Being that I just recently aquired a V8 LE model Pathfinder, what exactly the "AUTO" mode on the transfer case selection switch does suddenly became something I found worthy of some research and testing. I do go off-roading, so how exactly this "AUTO" transfer case is better/worse than the more simple model is important to me.
First off let's define some terms...
The "Transfer Case" is a bunch of gearing/electronics/clutches/etc that connects to the output shaft of the transmission. It has two output drive shafts, one going forward to the front differential and one to the rear diff.
The "Drive Line" is everything between the motor and the road, including transmission, transfer case, drive shafts and axles, etc.
"4x4" not only means the vehicle has 4 wheels and all 4 get power from the engine, but also commonly means that the front and rear axles are firmly connected and therefore rotate together. This is no problem when off-roading or on slippery surfaces (snow), but on pavement when you turn the vehicle it will cause a lot of stress on your driveline parts as the front and rear axle try to turn at different RPMs.
*SOME* 4x4 vehicles have unlock'able hubs, so that the front wheels can spin freely without rotating the half shafts, front differential, and front drive shaft when the transfer case is not engaged. The Pathfinder is NOT one of these vehicles, the hubs are always locked, the diff and drive shafts are always spinning, the only engagement/disengagement happens inside the transfer case.
"AWD", or All Wheel Drive, commonly means that while power is given to all wheels, the amount of power delivered to each axle and the RPMs they rotate at is variable, which allows you to drive on dry pavement. The downside is that the AWD transfer case system sometimes is not strong enough to send 100% of the engine's power to one axle or the other if you are stuck with a wheel up in the air. Also they tend to be heavier and more complex.
Some AWD systems use a geared differential (often Audi and Subaru for instance, just like the diff between the left and right wheels, but a 3rd one between the front and rear axles). Others use a clutch to allow power to be transmitted only as demanded, but also allow slipping to occur so that the front and rear axles can rotate somewhat independently.
For the clutch type AWD systems, usually one axle is always driven, and the other receives power when desired by the driver or computer. My old Murano for instance, FWD, with an additional drive shaft going back to the rear diff, and an electronically actuated clutch pack. The Skyline GTR on the other hand, always RWD, with a clutch system to send some power to the front wheels as needed. Because of it's part-time nature, the clutch system and that end of the drive line (diff, axles, hubs) is sometimes under-built to be lighter, though therefore with less strength.
The Pathfinder has two transfer cases, the ATX14B and the TX15B. Let's look at the latter one first, the simple non-Auto transfer case.
The TX15B has 3 options on it's selector switch, 2WD, 4H, and 4Lo.
2WD sends power to the rear wheels only, period, end of story.
Switch to 4H, and an electronic clutch engages locking the front and rear axles together. It's a clutch, so it can be smoothly engaged as you are moving, but it is NOT designed to slip once engaged. Front and rear axles are locked together, so use this mode on slippery surfaces ONLY or you may damage your driveline. I think there's ample evidence from off-roading Pathfinders that the clutch is fully able to send 100% of the engine's power to whichever axle has traction, sometimes even more than the front differential can take without blowing itself to pieces...
To go into 4Lo you must stop and shift the transmission into neutral, because a direct metal toothed geared connection is being engaged between the front and rear axles, and also before both axles a reduction gear for Ultimate Power and Control (tm). The gearing ratio of 4L is roughly 2.6 to 1, meaning if 1st gear topped out before at 30MPH, now it'll only do 12 MPH. Be aware that 4Lo also disables the VDC electronic stabilization system, though ABLS still functions.
The ATX14B adds "AUTO" to the above 3 modes, but also changes a bit how the 4H mode works. As before the ATX14B primarily drives the rear wheels, but it has a variable "wet" clutch pack driving the front wheels. "Wet" meaning the clutch plates are lubricated and cooled by the transfer case oil. This may seem odd, to add slippery oil to something designed to create friction, but the point is that because it's continuously variable and slipping ALL the time, as friction and heat is generated what wears IS the oil, not the clutch plates that would otherwise inevitably wear out and need replacement. Use Auto routinely? Be sure to change your transfer case oil as at least as recommended!
There are two hydraulic oil pumps in the transfer case which provide oil pressure forcing the wet clutch plates together as requested by the transfer control system. One is gear driven from the input shaft, the other electric to provide pressure when the vehicle is stationary and up to 34MPH. If you are set to 2WD the electric oil pump will not operate at all at any speed, which is the only and probably VERY small MPG benefit to 2WD. I'll test this some time for giggles...should be truly zero difference if driving over 34MPH.
Some have said that "AUTO" means power is only sent to the front wheels if rear wheel slip is detected, but this is not correct. Page 15 of the FSM Drive Line manual (http://www.thenissanpath.com/filelib/R51B/DLN.pdf
) shows that power is transmitted to the front wheels based on *3* simultaneous and dynamic conditions. One is rear wheel slip, yes, but also more throttle input immediately sends more power to the front wheels. Also when braking hard and the ABS is operating, more engine RPM (engine braking force) corresponds to more power being transmitted, from in this case, the front wheels to the engine.
Also when at a stand still there is some amount of connection force from the wet clutch pack even at rest, and it's easy to test. Jack up one front wheel, chalk the rear wheels, set the parking break firmly, and shift the transmission into neutral. Set the transfer case to 2WD and try to spin the front wheel, easy as pie. Set the transfer case to AUTO and the front wheel is noticeably harder to spin. Set the transfer case to 4H and you can't rotate the front wheel at all anymore, same for 4Lo.
Now turn the engine OFF and do the same tests. With no oil pressure both AUTO and 4H spin freely as 2WD, while 4Lo maintains it's lock even with the engine off.
Now does the TX15B stay locked up in 4H even when the motor off? I don't know but would love to find out, and I bet it does if somebody can do the above test.
Also, does the ATX14B's wet clutch pack, when in 4H mode, provide enough clamping force to send 100% of the engine's power to the front wheels when a rear wheel is up in the air? I don't know, and not really sure how to test it conclusively... However there are various forum members who off-road the Auto transfer case without issue, and also the V8s have the even larger and stronger front differential from the Armada, implying they have designed the front of the drive line to take a lot of power from the transfer case. However ultimately I think it doesn't especially matter, if you're doing some rock crawling and getting wheels in the air I bet you're in 4Lo anyway which is unequivocally always locked.