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-   -   2004 KJ Rear End Thump (http://www.jeepforum.com/forum/f28/2004-kj-rear-end-thump-1127318/)

lilngineer 11-11-2010 05:05 PM

2004 KJ Rear End Thump
 
Hi-

I recently purchased my 04 KJ with 97K on it and have discovered a problem that I could use some opinions on. I have noticed when I accelerated quickly off a slow roll (ie punch it), I get a loud thump, thud, slam kind of noise and feel from what appears to be the rear end. I also seem to notice it mostly when doing so starting to turn.

For example, I am leaving our condo complex to the main road rolling slowly, I see no traffic and hit the gas to turn out and get the thud.

It however only happens under this condition. Slow accelerations are fine.

Any ideas on what it could be? I am concerned it might be an issue with the rear axle. A local shop suggested control arm bushings. Thoughts?

Any help is much appreciated.

lilngineer 11-12-2010 04:10 AM

Anyone??

LibertyTC 11-12-2010 09:05 PM

Anytime I hear the word thump bang clunk or otherwise, I normally think of u joints related on driveshafts or a Trac-lok / diff in trouble. The GL5 fluid should be changed out frequently so pulling the cover and inspecting is a good idea as well
I would get this up on a hoist inspect the jeep.
I have had to replace my rear lower control arm bushings, where the diff was shifting a bit, the jeep at highway speed would push left when stepped on gas, and when gas released to coast, would drift right, with no clunks etc.
I have also heard that the rear parking brake drum can cause this effect, so while taking off lightly apply parking brake and see if it stops.

lilngineer 11-14-2010 07:15 AM

Well I took it to a shop and hopefully got a proper diagnosis. The guy was a former Chrysler dealer mechanic who seemed familiar with the KJs.

We went for a ride and he first thought it was a rear end posi problem. He put it up on the lift and found out it didn't have posi and upon further inspection found the front prop shaft CV boot torn with a cracked shaft. I could have swore the problem was in the rear end but maybe it was traveling through the driveline.

He said the front shaft spins in 2wd because it never disengages from the front diff.

So my plan is to remove the shaft and test drive it for the thump without it in place. Either way I guess it's time to find a replacement.

Is it reasonable to think this is where the thump is coming from?

lilngineer 11-14-2010 07:18 AM

Also as an FYI, I had the 4WD and transmission fluid service done yesterday. I also did rear pads, rotors, parking brake shoes and hardware the weekend prior.

LibertyTC 11-14-2010 06:14 PM

How many times have I heard about noises that are at the opposite end of where you think they are coming from. Some stuff is hard to note it's exact location, without careful inspections.
"He said the front shaft spins in 2wd because it never disengages from the front diff." TRUE!!!
Good idea to remove it and ensure there is nothing else to be replaced.

lilngineer 11-15-2010 07:03 PM

It looks like my intuition was right. I removed the front drive shaft yesterday. Although it does have a torn CV boot on the t-case side, I could see any cracking on the shaft at all.

I've been driving it with the front shaft removed and I still have the same problem. So I guess it wasn't the culprit.

So any other suggestions? Do I need to get it to a rear end shop to get looked at? Could it need a rear end rebuild of some kind? What are typical costs?

It's hard to reproduce as it's intermittent, but I seem to notice it most when turning out from a lot onto a main road with moderate acceleration. It definitely feels like a "slam". I never get it with light acceleration at all.

I'm stumped!

LibertyTC 11-15-2010 07:22 PM

Pull the rear dif cover and have a look in there.....did you have"the Mechanic" check your u-joints ,shock bolts tight?

lilngineer 11-15-2010 08:06 PM

What might be the problem in the rear diff?

BTW- I am mostly the mechanic on this one. The shop just took a free look at it for me on Saturday. (I also quickly took a look at the front drive shaft while on the lift. I asked him at that time if ge noticed anything else and all he commented on was the front brakes and a slight rear main leak.

I also asked about u-joints but he said they would be vibrating like crazy if they were the problem.

LibertyTC 11-15-2010 08:41 PM

I would seriously consider asking a dealership if they would charge you to have a service adviser do a ride along to help diagnose this problem. They might even throw it on a lift for you and exactly identify the problem though a proper inspection.

TexAcoon 11-18-2010 08:10 PM

From what you stated sounds like the same thing I had.

Replaced both rear shocks and it took care of the clunk...

Might look into that ..

davegt 12-11-2010 02:00 PM

rear control arm bushings stopped the same noise on mj jeep. the control arm holds the whole axel in place any movement in the bushings and the axel will move. i thought my tranny was slipping or my rear end was
shot. Also the you may as well replace the whole arm if the bushings are gone. the control arm doesnt seem to made very well. i have only 111,000 k on mine and the metal was bending very easy from rot and rust when pressing the bushings out and while putting the new ones in. Im looking for a new control arm now and it seems when you buy the arm u grt the ball joint and bushings for around $150-200

waywardtravel 12-20-2010 03:33 PM

Maybe your lucky and someone installed a detroit locker before you.

jimymc 02-01-2011 11:13 AM

What you may be experiencing is takeup of the slop in the drive train. Sounds like you are rolling in a downhill direction when you accelerate and all the rolling force is backed up in the drive train. The drivetrain is being driven by the wheels. When you accelerate the force from the engine is in the forward direction and any slop in the drivetrain is accentuated when it catches. Just the slight incline of a driveway into the road is enough to do this. It you don't hear this sound when accelerating uphill I would suspect this is the culprit. Unless it is really bad, I wouldn't worry about it. Most high-mileage vehicles will do this. I've got 128,000 miles on my Libby and it will do it. Just take it easy on the first part of the acceleration when you are starting out downhill.

brendanc 02-01-2011 01:14 PM

Quote:

Originally Posted by jimymc (Post 10895646)
What you may be experiencing is takeup of the slop in the drive train. Sounds like you are rolling in a downhill direction when you accelerate and all the rolling force is backed up in the drive train. The drivetrain is being driven by the wheels. When you accelerate the force from the engine is in the forward direction and any slop in the drivetrain is accentuated when it catches. Just the slight incline of a driveway into the road is enough to do this. It you don't hear this sound when accelerating uphill I would suspect this is the culprit. Unless it is really bad, I wouldn't worry about it. Most high-mileage vehicles will do this. I've got 128,000 miles on my Libby and it will do it. Just take it easy on the first part of the acceleration when you are starting out downhill.

You can compensate for this by increasing the RPMs in neutral/clutch depressed to match the speed. Then the "catch" doesn't occur.


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