Tow/Haul Mode, Stuck in 4th Gear - 2018 Mega Cab SB 6.4L Hemi

[Blueduck3285]

I was heading west on I-70 out of Denver to Grand Lake and put the truck into tow/haul when heading over the pass to Winter Park (I have a payload of about 2000 not including myself and my wife). As I was hitting those turns going uphill, dropping down to 10-15 mph, the truck stayed in 4th gear and would not downshift. It was a slog getting up the hill. Even when I tried to switching out of tow/haul and manually forcing the truck into a lower gear, it just stayed in 4th until I was down and had parked and shut the truck off. Once I restarted the truck, everything worked normally.

On our way home, I didn't want the same situation so I kept tow/haul off and manually locked out 5/6 and the truck operated as you would expect, downshifting when slowing for the turns to get into an appropriate powerband.

Would this be an issue with the ECU, that it is sticking in 4th when selecting tow/haul? Even out of tow/haul it stayed in 4th even when coming to a complete stop at a light. So long as I don't use tow/haul, it seems to function perfectly fine.

 

[Hugh Johnson]

Are you sure you didn't have a check engine light on and it kicked truck into limp mode?

 

[Burla]

So you have the 68rfe, I would see if you can duplicate it, I wouldn't worry as you know to go manual you can get it back. How old is fluid? I do believe RFE limp mode doesn't allow early shifting either, so it would be your limp mode.

Run Codes.

 

[62Blazer]

As mentioned above I would suspect a fault code was set which then triggered the "limp mode" in the trans. The limp mode will lock the transmission in 4th gear (depending on what model, but your profile states an '18 6.4 truck which is a 66RE and would be the case). Once the code is set and it goes into limp mode, nothing will change until you erase the code or cycle the key (which basically resets the code). I doubt the tow/haul function really had anything to do with it, rather there is something that happened that the trans didn't like. And that would have happened whether tow/haul was engaged or not.
I would suspect there was a code set you could go in and read, and would also suspect there was a check engine light.