Auto LJ bucks under gentle acceleration from slow roll - JeepForum.com
 
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post #1 of 9 Old 08-20-2014, 12:22 AM Thread Starter
Pentara
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Auto LJ bucks under gentle acceleration from slow roll

I'm hoping someone reading this might have an idea of what is happening, I don't know what to do next. I asked my Jeep mechanic and he had no idea.

I'll do my best to describe what I'm experiencing. For quick reference I run an 05 LJ with an automatic 42RLE transmission, 33" tires, stock 3.73 gearing and a 4" suspension lift. I've got ~120,000 KM on the Jeep.

This happens occasionally, usually when I'm in rush hour traffic. If I am rolling forward at around 10-15km/h and I press on the gas (gently or normally) to accelerate as traffic starts to move, as I speed up sometimes I hear a slight thump and the Jeep jolts / bucks forward. It feels like someone hit the Jeep with a hammer to speed it up.

If I had to describe what I thought was happening I'd have to say it feels like 2 gears are engaging (I dunno how an auto tranny works but I picture the concept of a manual tranny and the clutch pads) and one is spinning significantly faster than the other, so when they engage the faster spinning gear's teeth grab the slower spinning gear's teeth with a violent hit. The sound and feeling seem to come from behind me when they occur. There is never any grinding though, the above is just the impression I get.

I'm having 4.88s installed today so if this phenomenon goes away in the next 2-3 weeks, I'll post it for reference should anyone else ever experience this. But hoping that someone has been through this, I'd still love to know what it might be.

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post #2 of 9 Old 08-20-2014, 08:11 PM
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Autos have a torque converter that smooths all that meshing out. How old is your fluid? Is you're fluid at the proper level?

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post #3 of 9 Old 08-21-2014, 06:59 AM
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Quote:
Originally Posted by mike_dippert
Autos have a torque converter that smooths all that meshing out. How old is your fluid? Is you're fluid at the proper level?
I'd say this is the first step. Low fluid will cause jerky engagement. If your Luis level is ok then get the ECM scanned for transmission codes. They don't show up when you pull codes by cycling he key.

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post #4 of 9 Old 08-21-2014, 03:34 PM
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If your Luis level is ok
What's a Luis level?...

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post #5 of 9 Old 08-21-2014, 09:21 PM
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Quote:
Originally Posted by Knuckelhead
What's a Luis level?...
HA! Now that's funny. Autocorrect fail.

The word I wanted to say was fluid.

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post #6 of 9 Old 08-22-2014, 08:37 AM Thread Starter
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My mechanic took my Jeep for a test drive and said he felt what I was talking about, but that it was a normal thing with any automatic transmission. He said in any auto tranny the components are always spinning and unlike a manual transmission which has clutch plates, there is no gentle engagement in an auto tranny.

It doesn't fully explain what the cause is, but he did say it wasn't gears or gear ratio or any such thing dependent. Also when he opened up my axles and dropped the transmission pan for a fluid change, there were no metal splinters or pieces and he was amazed at how good my Jeep was condition wise for a 120k km Jeep.

I'm not sure I agree fully as I never experienced such hard engagements with previous cars I drove (Honda, Impala, Dodge, Corolla, etc). I will see if the problem persists over the next few days now that I have my Jeep back with new gears, LSDs and tranny fluid service. Another update next week.
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post #7 of 9 Old 08-22-2014, 09:19 AM
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I don't agree either. I've had your experience (I think) and it was due to low fluid level. I had hot fluid working out at a hose barb from when I added a cooler. I filled it up, and no more problems. I do get an occasional hiccup if I accelerate at just the right time after slowing down to a near stop. I'm coasting along, the trans wants to go into 1st, and I step on the gas. It causes a quick 2-1-2 gear change that sometimes feels harsh. It's not a regular thing for me though. 4.88 gears should help by reducing the load on the transmission.

.
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post #8 of 9 Old 08-22-2014, 11:14 AM
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I had a similar feeling thing happen to my XJ, also was not diagnosed by a Jeep Mechanic correctly. In my case, the slip yoke splines , where your rear driveshaft connects to the transfer case, were binding. As you came to a stop it would load up the entire drive train and hold the rear of the vehicle up a little, then as you drove away, it would suddenly release causing a thunk as the entire drive line unloaded and the rear of the vehicle dipped down. The fix was easy, as I'll had to do was remove the rear driveshaft and put a little grease on the slip yoke splines.
Turns out it only happens to automatics because they keep a rotational load on the driveshaft as you come to a stop. With mine, I could get it to stop doing it, if I put the transmission into neutral as I was coming to a stop, then put it back into drive as I was sitting there waiting to go. In really bad cases, the output shaft from the t-case and driveshaft have galled enough that both need to be replaced, but for most vehicles with this problem, applying a little grease to the slip yoke spines fixes the issue for 6 months or so, until the grease is washed away. Not sure if this is the same as you are describing, but I went to three mechanics with mine before I found one that figured this out (it's was a common problem with Ford trucks).

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post #9 of 9 Old 02-03-2015, 09:16 AM Thread Starter
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I finally diagnosed this problem. Had the jeep up on a hoist a little while ago having my axle fluids changed after re-gearing, and my mechanic did another inspection. This time he got real close to my suspension and noticed that it is the suspension which is the culprit.

I don't know the proper names, but basically my rear bushings (lower control arm?) were loose. When I accelerate, because I have so much torque from my 4.0L, the suspension pushes back and shifts about 2-3 mm against the bushings. This is what the hit I feel is, and why it got stronger over the past year as I drove more and the bushings got looser. He tightened them up again as much as he could (I need to replace them soon) and things got better.

Another thing he checked and tightened, though I don't think it was as significant, was the bolt connecting the centre angled support bar from the suspension to the frame. The bolt was super tight against the nut (he had to torch it first) but it wasn't tight compressed enough to be tight against the suspension. I list it here as another item to check, though I'm 98% certain the problem was the bushings. (Since I could see an area of wear from when they shift)
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