I'll repeat this again for more redundancy. I have a 99. For a couple of years, we went to Big Bear every weekend that was not winter and a few in the winter. Same trip along the toll road. Last toll booth is at the top of a long hill. With the cruise control set at 65 it would invariably downshift to second right even with a particular sign without fail.
I added several things one at a time. All the typical stuff, CAI, High flow filter, high flow exhaust. But only one thing at a time.
Each item would work very well the first trip over the hill and delay the downshift by a noticeable margin sometimes even pulling the hill to the top in drive. The next trip it would be right back to downshifting into second at the exact same spot.
32RH, geared to 4:10's on 33's.
Not an exact science by any means but clear enough to me that nothing I did was going to have an effect long term without programming changes.
We have the benefit nowadays of that being available, it was not back then in 99.
That is quite interesting. In terms of science, there seems to be negative feedback of sorts implemented in the programming that cancels out what is seen as anomalous to what the ECU had already learned and stabilized at. It would be really fun to understand how that is implemented.
@Wranglerfix is this something that is very obvious to you given what you know about the TJ ECU?