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I thought that the difference betweeen all-wheel drive and 4 wheel drive was that 4 wheel drive refers to having a low range. I also thought that having one wheel turning on each axle meant having a locked transfercase, i.e. the difference between high and high lock, or, if you have it, low and low lock, and certainly, high and low lock. Please correct me if I am wrong.
4WD is locking the t-case. It has nothing to do with high or low range. In the stock Hummer the only mode available in low range is 4WD, while in high range both 4WD and AWD are available.
-- very long answer follows --
The terms All Wheel Drive (AWD) and 4 Wheel Drive (4WD) are really "terms of the art". That is, they have special meaning within a context (art) that may or may not be the same as the common (or lay) definition of the word(s). In this case, they are not the same.
(The terms "power" and "torque" are also "terms of the art". I am using them here in their "lay" definitions.)
AWD is a term applied to a vehicle that drives both front and rear axles by means of a differential between the two drive shafts. In our trucks, that differential is in the transfer case. (This is separate from the differentials that are in the front and rear of our trucks.)
The term 2WD is applied to a vehicle that drives only one axle, either front or rear. The term FWD (Front Wheel Drive) is sometimes used for those vehicles that drive only the front axle.
4WD is a term applied to the mode where both the front and rear drive shafts are driven without a differential between the drive shafts. In our trucks, this means locking the t-case differential. In non-AWD vehicles, this usually means engaging the (otherwise completely not driven) front drive shaft. (I am not aware of any 4WD-capable vehicles that drive only the front axle when in 2WD mode.)
In the simplest sense, a differential takes input torque from a spinning shaft and splits it between two output shafts. The differential applies all (or most) of the output energy to whichever output shaft spins faster or more easily. This is useful whenever the two output shafts must turn at different rates, such as when a vehicle is turning.
In AWD, the center differential splits the output torque between the front and rear driveshafts. Whichever spins faster or more-easily gets the power. Similarly, the differential on each axle sends the power to whichever side spins faster or more-easily.
When in AWD, if you loose traction on one wheel, here is what happens: all the power sent to that axle is then sent to the slipping wheel, and all the power from the engine is being sent to the drive shaft for that axle. Because of this, only the wheel with the worst traction is being powered.
When in 2WD, if either wheel on the driven axle looses traction, it is given all the power.
When in 4WD, both axles are driven equally, regardless of traction at the wheels. On each axle, all of the available power is given to the wheel with the least amount of traction (or that is turning faster). Therefore, if one wheel on each axle has lost traction, the vehicle will no longer be able to move. This commonly happens off-road when one front and the opposite-side rear wheels are on high spots. The other wheel on each axle spins freely, and the truck will not move.
One more note about differentials. The total rotation in (from the drive shaft) must equal the total rotation out (to both axle shafts), factored by the differential gear ratio of course. Here is what that means: if you both wheels on an axle are moving at 10 mph and one of them stops moving, the other will spin at 20 mph (assuming that the engine and tranny speed have not changed). This means that if you are driving along and one of your wheels begins to spin, it could be spinning twice as fast as it was turning. In fact, if you begin to roll backwards, the spinning wheel will turn even faster.
The reason that differentials are needed is because when a vehicle turns, the outside wheel must travel farther than the inside wheel on that axle. If the differential were locked at that time, both wheels would be forced to turn the same distance. This can cause steering and handling problems, particularly on a front axle.
The reason that a center differential is needed in an AWD vehicle is basically the same. When turning, the front axle must travel farther than the rear axle, and so must turn at a different rate. When in 4WD, the front and rear axles must turn the same amount, and again some handling issue may be noticed. In good traction, you will hear tires chirping as they are forced to turn faster or slower than the vehicle is moving. In lower-traction situations, you will probably notice the vehicle will tend to skip or slide to the outside of the turn. This causes the turning radius to be slightly larger in 4WD than in AWD.
Because of the weakness in the way that differentials distribute power (i.e. to the wheel with the worst traction), a variety of devices and systems have been developed to overcome this weakness. One of the most talked-about is the "locker", or locked (axle) differential. Locking a differential forces both wheels on that axle to turn at the same speed, regardless of traction. This can greatly increase the handling problems during turning. On high-traction surfaces, the tires will chirp even more. On medium-traction surfaces, the turning radius will be even larger. On low-traction surfaces, the turning radius may disappear altogether. A fully locked truck on slick surfaces tends to go straight regardless of how you turn the steering. This is why the Detroit and other automatic lockers are generally not used in the fronts of vehicles. This is also why the "safety system" in the AMG e-locker setup will turn the front locker off if it is left on for too long (1 or 2 minutes, if I remember correctly).
Other "limited slip" system have also been developed to help with the limitations of standard differentials. In high-traction situations, these differentials allow the axles to turn at different rates. When there is less traction, some power is given to each axle regardless of spin speed (as long as the speeds are close to the same).
Technically, the Torsen differentials on the Hummer are not "limited slip" differentials. They actually sense the difference in torque between the two wheels, and apply power accordingly. If the two sides have similar (but not identical) speed, both will be driven. If they have much different speed, the differential will drive the faster one almost exclusively.
Another system used is the "traction control" system. TT4 in our trucks. This system applies the brakes to a spinning wheel. This makes the other (higher-traction) wheel on that axle spin more easily, so the differential sends the power that way instead. BTM is a variation on this same principal - with brakes applied to both sides of an axle, the differential sends power to both sides, effectively locking the differential. The torque-sensing capability of the Torsen diffs makes BTM much more effective than it would be on an ordinary "open" differential.
During BTM, you will see both wheels on an axle turn at the same rate, even though one may be up in the air. During TT4 operation, you see one wheel stop (usually briefly) as the other wheel on that axle turns.
In order for TT4 to know which wheel(s) need to have the brakes applied, a sensor is installed at each wheel to measure the speed of that wheel. The computer then compares the speed difference between the 4 wheels and makes a decision. The computers are easily confused if the speeds of all the wheels is changing rapidly. That is why a steady throttle / engine rpm is required for TT4 to work correctly.
One more note about TT4 -- it is also an ABS (Anti-lock Brake System). The way that the computer knows whether to apply the "traction logic" or the "braking logic" is by whether the driver has applied the brakes. If the brakes are on, it is ABS. If the brakes are off, it is traction control. TT4 traction control WILL NOT WORK if the driver presses the brake pedal.
For anyone who has made it this far, my apologies if I have been too long-winded. This is a complex and commonly misunderstood topic. I hope I have helped shed some light.
That is one of the best explanations I have ever seen typed.
For those of us that need to see animated graphics showing you what Dave described, go to this site:
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