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Serious Thump After 4HI Disengage

4K views 33 replies 12 participants last post by  Gold Miner 
#1 ·
Puzzled. (32rh 4.0L)
This happened once yesterday and then once again today.
Same circumstances both times.
While driving 50-55 in 4WD HI on snowy Hwy... Hwy turns to bare asphalt so I slip the 4WD lever down to 2WD. I've done this thousands of times since I've owned the Jeep with no problems, smooth as pie.
But once yesterday and once today.... everything goes as planned and nothing even feels weird BUT 7-10 seconds after I put the shift lever down I get an audible "thump" and I can feel it in my feet on the floorboards. Just one thump and something I've never heard before but scared the BJesus out of me twice now. Everything seems to be working well.
What the heck could it be? I need to address this before bad things happen.
Help and advice appreciated! I can wrench.
 
#2 ·
Same thing happened here with the snow last weekend. Like the OP stated, other than the thump, the Jeep seemed normal. 06 LJ here...
 
#3 ·
I had the same issue this past week, did it twice on the same day, and it scare the Jesus out of me!!! I noticed that if I shift it back to 2wd before leaving the ice/snow on the road it was smooth, but if I waited to be on concrete or asphalt it would do that loud stump about ten to fifteen seconds later. My thoughts are that the rear axle is rotating at a different speed than the front one and is creating a bind that eventually will result on the loud stump. I think this because I have a newer pair of tires on the front versus the old worn out pair on the back, difference on the diameter of the wheels will cause this. Just my .02.
 
#4 ·
Only happened to me once. About 6 miles after replacing my ujoints. I pulled into a Steep ditch to talk with my neighbor out in his field and actually needed 4wd to get back up, disengaged 4 as I crested the top then it popped on me at like 20mph going to highway speeds. Never done it since even though I use 4wd everyday even at highway speeds to combat washboards when there's oncoming traffic. i guessed it was just torqued into gear. Kind of like push button 4wd vehicles refusing to shift into 2wd until you put the thing in reverse to remove the load.
 
#6 ·
I had this issue but coming off the trail. Riding in 4wd and had problem getting into 2wd. At low speed I could feel I was still in 4wd so I had to drive in reverse about 15' to get the gear to catch if I didn't realize it I would get that thump. PO had a 3" BL and I now have a 1.25" BL if that helps.. What do you guys have?
 
#7 ·
I have a 2" SL which shouldn't have any effect, never has anyway.
Also, I don't think it could be tires. Rear tires are a little more worn (after rotation a while back) but not significantly.
All 4 of my tires are KM2 and in fairly good condition.
I have to commute 110 miles each way for the next few days and this involves two snowy mtn passes twice each day. Most of the commute is dry but the passes require 4HI. For now I'm disengaging at about 30mph and not experiencing the thump. Pretty sure it'll thump again at 50-55 (10 seconds after disengage) but I'm not going to try it until I know what's going on. Can't afford to break down that far from home.
 
#9 ·
Had a light bump and delay getting out of 4WD last week so I pulled out FSM and they suggest checking the fluid level so I'm gonna change it out tomorrow to see if it happens again.
 
#13 ·
Just curious when you guys shift on the fly do you slightly back off the gas when you shift into or out of 4H? It might be worth a try, I had a similar problem with a 4x4 Ford PU and that seemed to eliminate the problem.
 
#15 ·
That's fine; you just had a bit of bind built up from a turn or wheel slip that released when the chain slid back off the front output shaft. Slop in the chain, driveshaft, u-joints, axle shafts, hubs, and control arms lets tension build up between the front and rear output shafts; when you release front output shaft it "unwinds" and makes a thump.
 
#18 ·
Yeah, I try to relieve pressure on the gears. This'll be something I watch out for in the future. I definitely try to unload the driveline when transitioning to and from 4wd.

06 LJ, stock height, auto, 31's
 
#22 ·
I know, and I meant to replace them before winter set in but got extremely busy and it fell off the short-list. They don't look or feel bad but they are ready based on mileage alone. Definitely might be the culprit. Stock CA bushings are pretty worn all around too and I plan to replace all CAs also. These things may have to wait until spring though unfortunately. Until then I'm going to keep disengaging at 30mph or slower. It doesn't make the thump at lower speeds.
 
#23 ·
It's the 7-10 seconds that has me curious, but I don't know enough about transfer cases to be any help. Could it be as simple as a chain hang up internally that sorts itself after a bit?
 
#26 ·
Well, here's a new twist.
Woke up to a 1ft dia puddle under my skid, couldn't ID the fluid.
Too damn cold out to wrench right now so I dropped it off with my mechanic so he can put it up on the lift and diagnose it. I'm guessing a TC leak somewhere. He should be calling me in the next couple of hours with the verdict.
 
#27 ·
Update.
Just got a call from my mech. Said that it's a transmission seal and he'll have to drop the TC to get to it. Est cost $210 and that's a good deal in my opinion. I'm kinda remote so the seal won't be here til later today or tomorrow. Cabin fever, byaaaaaa! If it were summer I'd attempt the repair myself but the $210 will be worth it in this case.
I'm not sure if this is associated with the thump I described or just a coincidence. I'll know once she's done.
 
#29 ·
You can shift "on the fly" from 2hi to 4hi, you need to stop, or mostly stop, to shift to 4low.
 
#33 ·
For 4 low, mine goes in sitting still but can take a little movement to get it back out... Just enough to unload the gears
 
#34 ·
Got my Jeep back from the mech already and my transmission seal has been replaced, no more leak. He didn't think that had anything to do with the thump issue. I haven't had a chance to test it on the road yet. I have a 200 mile round-trip commute tomorrow so I'll have a chance to try 4HI to 2WD at hwy speed. Pretty sure the delayed thump will still be there. I'm still leaning toward front DS u-joints next if it is... although I'm not sure if that would cause a delayed thump in the drivetrain like that either. I replaced the rear DS u-joints last summer.
 
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