NP231 Cracked! - JeepForum.com
 
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post #1 of 4 Old 12-08-2019, 08:11 AM Thread Starter
JabaThaHut
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NP231 Cracked!

I recently picked up a "new" TJ, which I knew had a few issues. One unknown issue became apparent with an intermittent metallic contact noise of some sort which was only occurring during engine braking at low speeds (4.0/AX15).

The TJ had a dented driveshaft on the front, which must've had a slight bend in it since you could feel it over 40mph. When I got it home, I popped the front driveshaft off and that problem vanished, but after driving it a bit more, I realized something else was going on that didn't show itself during my test drive (I did check all transfer case positions during my test). I drove for about 2 weeks without the front shaft on after checking fluid levels in the transmission and transfer case. I typically wouldn't hear anything until everything warmed up and it would really only make noise under 15mph while decelerating in gear and it seemed to match driveshaft revolution speed. It was rather infrequent and sometimes it would be barely noticeable while other times it would sound absolutely terrible! It sounded like it was right beneath the stick and I could feel it when I heard it. I also noticed it was sometimes worse if I was turning, which was throwing me off a bit. It's hard to describe but it sounded similar to a dry bearing that was locking up. I'd get a bit of a grinding/scraping sound sometimes accompanied by a screeching. I started to get a little more worried since the frequency of occurrence increased, but I put myself more at ease when I noticed one time I was able to get the sound to change if I moved the 4wd lever a little. I was much thinking my problem was in the transfer case than the output shaft of the transmission!

The hunt for a used NP231 began. The day after I picked one up, I was getting off the highway (of course it was 11:30pm) and decelerating on a curved ramp when the noise began earlier, got loud, and was accompanied by a nice clunk, and a loss of power to the rear wheels. I rolled off into the shoulder and was guessing I was gonna call for a tow. I was still riding without the front shaft (I picked another up but figured I'd wait to install after the "new" transfer case went in) so I couldn't test if power would go to the front. I tried cycling the case to low range and got power to the rear! No unusual noises or difficulties shifting through into low. After that test, I tried to push the lever back to 2 high, it went, but the case was still in low range and now the lever was stuck in that spot. I finished my drive home in 2wd low and drove like this for a solid 2 days until I had time to swap the case. Aside from the low range whine and me not wanting to go over 40mph, it drove perfectly fine! I just started in 3rd and shifted to 5th right way

When I had time to swap the case, I found the darn thing had cracked right open (I'm sure that was part of the definitive final clunk I heard). I'm assuming it let all it's fluid out when it cracked. I'm pretty impressed I drove 2 days with it in this condition without a peep or hiccup from that thing!

I haven't split the case open to take a look, but what do you guys think was wrong with it when I bought it? What do you think happened? I know it's pretty simple in there so I'm wondering if there was a broken snap ring that was allowing some movement? It never made a sound under acceleration, only under deceleration at slow speed (coming to a stop). Of course I tried searching all over the place the first time I started hearing it but nothing I found really seemed to be the same situation.

Thanks!

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Eric
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post #2 of 4 Old 12-08-2019, 08:52 AM
NashvilleTJ
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My guess is that whatever caused the damage to the front driveshaft is also what cracked the case. Perhaps a hit to the front axle, or something similar. Those cases also crack, but more often split apart, when the double cardon joint in the front driveshaft seizes. Either way, you’re on the right path with a new TC and driveshaft. Just be sure to check the shaft to ensure that all the unjoints move freely and are well greased.

Good luck! Let us know how it goes.

Jeff

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post #3 of 4 Old 12-08-2019, 06:54 PM
WSS
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AS Jeff mentioned, it was probably a good hit from the front. Looking all around, the crack looks like a push from the front DL.

Not sure where you are headed with this but a Atlas II is about $3k for entire project. You would be able to utilize the .307's a little better with a 5:1 or 4:1 TC. Just sayin.

WSS

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post #4 of 4 Old 12-09-2019, 07:03 AM Thread Starter
JabaThaHut
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Quote:
Originally Posted by NashvilleTJ View Post
My guess is that whatever caused the damage to the front driveshaft is also what cracked the case. Perhaps a hit to the front axle, or something similar. Those cases also crack, but more often split apart, when the double cardon joint in the front driveshaft seizes. Either way, you’re on the right path with a new TC and driveshaft. Just be sure to check the shaft to ensure that all the unjoints move freely and are well greased.

Good luck! Let us know how it goes.

Jeff
hmm...I didn't really think of that scenario but that's definitely plausible! I guess I may never know.

I threw the other transfer case on there, of course after dealing with 3 out of the 6 skid plate bolts spinning and needing new nutserts. Before installing the "new" front driveshaft, I swapped out the 3 U joints for new 5-1310x joints and did the same for the rear shaft to make sure all that was good. I see a little seepage in the front pinion seal, but I'm gonna be swapping out that D30 soon anyway. The yoke has no play and there's nearly no backlash at all.

I messed up though and ended up with a front driveshaft from an automatic. Its a little shorter but seems to have enough spline engagement to not be worried about the pieces coming apart under suspension travel. What's your opinion there? When I swap the front axle out, the new one will be a HP D30 so that'll help with this a little I think.

Quote:
Originally Posted by WSS View Post
AS Jeff mentioned, it was probably a good hit from the front. Looking all around, the crack looks like a push from the front DL.

Not sure where you are headed with this but a Atlas II is about $3k for entire project. You would be able to utilize the .307's a little better with a 5:1 or 4:1 TC. Just sayin.

WSS
I can definitely picture what you're saying. I wonder what it was? I also see a dent in the front pipe right in that same area.

I'd love to throw an Atlas II in there, but unfortunately that just isn't in the budget. I found a NP231 with decent mileage on it for $200. I have a set of axles I'll be throwing on soon (HP D30 front and 8.8 rear with 4.88 gears) that'll definitely help out with my crawl ratio and anemic 3.07/33" current combo!

Eric
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