Originally Posted by mikeyc
There are alot of different 4x4 and awd systems out there. Jeep alone make several types. Could someone explain to me how the 4x system on a 94 Sahara works. I don't have rear lockers, just a plain 4x that would be on a stock sahara. What gets the power, how much, what happens when slipping occurs, etc?
If this has been covered and I just couldn't find the previous post could you point me to it. Thanks.
WARNING! BOOK AHEAD! Jeep 4wd 101. lol Sorry if this seems a little more basic then you wanted but you did ask.
Ok, the 94 sahara, and just about all jeeps for that matter use a part time 4wd system. I'm sure you've seen the short little stick on the floor of the jeep just in front of where the cneter console is or would be? that's the selection lever for the transfer case. In the stock form, the case has 4 possible positions. 2Hi, 4Hi, N, or 4low. When the case is in 2hi, only the rear wheels of the jeep get power. The front wheels and the front drive shaft just free wheel. In 4hi, you have both axles getting power, like Canuk said, split pretty much 50/50. N is obviously Neutral, however it's not the same as putting your tranny in neutral. The transfer case itself has it's own neutral, so you could technically leave the jeep in gear idling if your case is in neutral (that's a good way to warm up your transmission in the winter time BTW. put the jeep in 2nd or something, the case in neutral and let it warm up like normal. helps ease the hard shifting on those really cold nights). 4low is the same setup as 4hi with one critical difference: there's a 2.72 to 1 gear reduction on low range, accomplished by the planetary gears in the transfer case itself. This reduction increases the torque applied to the wheels from the engine almost 3 times, at the cost of top speed. Most transfer cases use a reduction somewhere in this range, however there are cases out there, built exclusivly for off road applications that have reductions of 4 to 1 or greater. the lower the reduction, the slower you go (which is great for rock crawling) and the more torque or pulling power you have. Again, in 4low both axles receive equal shares of the power until one begins to slip.
To accomplish 4wd, the t-case has two shafts, the main shaft and the front shaft. The main shaft spins all the time, regardless of what range or mode you're in. This is what drives the rear axle in 2wd and in 4wd. When 4hi is engaged, the shift fork moved by the selection lever locks in a gear to the main shaft. That gear drives a chain which then turns the front output shaft, causing the front drive shaft to turn the gears and shafts on the front axle. Note that because both axles are coupled by the chain, both will spin at the same rate. This is why it's not good to drive in 4wd unless there's snow or ice on the road. If one wheel tries to turn faster then the other, like in a turn for example, the slower wheel will have to spin to catch up, or else the chain will stretch or bind. On slushy roads or in the dirt or mud off road, the wheel will slip enough that this won't cause much of a problem, but dry pavement is a different story. In 4low, the driving gear that turns the main shaft is changed so that it now drives a planetary gear which in turn drives the main shaft. the planetary provides the large reduction in much less space then a similar reduction done with standard gears.
So now you know how to get both drive shafts turning but there's still one other component Chrysler decided to throw in (in all their great wisdom
) and that's the infamous front axle disconnect. All it is, is a shift fork that slides a coupling collar onto both ends of the two piece axle shaft on the passenger side of the axles. The way an open diff works, is that it will transfer power to the wheel that has the least amount of resistance. the two piece shaft on the one side of the axle means that, if the fork doesn't couple them, then the diff will see that side as having the least amount of resistance (slipping) and thus will send all the power there. This is why so many YJ owners have problems with their 4wd. the fork fails to move, thus the collar doesn't connect the shaft pieces together and you have a front axle that doesn't spin in 4wd. The reason the fork fails to move is because of the vacuum motor that moves it. or more often then not, the vacuum lines that feed said motor. When you shift the t-case into 4wd you also activate teh vacuum switch on the case itself, which sends a vacuum to the driver side of the shift motor diaphram. The other side of the diaphram is opened to atmospheric pressure, and the resulting pressure difference does the rest, pushing the diaphram and the shaft that it's attached to over to the driver side. The shaft is connected to the shift fork which moves the collar and viola, you axle is locked in. from your axle, vacuum pressure is sent to a simple vac switch mounted on the firewall which then turns the 4wd indicator light on in the dash. When you shift back to 2wd, the vac pressure is reversed so that the passenger side of the diaphram is now under vacuum, and the driver side is opened to the ambient air. This pushes the fork back to it's normal position and disengages the front axle. if at any point, one of the vacuum lines frays or cracks, creating a leak (air rushes in because of the vacuum inside the line, thus reducing the overall pressure difference or eliminating it entirely) then the whole system will not work. The shift motor itself (that diaphram I mentioned) can leak as well. Or the vacuum switch on the t-case can go bad. Any of these will render your 4wd inoperative becuase the axle shafts won't be connected. Apparently this is all done to reduce wear and tear on your differential and t-case (if your shafts were locked all the time, your front drive shaft and diff would constantly be spinning even in 2hi, thus reducing gas milage and increasing wear) however the TJ's all use a one piece shaft on the front axle as do many other 4wd vehicles on the road, so apparently it wasn't worth the headaches it caused
You may have noticed that the open diffs can cause you to lose total traction on either axle or both (Canuk pointed that out too) so really you only get a true 2wd system, even in 4wd unless you have lockers in one or both axles. The lockers do just that, they lock both wheels together causing them to spin at the same rate all the time (newer technology is allowing them to spin at different rates during turning but for the most part they spin the same all the time). In this case, if one wheel is spinning, you still have the other side of the axle spinning as well. With one axle locked you have a true 3wd system (2 spinning all the time with one axle varying between the two wheels) and with both axles locked you have a TURE 4wd system with all wheels constantly turning together. Limited slip differentials do something in between, allowing the power to be supplied to either wheel in varying degrees. Say 80 to the spinning wheel but still about 20% to the non spinning. At some point though, it's still possible to lose traction on one wheel of the axle due to the other spinning. It's also possible sometimes to fool the limited slips and the open differentials into thinking the spinning wheel has traction again by gently applying the brakes. the added resistance of the brakes will force the diff to start sending power back to the side of the axle that is stationary. This trick is best used when you're only a little stuck, like in a patch of slick mud that isn't deep, just slippery.
That's basically all there is to it. Any questions class?