DrewMogz

Hey guys, been having a problem shifting into FWD lately with my 1990 XJ stock 4.0 Automatic Transmission and Command Trac 4WD I believe with the 2H, 4H, N, 4L options for 4WD.


I used to be able to shift it into 4H without much problem at any speed relatively easy. However, the past month or so when I try to enter into 4H, I will ended up either not being able to get into it or having to abort trying due to it wanting to grind. The only way I've been able to successfully get into 4H is by nearly coming to a complete stop and then doing it. I'm also not keeping my foot on the accelerator when shifting it, I always take my foot off the gas if that matters.


I've checked the transfer case fluid today by removing the topmost 1"3/16 drain plug nut and fluid somewhat started to come out at the bottom, so it must have enough in there. I'm not sure what else to check problem wise and was hoping someone out there might be familiar with this problem. Thanks for the input in advance.

Oswalla

I believe you have the vacuum actuated system as I do. Vacuum actually pulls the fork to lock 4wd in. You can inspect the lines. Upper Shift linkage needs to be inspected too.

A search of the archives here will give you options to eliminate this vacuum system if you feel the need. The transfer case is NP231, search that term, tons of goodies to read. Pay attention to Cruiser's post

cruiser54

I'll bet the front axle is not engaging. Do this:

https://www.cherokeeforum.com/f51/fr...ss-91-a-38629/

DrewMogz

Appreciate the input guys. I checked my vacuum lines before posting, to make sure it wasn't a problem with that. All of my lines and other functions seem to be in pretty good shape and I would've thought if it was a vacuum problem that it wouldn't work period and not just at speed.


I took a look at the write-up that cruiser linked me too, I might have to give that a shot if all else fails. I don't see much downside to having a 1pc constant axle and I wouldn't mind bypassing my vacuum system, which one day will be a problem I'm sure.


It seems like I can shift into 4H no problem at speed if I'm on level ground or snow, but as soon as I try to do it when climbing my road w/ ice is when it gives me fits. I don't know if it's because it's an incline and not level road or what the deal is. For what it's worth, I've started to notice a small drip or two of ATF coming from the transfer case. It's not much, as I've seen it for a month or two now and when I checked the top most 1 3/16th drain plug, some started to come out of the bottom of it. I haven't been able to located the cause, it almost looks like it's somehow just coming through the 2 halves of the casing where they meet but all the bolts are tight so I don't know if the gasket is somehow going bad but I don't think that's the cause of this problem.

nujeepguy

this is new to me. i read it twice. dont get the concept of whats happening? just trying to learn stuff thats new,unknown.
any other help?
i am a nu jeep man !!

cruiser54

The axle disconnect is totally unnecessary and troublesome.
When the lever is pulled, vacuum is supplied to the front axle to engage it. If it doesn't engage, the front driveshaft doesn't spin. If the front driveshaft doesn't spin, but the rrear one does, the difference in speeds will not allow the transfer case to mesh properly.

DFlintstone

Drew, the vac disconnect (disco) on your front axle has nothing at all to do with any grinding or trouble shifting the transfer case. (seems at least one poster has his disconnect working perfectly). Mine will normally go into 4 high "on the fly" with no issues as you report yours did. All I could guess is it's really cold? Doesn't sound like linkage, so sounding like the TC it's self being stubborn. Maybe see if it still does it warm. Maybe go ahead and change the fluid, check the drain pan for any sign something is not right. I do need to coddle and nurse mine into 4-low if I don't want to grind something.

DrewMogz

I appreciate the input and was thinking it had to do with just the transfer case as well. It's been pretty cold here lately but I've been experiencing the problems even after having driven on the highway for a half hour or so and seems to do it the most when I'm on some sort of incline/decline or possibly when I'm decelerating, at which point I will pretty much have to come to a stop to get it in.


I might have to change the fluid but wasn't sure if it was needed since it looked pretty clean when I took out the drain plug to make sure it had enough fluid.


I also went ahead and looked at the linkage while it was parked and had someone shift it in and out which it did easily and the linkage all looked great.

DFlintstone

Yea, even removing your front drive line might not affect that. Cruser there, (my bud I was poking fun at), or Freedgr or others likely could be more help here than me. I do wonder. If the disco was somehow jammed engaged, that's OK and the result of the mod suggested, wonder though if that could change our easy shift on the fly. Now the drive-line would be spinning, locked with the front wheels. Again it's OK to have the RF axle one piece, newer are built that way, but I don't know if with that locked on if it shifts the same way. Just wondering, don't know, all three of mine have functioning discos.

DrewMogz

I just figured if I did lock the front axle to fix an intermittent problem, I would be looking at more wear on my front tires and not as great of a turning radius, right? Not only that but there has to be something less severe to do to fix this problem, just not sure what the exact source of it is...


Also, am I always supposed to take my foot off the gas pedal before shifting into 4H or should I keep my foot on the gas?

cruiser54

2 things:

CAD STINKS


The big issue is that the CAD system only fails when you need it the most.

Kinda like when you don't realize your wiper blades need replacing until it rains....

I know somebody’s system works and he advocates keeping it stock, but my exposure to the failures is probably hundreds of times greater than his, as a result of being Service Manager at a Jeep dealership from 1980 through 1992, and being a current XJ and MJ owner.

CAD stinks. Every Jeep I work on with it, I eliminate it. Guess what I find in the process? 25 year old busted vacuum lines, bloated vacuum line connectors, bad vacuum reservoirs and piping, bad transfer case vacuum switches. In other words, had the person gotten into a sticky situation and needed 4WD, they would have been screwed!!!

That's the tip of the iceberg. The factory went through 3 revisions of the shift fork also. The fork issue is non-existent if you just slide it over and leave it.

See the link below for complete instructions. Takes less than 1/2 hour.

https://www.cherokeeforum.com/f51/fre...ss-91-a-38629/

http://cruiser54.com/?p=117
,

DrewMogz

Thanks again for the help, going to try that next time I shift it into 4H. Will locking that front axle impact the wear on my front tires much or my handling/turning radius?

DFlintstone

No. Locking it just makes the driveline and more stuff in the TC spin when it doesn't need to. With the 4-wd disengaged at the TC, the front freewheels. Again, your issue likely has nothing at all to do with the disco.

cruiser54

Not at all. The factory went to NON disco in 1991 with no issues. Models from 84 to 90 with Selec-Trac were non disco. Just eliminate it and don't look back.

DrewMogz

Thanks for the input, I was reading somewhere about the type of fluid that I'm going to need to refill it with, I'm sure it was in that write-up somewhere. Thanks again.