- or high-range.
When in four-wheel drive, the front and rear axles are linked directly together and turn at the same speed. However, here is where a problem comes in-actually, a couple of problems. It's nearly impossible for the axles to turn the same speed due to variations in tire diameter caused by uneven tire wear front to rear, or differences in air pressure, or weight distribution, front to rear, which affect overall tire diameter. Then there is the problem encountered when turning-no matter how slightly: There is a difference in turning radius of the front and rear axles, which cause a difference in axle speed. These reasons are why this type of system can only be used in low-traction situations where tire slippage can take place and makes up for this difference in speed. When four-wheel drive is used under high-traction situations (e.g., pavement), the tires must be able slip in order to equalize axle speeds. If the traction is so great that the tires cannot slip, guess what happens? Yep, something has to break! This could be a twisted driveshaft, broken U- joint, or even driveline gears.
Full-time four-wheel drive: Generally speaking, this type of system is a direct cousin of the part-time system. As the name implies, the transfer case is always in four wheel drive. It has, depending on manufacturer and application, either a set of differential gears not unlike those in a rear axle, or a clutch pack, like a limited slip would have, within the transfer case. This allows for a "differential" in speed between the front and rear axle. There is also a four-wheel high and low-range that totally locks the differential action out, usually referred to by Hi-Lock or Lo-Lock.