NP231 Rattle

I have a 1994 YJ with the 4.0/aw4/Np231 out of a 1987 Cherokee. The donor XJ had 300k miles on it. When I first got this thing on the road, it had a horrible rattle that sounded like the transfer case chain any time I lifted off the throttle at highway speeds (40+). I bought another NP231 from a local wrecking yard and installed it. Now I have the same noise, only not as loud. It does not do it under acceleration, and it quiets if you downshift while slowinig down (but it's still there). It's at it's worst when you just barely lift on the throttle so that the vehicle is essentially coasting in gear. Did I just get another bad Tcase? Did I mis-diagnose this entirely? An ideas?

It sounds like throttle lift-off driveline vibrations. What lift, shims, TCase dropdown are you running? I get a slight vibration in the same situation, but I know what it is. I'm doing the PORC SYE this week to solve it.

Check your u-joints and the rear bearing on the slip yoke output before you replace the entire unit again. Complete master overhaul kits (no chain) are around $100 and include all the seals and bearings. It may be a good way to go for you since you have a "spare" unit.

Good luck.

I'm going to replace it again anyway. The yard I bought the case from guarantees them for 6 months. They have already pulled one for me to pick up today. The lift is a SOA with RE 1.5" springs, and the engine and tcase are custom mounted, as this trans was never offered in a YJ. I only have a slight vibe at about 40mph, which should go away when I can scrape up the cash for a SYE. Are there any tests I can run to isolate the problem before I pull the case out? MAybe drive with no front shaft, or run it in place with no front or rear shafts? Will this tell me what the problem is?

The odds are pretty good that with the long trans, already short rear DS of the YJ, the SOA, and 1.5" lift you are pretty much destroying the TCase rear bearing with vibrations. I would hold off driving (if you can) until you get a SYE installed and the driveline angles ironnd out.

PORC HD SYE for the 231 is $179 delivered. Still cheaper and easier than swapping cases every few months.

My .02

The driveline angles are in pretty good shape. I drove it this way for a year with the original drivetrain with no noises. The noises started immediately after the swap, and were still there with the new tcase, just a little quieter. I think that I have just got a couple of bad cases in a row, but since I have already sold my original 231, the only way to be sure is to try again.

And yes, I will be keeping the driving to a minimum until I get the SYE. I just want to finish sorting out the other issues.

Just curious, how difficult is it to pull the transfer case?
I think I may need to tackle this on my XJ soon

If you move slow, it can be done in a little over an hour - at least on my YJ.

Took out the Tcase last night. Started comparing it to the "new" one (chain slack, noises, etc.). They appeared to act and sopund the same. Crawled back under the Jeep and rotated the output shaft on the trans - voila! The same rattle that I hear when coasting. It sounds like ti may be tghe sprag. Anyone else had this problem?

Still fighting this stupid thing. I have now put three NP231's and two AW4's in this thing, and the noise is still there. It has to be coming from somewhere else. Do any of you have an idea where to look next? I even dismantled Tcase #2 looking for clues, and everything looked fine.

I'll say it agian...SYE...SYE "lift off drive line vibration". Get an SYE and you'll have no more troble.:gee: