How exactly does the AWD on the Dakotas work? I have an 02 QC 4.7 AWD, I was reading the MPG thread, and it seems that having AWD does not have much affect on gas mileage compared to a selectable 4WD. Are all tires constantly getting power, or how does it work?

 

my understanding is when your just tooling around town its rwd, but if you launch from a stop or go offroad it moves power to the front.

 

AWD systems work by having a differential in the transfer case, i.e., a center differential. The center differential works the same as the differentials in the frontend and rearend (allowing the front and rear wheels to turn at different speeds without stressing the driveline). Torque is split evenly among all 4 wheels, in the same way that with an ordinary 2WD with an open differential, the torque is split evenly between the two wheels.

However, AWD systems suffer from the same problem as any open differential system; i.e., in cases where there are differences in available traction among the wheels, the wheel with the least traction gets all the power so to speak, so that if a wheel loses traction and starts spinning, the other wheel will do little-to-nothing. In the case of an AWD system, you will have 3 of the wheels which normally get power, doing little-to-nothing if one starts to spin.

Fortunately your truck has the ability to lock the center differential if you so choose (a feature which most AWD cars, minivans, etc. lack), in which case it performs like a normal part-time 4WD system, meaning that even in the worst cases of low traction, you will always have at least one wheel in front and one wheel in back spinning.

Systems like on your truck were popular in the 1970s on fullsize trucks from GM, Ford, and Dodge. It was called "Fulltime 4WD" back then, and they used the NP203 transfer case. Those were gas hogs compared to their part-time 4WD equivalents, and many of them were converted to part-time 4WD using a set of "locking hubs" and a conversion kit for the transfer case. The reason for this is that the part-time 4WD versions of the same trucks had mechanisms to selectively engage and disengage the front hubs from the rest of the driveline (i.e., "locking hubs"), so with the front hubs "freewheeling" when in the unlocked position, you had significantly less rotating mass than when the hubs were locked, thus better gas mileage.

So why do part-time 4WD Dakotas not get better gas mileage than AWD Dakotas? Because Dodge provided no way to "freewheel" the front hubs on the part-time 4WD trucks; i.e., the front hubs are permanently engaged to the front driveline, so the front axles, differential, and driveshaft are always turning when you are driving, even in 2WD (though they are not directly powered, because selecting 2WD disengages the transfer case from the front driveline). That rotating mass is hogging gas, whether the engine has to directly power it or not; thus little-to-no difference in gas mileage between driving a part-time 4WD Dakota in 2WD mode, or driving an AWD Dakota.

If part-time 4WD Dakotas had "locking hubs", then you would get significantly better gas mileage from them while "freewheeling" the front hubs as compared to an AWD.

 

Wow,.. Good explanation Maxim.

 

So basically the only difference between the 2 is that if an AWD dak is in a low traction system, power can get sent to front tires. To get power to the front on a selectable t-case, you have to put it in 4WD. And on selectable 4WD and AWD daks, the front driveline is always engaged, just not getting power all the time?

Good explanation, thanks for that.

 

Don't forget the differential in the transfer case. Without that you couldn't drive your AWD on dry pavement.

 

At last, someone with a proper explanation! I got fed up trying to explain that there will be NO difference in MPG between a 4WD and AWD Dakota, (all other things being equal). The important word here being DAKOTA. The lack of free-wheeling hubs means that everything in the front drivetrain is always turning. Doesn't matter if its directly from the transfer box (AWD) or via the rear wheels pushing the truck along and therefore turning the front wheels (4WD), it all uses the same amount of gas!
Personally I wouldn't be without my AWD. What's the point of carrying all the extra weight and friction for a system that you can only use off road or in snow? Even with its "open" centre diff, the traction available on wet roads is astounding (can't break traction from a standstill in a straight line) and makes a real difference pulling into traffic from side roads! Until a wheel breaks traction, each wheel only gets 25% of the torque so is far less likely spin. If only they made a TruTrac for the center diff, it would be perfect!

 

That's funny; I was thinking the exact same thing. Furthermore, three TrueTracs (or similar worm-geared LSDs such as Torsen or Quaife), front, center, and rear, would make for the ultimate fulltime 4WD system in my opinion. Even better would be if they could have the function to manually lock each one independently from inside the cab, i.e., a separate locking control for the front, center, and rear (though with three TrueTracs you would rarely need to bother with locking any of them).

 

With AWD, torque is divided equally between all 4 wheels when driving; not just in low traction conditions. This can be done on dry pavement without stressing the drivetrain because of the center differential which allows the front wheels to turn at different speeds than than rear wheels. With a part-time 4WD system there is no center differential, so when 4WD is selected, the front and rear driveshafts are locked to each other so it can't allow for different rotational speeds for the front and rear wheels, which is why you shouldn't drive them in 4WD on dry pavement.

Correct.

Correct, except your statement only applies to part-time 4WD Dakotas. With AWD Dakotas, the front driveline is always engaged and always getting power; though in cases where one wheel loses traction (e.g., if one tire is on ice and the other three are on tar), "torque reduction" comes into play (which is a problem inherent to open differentials), which results in the wheel that lost traction spinning wildly and the other 3 doing nothing to propel the vehicle.

When I first got my license as a teenager in the early '90s, the first vehicle I drive around in was my father's 1976 Chevy fulltime 4WD 1/2 ton pickup. At the time I didn't understand how the system worked. It certainly got great traction on snowy roads without having to move the transfer case shift lever to anything different than you used for normal dry pavement driving.

One time I was driving up an icy dirt road that had a thin layer of powdery new-fallen snow on top of the ice (very slippery conditions). Being an inexperienced driver at the time, and seemingly having great traction, I barrelled up that road at like 45 MPH. Needless to say, I soon lost control of the vehicle and ended up perpendicular to the road with my front tires sitting down in the ditch on top of frozen ditch water, with the rear tires up higher on the road. This created a condition where only the front tires would spin and the back tires did nothing. I thought Dad's truck was broken, because why else would it be acting like a front-wheel-drive? I didn't know what the point of the 4WD lever on the floor was, because it was "fulltime 4WD", so I was already in 4WD; what did I need a lever for?

Of course, I now know that the truck was operating correctly, and the reason only the front tires were spinning is because they had less available traction than the rear tires (and the reason both front tires were spinning is because they both had equally poor traction due to both being on the same ice sheet). If I'd shifted to 4H or 4L Lock I would have forced some action from the back tire(s) too (because it would have locked the center differential), but I knew nothing about that.