I can't seem to grasp what happens when you take the transfer case (NP231) out of 4WD and go back to 2WD. I'm assuming the front driveshaft is disconnected from the transfer case with some sort of mechanical linkange inside the TC, but do the front wheels continue to turn the front axle, diff, and driveshaft as the vehicle moved forward, or is there a vacuum-actuated disconnect system in the front hubs or diff?

Sorry for the stupid question, but I can't exactly stick my head down there while I shift the TC driving down the street.

 

On your 98 there is no vac disconnect in the axle. Basically the tires are turning the axle shafts which turn the gears which turn the drive shaft, then at your transfer case the driveshaft is disconnected from the drivetrain inside the transfer case.

 

In most cherokees, when your in 2wd, your hubs are always engaged and spinning. They spin your shafts, shafts spin the gears, pinion spins the drive shaft. That's why some axles have locking hubs so you don't spin everything going down the road in 2wd.

Never really took apart a t-case before but there's a shift fork that engages and disengages the power going to the front axle. Someone will probably chime in with more info.

 

That's right. The front driveshaft still spin even though it's mechanically disconnected inside the transfer case.

 

I guess that helps explain the crappy gas mileage these things get.

 

It certainly doesn't help, but I had a 96 with the front ds removed since the tc didn't work and I was lazy cuz it was a beat up xj anyway - and it really didn't get much better gas mileage from what I remember, maybe 1 or 2mpg better at the most I would think, if anything.

 

Jeeps just don't get good gas mileage. Mine gets 13 mpg no matter what gears or maintenance I did on her.

 

Even if you DID have the Vac Disco, the left axle, the spider gears and some other will still spin. My .02, nothing meaningful is lost in mileage. But! During the lifetime of the Jeep, having the front driveline, and all it's "stuff", (front pinion, u-joints, whatever turns in the transfer case), spinning full speed, during ther lifetime of the Jeep there will be a tad more wear on those components. Not allot, it's just freewheeling, but a tad.

I get 20 or better highway, with 280K on my Stock 90! (746 injectors though)

 

This is really the only thing that will ever make a difference past routine maintenance. I should have gotten around to doing it already but just haven't had the time. Top of my list for springtime.