I have a strong driveline thump when accelerating in 4th gear (torque converter locked up) on highway when going up slight inclines. It only happens when accelerated slightly between 55 and 60 MPH, and the torque converter is locked up. The thump feels like someone is kicking the bottom of the seat when it happens, like something is binding. I cant figure it out, brought it to two dealerships, and they just charge to reinstall the most recent transmission software.

My jeep is a 2008, 2 door, 42RLE 4 speed automatic, well maintained since new.

What has been looked at:

- Engine mounts "look" good. Some sponginess in drivers side mount and in tranny mount, but told acceptable.
- Control arm bolts all tight
- Tested control arm mounts with pry bar, little to no deflection.
- Body mounts all tight
- Put in a new passenger side axle shaft
- Transmission fluid and filter changed 34,000 miles ago (55,000 kms). Fluid looks great, red, and smells new.

Other:

- I had to heat the transfer case to get the drain plug out, it was seized, around the time this stated.
- One garage jacked the jeep up by the lower control arm mounts... but they still "look" ok.

Does anyone have any ideas? Anyone ever have this issue? A bad torque converter would manifest in other speeds and gears wouldn't it? Which makes me think something else in the driveline.

 

You've certainly covered the basics. Have you pulled the rear driveshaft out and drive it in 4h, essentially fwd, to rule out something happening with your rear driveshaft? The control arms were my first thought and even jacking on them really doesn't contribute to damage.

 

You said the control arm bolts are tight but are they torqued? I had a similar issue ('08 2 dr) with my lower control arms but at low speed. Torqued the lowers to 125 ft lbs (spec) and that resolved it. Just a thought

 

Ya, torqued them as tight as I could. The upper control arm bolts were harder to leverage to tighten though. I will double check them.

I was worried about rear diff lash being off, but dealership didn't think that was the issue... Is there a how to anywhere on how to inspect and measure gear lash, where to measure it etc?

The only other thing I can think off, is that when bought new, the factory tranny lines where never hooked up, and I once the tranny heated up, literally in the first 20 miles off the dealership floor, it spewed out all the fluid, and I had to coax it, while in 4th, to get to exit. Could that episode have prematurely warn the 4th gear friction plates causing it to slip now?

At a loss. If it was a torque converter issue, wouldn't the thump, when accelerating, and locked in 4th gear, happen in other gears, not just that one? As soon as it locks in 4th, and I accelerate very slightly, it thumps hard...

 

You could try to put the vehicle in low, put your foot on the brake and tap the gas pedal pretty hard a couple of time to see if you can reproduce this. It does sound like a trans mount is coming apart. When these mounts come apart you really cant tell until you take them off and inspect them as sometimes they start to pull away from the metal mount and while compressed under the weight of the trans.

 

You could also just crawl under the vehicle with a pry bar and try to push the trans up off the mount and see what happens.

Another crazy idea is your exhaust could be hitting the bottom of the cab under this condition. Try to push your exhaust up be hand to see if this it hitting.

 

These are good suggestions, and I have done these. Surprising amount of movement in the tranny mount, felt soft, but the dealership said it was spec. Exhaust looked close and in a few areas on the insulation it looked like something pressed up, but I could not mimic it.

 

Update to my original post on the thumping when accelerating:

• Dealership added an updated transmission file for the 2008’s tranny. I didn’t know one existed. The issue got worse at first. The shifting now holds the gear higher, revs up a lot quicker, less geared towards MPG, and more towards accelerating. So less reliance on the torque converter (it’s not dropping low right way to save fuel like the original Jeep tune). In the end, it didn’t fix the issue, but may have contributed or helped, a little.
• Inspected the E3 plugs and found that the plug terminals where spinning and loose on three of the plugs! Tightened them with pliers, added dialectic grease to the ceramic and reinstalled. (Only have 10-15K on the E3s)
• Inspected the plug wires. All were dry and a little loose (time for a new set). For now I stuffed some dialectic grease inside to insulate better.


Problem solved! I don’t know if it is coincidence, but the problem is gone now. Lesson learned. I’m going back to stock jeep plugs, and will be putting some new wires on the jeep. The heat really destroys them.

Hope this helps others with the same issue.