2008 4X4 6.4 Lariat crew cab with 36K miles used primarily for towing 13K lbs. fifth wheel travel trailer. So far radiator/hoses/expansion bottle, bed plate seal re-seal, and front crank seal replaced under warranty. I now have a hard shudder when downshifting from overdrive under load (climbing TN hills) at 65+mph in “tow mode”. This is an intermittent condition, does not happen on every downshift. I cannot really tell if this is drive-line shudder or transmission chatter, it’s almost like an injector is miss firing or not firing correctly. Starts off with a fast shudder then smooths out to normal. Seems to happen more when the cruise control is engaged. When I first starting towing with this truck I had a slight shudder when accelerating from dead stop but was never as severe as this. I corrected most of that shudder by installing air springs on the rear and leveling the truck when the hitched to the fifth wheel trailer. Do any of you experts have any suggestions or any idea what’s going on with this truck before I take it to the dealer?

 

I think you have ruled out the driveline angle with the airbags.

TN has some hills that will cause a downshift alright. I just drove the I40 route yesterday.

I use cruise and set it at 65.

Have you ever serviced the tranny?

 

I haven't serviced the transmission. They added a few quarts of fluid when they repaired the bed plate leak (per the invoice). Do I need to drop the pan and change the filter with the miles I have?

 

I think I would try a tranny flush only.

 

Even with air bags you can still get a little spring wind-up which dramatically changes pinion angle. Also, a damaged motor or transmission mount can cause this problem.

 

Went by my local dealer and checked on a transmission flush and they didn't recommend it until I'm closer to 60K miles. They pulled up the schedule and printed me a copy which stated 60K was the interval for severe duty. I told them about the shudder on loaded downshift and he said it was due to the drive line loading up putting a monetary twist in the differential to drive shaft connection. Thinking back, I may have had a little less pressure in my air bags than I normally carry. I'm pulling the fiver again in a couple of weeks, I will make sure I run enough pressure to level the load and see what happens.

 

Back in January the dealership performed open heart surgery on my truck to repair a leaking bed-plate. The engine was out of the truck to do this procedure. I wonder if the motor mounts were tightened down when the engine was re-inserted into it's proper resting place? I will check if I can get to them. It would not surprise me if they were loose, I've found a bunch of stuff not fastened that should have been.

 

Keep us in the loop, abubba1. I'll be here if you need me to step in and assist.

Crystal

 

I'm see I'm not the only one with the "shudders".
I finally made a trip with the 5th wheel this weekend. Around 325 miles with some fairly steep grades between Camp Wood and Leakey, Texas. The RV weighs around 12,500. Normal acceleration from a stop on a level street induced a serious case of the shudders.
I had the trans fluid changed at about 55k and I've got about 62k on it now. It didn't shudder until the heavy load.
BTW it, the truck, decided to clean the exhaust filter under while I was going up a steep incline and sounded like it was going to come unglued and was really struggling to maintain the 45 mph. I was a little disappointed that it was a struggle to climb that hill.
Conditions: 87 deg, 800 +- above sea level, 3:73 gears, bone stock 6.4 CC, 4X4, auto trans.
Any Ideas will be most appreciated.

 

You may have another issue with the programming. I could always maintain speed, even during the regen. While the regen does reduce the avail HP it is not that much.

I towed a 15K 5er.

 

Just got back from pulling the fiver another 500 miles. This trip I ran five more pounds of air pressure in my air bags than I normally do. With the tranny in tow mode and cruise set at 65 mph I had plenty of hard downshifts pulling over these Tennessee hills. The truck never shuddered during these downshifts like it did before on my last trip. The extra five pounds in the air bags did the trick but something has changed in my drive line that that requires this extra air pressure. Not sure if it's worth the worry, if it ain't broke don't try and fix it. I'll just adjust the pressure up higher if the problem returns.

 

Great info here. The hard downshifts I have gotten used to while in Tow/Haul. Justs got back on a whirlwind trip to Colorado in my 08 and had several downshifts that were hard especially going down Raton pass. I noticed some shuddering, I will try your trick and add just a little more air...