18 2500 auto, 6th gear at 60mph stutter

[joeyf86]

hey guys, i got a pretty gnarly tranny stutter at 60mph while in 6th, happens on incline.
I can keep it in 5th and have zero issues, or if I feel it i can give it more throttle to have it kick down and it clears right up til it tries to shift back.
besides typical power mods, its leveled on 37's. I'm curious if you all think its because of the bigger tires or if I may have a tranny/tc issue?
Been thinkin its probably because of the bigger tires and getting a regear to pull stress off the tranny and bring it back to a more suitable gear ratio.
Truck has 42k, about 2k with mods

 

[buckeyexx]

What trans do you have and what gear ratio?

 

[mtofell]

I'm assuming 6spd? I'd say it's normal. The 66rfe has terrible programming and absolutely refuses to downshift when it should. I wouldn't say the tires are the cause but they certainly don't help. My truck with stock tires often does the same thing (but practically in every gear). I very often shift manually or just hammer the gas to make it shift.

 

[joeyf86]

It's got the 68RFE with a 3.42 axle

 

[HEMIMANN]

Yeah, if you're not into swapping your trans valve body, try a different trans oil. @Burla had transmission shudder and got it settled down.

I think you can use Red Line C+ATF (Mopar ATF+4 spec). I plan to go that route soon for my 66RFE, which is a 68RFE "light". I already modded my PCM, otherwise I'd swap my valve body too. This requires a PCM re-learn.

 

[Burla]

Overdrive is the first thing you loose with these rfe, especially with the Cummins. If you don't have warranty left, lok into valve body upgrades, sooner the better. Use pedal to help chnage gears with that cummins, the torque just eats the rfe's. My brother went through 3 transmissions with his cummins, back then I believe it was the 48.

They have kits, or you can just buy a built valve body with all of the upgrades. The fact it is a young truck, it really makes sense to attempt this imo. Get a metal thread upgrade and look up our synthetic filter upgrade for the rfe thread on RF.

Product Overview: ATS 68RFE Valve Body and ATS Co-Pilot

Clint Cannon, President of ATS, describes the inherent weaknesses of the 68RFE (545RFE, too) valve body. This is a comprehensive overview of the numerous pro...

youtu.be

 

[Burla]

My situation was low rpm shudder, not the same as yours, but after you do all this also upgrade to redline c+ transmission fluid. If you just do the fluid and not the VB, it is 8 quarts plus synthetic media filter and mopar oem sump filter, for the sump the best option if oem, it is synthetic anyhow, for the canister read the thread here on RF>

Keep it in 5th til you fix it.

 

[SouthTexan]

Never ever ever tow anything above 7.5k lb in 6th with a 68RFE and 3.42 gears if you are towing below 70 mph. Locking out 6th and towing in 5th is not only easier on the trans, but it is also easier on your fuel mileage since 5th (which is still an overdrive gear) puts your right in the sweet spot rpm between 1,600 and 1,800 according to the BSFC map of the Cummins. Especially if you have larger and heavier tires than stock that are already putting a lot of stress on your trans unloaded. On the 68RFE(an most truck 6 speeds) with 3.42 gears, 6th is unloaded overdrive and 5th is towing overdrive.

EDIT: Sorry, for some odd reason I thought the OP said it was when he was towing.

 

[theviking]

Just an FYI. I had the same type of shutter for years. Tried custom (HF and JG) tuning, new aftermarket torque converter, swapped fluids, even new shocks. Nothing eliminated it. Would have to get off the gas and back on to stop it. Then during my cam swap the EGR accidentally disconnected itself (story I'm sticking with). Never had the shutter again. Not even a hint of it. So take it for what it's worth, but I blame something in the EGR system.

 

[SouthTexan]

Here is the BSFC of your 2018 Cummins. As you can see, the dark blue sweet spot when towing is between 1,600 and 1,800. With 37 inch tires, you are probably at a little under 1,300 rpm at 65 mph which puts you in a window where you are likely to get less fuel economy versus 5th gear putting you just under 1,700 rpm which is the best spot to be at for power and fuel economy.

 

[mtofell]

I agree with Southtexan here. Low RPMs are good for MPGs.... to a point. Drop to 5th and all your problems will likely go away. Your increase in tire size is (somewhat) similar to towing a trailer or other load. The 3.42s are already aimed at light loads at high speeds. Nothing at all wrong with dropping to 5th. You'll likely see increased MPGs and a better overall experience.