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  #37  
Old 08-13-2012, 08:25 PM
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Re: Rear bump feel when accelerating

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Originally Posted by Suther View Post
Comming back to the start thump, I agree it feels like the driveshaft is annoying to me but currently on a low priority. Maybe its a tolerance issue in manufactering it or Some wear. Let's see if it gets worse with mileage.
I have 4k miles and start recognize it.
I have 18k km on mine and it always had the thump. I thought it was the Hill Start assist but leaving that off doesn't help. I have the QL air suspension and thought that was the culprit but I don't think so anymore. The thump doesn't happen if you accelerate very slowly from start and it doesn't happen when the engine and drive train is cold. It pretty much always happen if everything is warmed up and I floor the gas pedal from standstill. I am convinced it is something in the drive train but I don't know exactly what. When I had my dealer look at it they came up with some obscure explanation of a known jump when 4WD engaged and turning the vehicle. That's not it either, as the thump occurs going straight on non-slippery surface. I have thought that maybe this gearbox disengages completely at stand still and therefore thumps when re-engaging but I don't know.
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  #38  
Old 08-13-2012, 09:33 PM
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Re: Rear bump feel when accelerating

Just one more thing: The jack can possibly slide back and forth in the foam holder. Worth putting in a rag to make sure it dosn't give a bump at braking/accelerating.
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  #39  
Old 08-14-2012, 05:52 PM
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Re: Rear bump feel when accelerating

I agree, it occurs with increased accelerating from a stop. It feels like the wheels would slip (or something else) and then suddenly get grip and engage. Isn't the max torque applied through the torque converter by accelerating from stop?
Would it then be possible that something in the gearbox or transfer case slips, or the software engages to late to balance the power flow?

I looked this up and it looks like there could be some software latency; "The Quadra Trac II's two-speed transfer case uses input from a variety of sensors to determine tire slip at the earliest possible moment and takes corrective action. The system also uses Throttle Anticipate to sense quick movement in the throttle from a stop and maximizes traction before tire slippage occurs. When wheel slippage is detected, as much as 100 percent of available torque is instantly routed to the axle with the most traction."

However, I don't know how the whole system works, so who could give some advice on this? I can't reproduce it all the time, so it is difficult for the dealer to work on it.
I also wonder if it is related to the rattle/clunking in the back as the whole suspension feels just unprecise.
Maybe we should start a poll to find out in which 4x4, engine and suspension combination this occurs?

Mine QT2, V6 and self-leveling shocks.
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Old 08-14-2012, 07:46 PM
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Re: Rear bump feel when accelerating

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Originally Posted by Suther View Post
The system also uses Throttle Anticipate to sense quick movement in the throttle from a stop and maximizes traction before tire slippage occurs.
You might be onto something there. It would then be a transfer case clutch clunk, or....?
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  #41  
Old 08-14-2012, 08:56 PM
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Re: Rear bump feel when accelerating

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Originally Posted by Suther View Post
I agree, it occurs with increased accelerating from a stop. It feels like the wheels would slip (or something else) and then suddenly get grip and engage. Isn't the max torque applied through the torque converter by accelerating from stop?
Would it then be possible that something in the gearbox or transfer case slips, or the software engages to late to balance the power flow?

I looked this up and it looks like there could be some software latency; "The Quadra Trac II's two-speed transfer case uses input from a variety of sensors to determine tire slip at the earliest possible moment and takes corrective action. The system also uses Throttle Anticipate to sense quick movement in the throttle from a stop and maximizes traction before tire slippage occurs. When wheel slippage is detected, as much as 100 percent of available torque is instantly routed to the axle with the most traction."

However, I don't know how the whole system works, so who could give some advice on this? I can't reproduce it all the time, so it is difficult for the dealer to work on it.
I also wonder if it is related to the rattle/clunking in the back as the whole suspension feels just unprecise.
Maybe we should start a poll to find out in which 4x4, engine and suspension combination this occurs?

Mine QT2, V6 and self-leveling shocks.
Your description is right on with what I experience....it will never happen with a gradual take off, but more times then not with quick acceleration.

Qt2, v6, QL suspension. I do not have any of the rear clunking that other folks have mentioned. It's strictly during acceleration and tends to happen more often when turning and accelerating.
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  #42  
Old 08-15-2012, 04:23 PM
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Re: Rear bump feel when accelerating

@F500; no experience at all my first 4wheel SUV (have been green in the past). So my thougt is, the system senses the desire for more acceleration (throttle sensor), decides from stop to put more traction to the rear, do to the stop it can't do anything but starts the transfer on the first turns of the shaft (or the software has some delay in it) which then engages the rear axle more causing the thump. Could also be that too much torque is then applied to the back causing a short slippage in the rear. From my feeling it is definitly in the rear. Despite I want to make a turn, then it feels like the inner wheel has that thud. Maybe that gets more torque then.

Who can tell how the software works, best solution is to write everything up and give Chrysler Customer Care this copy for their engineers to investigate. only react after.

@ajs800; Mmmh, that rules loose suspension out!, I haven't reread every post but it looks like only QT2 is affected.

