tetrad

As I understand it, the reason why you can't run part-time 4wd on dry pavement is because the gearing for the front axle is something like 3.50 and the gearing for the rear axle is something like 3.55. So if you cruise around without any wheel slippage, eventually your axles will be at the wrong rotation and pop back through the driveshaft to destroy your transfer case.

The gear ratio is a ratio between the number of teeth on the pinion gear and the ring gear, which both live in the differential. So my question is, assuming we actually had the gearing for front and rear equal (ie 3.55 and 3.55), is there any reason why you couldn't run "part-time 4wd" (ie on my NP231) all the time? Or is there another component of the gear ratio that's being generated by the transfer case? (ie in part-time-4wd the front vs rear driveshafts actually spin at slightly different speeds)

Basically, I want full-time 4wd... is it possible to do this by re-gearing rather than getting an NP242? If I did get an NP242, would it be a drop-in replacement, or would it basically involve replacing my driveshafts and re-gearing the differentials anyways?

Metmknice

In order to run 4wd you have to have the same gears/tire size on both axles. In part-time 4wd the front and rear axles spin at the same speed. The 242 has a differential inside it, allowing full-time/awd. Means that the tires can spin at different speeds allowing cornering.

Firehawk068

Your front axle gearing, and rear axle gearing are the same 3.55:1
If they were different ratios, the vehicle wouldn't move at all.
Part time 4WD locks the front axle to the rear axle.
The reason behind not wanting to use it like this on dry pavement has more to do with differences in tire diameter between the front tires, and the rear ones(the tread wears at different rates. even a minute difference) and the difference in wheel speed while turning between the front wheels and the rear wheels.
This causes a binding force in the driveline that affects ring and pinion, u-joint, and transfer case and chain wear.

The NP242 transfer case has a center differential when used in Full-time 4WD mode. This allows for use on dry pavement, where differences in tire diameter, and wheel speeds wont bind up the driveline.

I can tell you 1st hand, that it works nice.......especially when your commute includes some snowy/icy road sections AND dry pavement.
Granted, it will still spin a tire if you got one on ice, and the other 3 have grip, and you give it alot of throttle, but provides alot better forward grip than 2WD mode................And I like the fact that I can switch between locked, center differential, and 2WD while cruising down the road at any speed.

I really couldnt tell you how easy a drop-in 242 would be. You would have to do some research on the input/output spline counts. Also somewhere around '94 they changed the design of the tailhousings on these and the 231's.
For a Daily driver that sees only occasional trail use, the 242 works great!
But for a Trail-use rig that sees moderate road use, stick with the 231. You can still shift between 2WD, and 4WDhi while cruising down the road if you should encounter some slippery road.

cruiser54

Excellent info ^^^ there.
To install the 242, you need the indicator plate on the console, the shifter gate under it, and to have a non-disconnect front axle or make yours that way in 1/2 hour.

tetrad

Awesome! Thanks for the info guys. I'm gonna kick the idea around my head a bit and see what pops out.

It's my DD, and every day I've got to pop it in and out of 4HI a bunch of times. I'll go from a clear road to a side road covered in snow and/or ice and back within 5 minutes, and I feel like in the long run that won't be great for the transfer case. But the first 240k odd miles it didn't see much 4wd use, so it does make the switch snappy and clean... maybe it's not as much of an issue as I think

acem

I have seen a 4wd operate with different front and rear axle ratios. The chain in the transfer case made terrible noises from jumping and slipping before it broke... Thx-Ace

cruiser54

Shifting it into and out of 4wd isn't gonna wear it out. The 242 is my choice. i've got one in the 88 XJ and I've got a spare out in the shed I'd love to swap into my 90 MJ. Too lazy and have too many other projects to do.