Help with driveline "BANG" issue

[Jim]

Howdy folks, I'm looking for some input and guestimations...

[1990 Jeep YJ with NP231 transfer case, driver's side front driveshaft, that has a TeraLow 4-Low kit with SYE]

For some time (a year) I've been encountering an occasional BANG from the jeep. It feels solid, substantial, and from under the center of the vehicle. It's something that has been happening more often and is on my list before the season gets into full swing. Aside from the noise and sudden jolt, everything operates normally.

The first time I noticed it was on dry pavement at about 50MPH (forgot to disengage 4WD when jumping from Bunce to Rock Creek). It had two loud BANG's in quick succession. I attributed it mainly to my mistake of being in 4WD on dry pavement... until it would occasionally happen while out on a trail.

In my mind I kept thinking "it must be the chain jumping a tooth" and a few days back Jay (jaymoto) came up and we pulled the case and opened it up. It looked clean and fresh inside (plus, there were no leaks from the case / not cracked). I anticipated copious marks on the inside of the case from a slack, rubbing chain, but no. The case was 98% clean with only a scant mark from the chain and a scant mark on the engagement fork from chain rub.

Christopher (Fordboy77) PM'd a time or two and came with a comment that he encountered an oddity with a front U-Joint. It felt good but upon dis-assembly had wrecked bearings. While I've not removed the front driveshaft U-Joints or axle U-Joints, I do believe I have one on the driveshaft, near the xfer case, that is at issue as the cap is wiped clean/shines under the snap ring so the cap must be moving.

Any thoughts on what could be causing a substantial (sound and feel) bang from under the center of the vehicle? How tight does a chain need to be to be in "good" condition?

I ordered an xfer case refurb kit with chain and bearings (though existing bearings feel perfect) and will replace U-Joints up front too (rears were tended late last summer). Should the chain be my first suspect? Could any other part be at issue? What thoughts say ye?

Thx,
Jim

[Robert B]

i would love to help but sadly would have to be in the vehicle when it happens to try and get a hint of where to look and the exact feewl of it ............so im no help minus rip it all apart and check everything

[Java]

my guess would be ujoint bind, I'd start by pulling the front shaft and replacing all the joints; keep whatever is good, then you'll have some spares to carry. In my experience transfercases are too simple to do intermittent stuff. The np231 chains can stretch, but it takes a lot to get them there and when they do you often will hear it dragging on the inside of the case. it has to stretch a LOT to jump a tooth with the OEM gears, IDK if the 4-1 changes that. I think you would have noticed a loose chain right away when you opened it.

[Robert B]

nah the 4-1 is only the planetary gears in the front half chain is the same ...so are the chain gears

[glacierpaul]

Jim, I have the same year YJ, and last year the carrier inside of the the front diff. came unbolted, trashing the diff. Have you opened the diff's just to look? I also had Bill make me an upgraded shaft and got the upgraded u-joints as well, better than the oem stuff. I plan on the SYE at some point when I do a rear locker.

[Jim]

glacierpaul previously posted:

[carpenle]

I talked to the guy that I get my XJ parts from, he stated that he has never had a bad 231 but has had a bad 242. Not sure that this really helps, but I would say U joints.

[ColoJeeper]

Do you have a locker in the rear axle, and is what you are hearing the locker unloading after it has bound up some? Detroit lockers are notorious for this.

[carpenle]

I know Jim has ARB Air lockers.

[ColoJeeper]

How about the simple things like motor mounts, transmission mounts and t-case mounts? Is there any movement in the drive line when you rev the engine?

[scout man]

When I trashed my spider gears I had something similar (although much more frequent). I lost several teeth an all the spider gear components, and every couple rotations they would meet tooth-to-tooth and there would be a violent bang that could be felt throught the rig as it jumped into the next tooth gap. Now, mine we real trashed and it happened every rotation or so, but if you just broke one or two teeth, its possible that this only happens occasionally, but would likely keep getting worse. I would crack open your diff and just make sure there isnt any metal floating around, and take a quick look at all the teeth, and makle sure things are still tight in there.

[Jim]

Levi's correct - ARB's front and rear. I have NO issues, ever, in 2WD so I'm to believe the driveline for the rear (which includes motor mounts, tranny, xfer case in 2wd mode, rear u-joints, rear diff, etc.) are OK

It only happens when the front driveline is engaged. I can run in 4WD mode (xfer case) but disengage one (typically driver's side) wheel lockout and it does not happen (there's essentially no strain on the front drivetrain). No clue if it happens with front diff locked or not (I don't roll with it locked for long - no need).

I'll open the front diff cover (maybe today) to get a look inside. I do hope the front ARB doesn't need replacement.

[Rick]

Jim previously posted:

[Jim]

I pulled the front diff's cover and found metal bits laying inside. The ring gear did show a couple damaged teeth. The ARB has "pins" at the case exterior and one of the pin ends looks bad - just like my rear ARB looked when it was toast internally.

I could see putting an open diff into the front axle for the season and saving / doing an ARB again over next winter.

I'll probably call the local 4x4 shop and see their thoughts re cost for open diff reinsert / rebuild with thoughts to doing ARB next winter vs. doing ARB now and suck up the cost now.

[Chris]

Sorry to see this Jim, I say pick up a used open and toss it in the front for the Moab run.

[jaymoto]

I second that!

[Rob]

Chris previously posted:

[Hypoid]

Jim previously posted:

[Jim]

Hypoid previously posted:

[tonkatoy]

Good to hear you found the problem. just sucks it is not a easy fix