Any QT1 or QD have this issue?
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  #43  
Old 08-27-2012, 08:11 PM
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Re: Rear bump feel when accelerating

Had my 2011 Laredo 3.6 QT2 and QL in for 12 month service at the local Jeep dealer.

This time I had been granted a service tech for a ride upon arrival with the transmission at full operating temperature ca. 75 deg. C. The bump was easily reproduced many times in a row and the service tech was not surprised. He said that all V6 petrol models (they only come with QT2 in Australia) had this bump. V8 and diesel models doesn't have it. His opinion was that it had something to do with the rear differential. He then switched the Selec-Terrrain to Sand/Mud and the bump disappeared (normal ride height). He said that it bumps only in rear wheel drive mode and it is "normal" for this model.

So, at least the dealership have gained some experience and acknowledge the bump/thump. It would of course be better if they could make it go away and the Service Advisor (not the tech) said there was a TSB for it. I asked for a copy of the TSB or something in writing about the problem. Later in the day when I picked up the car he said a software update had been applied to "alleviate" the bump but it could not be removed completely. As for the TSB he was not allowed to give me a copy, and he also said that not all TSBs are available on the internet. They applied several other software upgrades that I wasn't aware of existed, so I don't know if this is all BS or the truth. However, upon testing the car afterwards the bump is still there, but maybe it is lesser, it is hard to quantify really, but not gone!
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  #44  
Old 08-27-2012, 10:14 PM
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Re: Rear bump feel when accelerating

Mine also has the bump when up to temp and mine is a Diesel, does not bump in sand mode or when cold.
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  #45  
Old 08-28-2012, 12:46 AM
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Re: Rear bump feel when accelerating

I actually think it's caused by the traction control system, and in particular the BLD feature. It's a similar noise and feel to when you accelerate on incline after the brakes have been holding it. You also get the same noise if the traction control tries to stop a spinning wheel, like during hard acceleration in a corner.

It just sounds like the brakes havent let go completely by the time you hit the power and then when they do, you get the bump noise and feel.
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Old 08-28-2012, 10:07 AM
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Re: Rear bump feel when accelerating

Sounds all like a software issue, the transfer case controller against the ESP/simulated LSD one. Maybe the software developers didn't talk to each other

Seriously, there might be thousands of situations they couldn't have thought about causing this lack of coordinated effort of slip control and transfer of torque working against each other. So let's expect several updates in the future (you would hope). What makes me curios is, why doesn't the V8 or CRD with QT doesn't have the issue? Different torque curve?

I also found (in the internet) that the same effect is related to an old and well known problem; the yokes of the driveshaft allowing its extension or reduction when the front or rear suspension moves can cause the same feeling. So if you accelerate heavily, the car goes up in the front extending the length of the front driveshaft. If that doesn't go smooth you feel a thumb. Fix against that is lubrication, just grease the driveshaft using the grease/zerk fittings oh, right we don't have them anymore, maybe there is a driveshaft service somehow?
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Old 08-28-2012, 06:53 PM
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Angry Re: Rear bump feel when accelerating

I have a 2011 Overland w/HEMI 9k miles. The symptoms of the rear clunk or bowling ball in the trunk are present on mine and it has been getting worse as vehicle ages. As much of a pain in the ass it is, keep taking it in, keep calling Jeep customer service, and keep posting here. We cannot accept this bull$%^ of "itís a characteristic of the vehicle" just because they canít figure it out. At this point, I miss my 2000 Durango 4WD with 120k miles. Never had these clunks. I am regretting the 45k purchase. I am taking it in for the thirds time last this week

I will be taking it to another dealer this time that deals with more of these. I will start a website, start the lemon law process or do whatever I have to until they fix this or buy the vehicle back. I will NOT drive a 45K SUV with clunks. Please post to this thread if you are experiencing the problem so we can at least have something to point Jeep at.
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Old 08-29-2012, 12:25 AM
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Re: Rear bump feel when accelerating

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Originally Posted by ov10mech View Post
I have a 2011 Overland w/HEMI 9k miles. The symptoms of the rear clunk or bowling ball in the trunk are present on mine and it has been getting worse as vehicle ages. As much of a pain in the ass it is, keep taking it in, keep calling Jeep customer service, and keep posting here. We cannot accept this bull$%^ of "itís a characteristic of the vehicle" just because they canít figure it out. At this point, I miss my 2000 Durango 4WD with 120k miles. Never had these clunks. I am regretting the 45k purchase. I am taking it in for the thirds time last this week

I will be taking it to another dealer this time that deals with more of these. I will start a website, start the lemon law process or do whatever I have to until they fix this or buy the vehicle back. I will NOT drive a 45K SUV with clunks. Please post to this thread if you are experiencing the problem so we can at least have something to point Jeep at.
If you are talking of the constant clunking/rattling while driving you might look at this thread http://jeepgarage.org/showthread.php?t=37484&page=7
If you are talking about the thumb when accelerating you are indeed right here.

Anyway, don't regret the purchase. Sure the car has same issues but it is one of the best looking and driving SUVs in that price class. You can have the same issues on other brands as well, but paid more the 60k. Stay confident and help your dealer to help you. Even when dealer ignore forums like this, they help you to understand things better.
I am sure they will find a fix and so long, stay patient and think positive. It is a great car and you made a magnificent choice in buying it!
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