Cruise and not shifting to 8th

[JTdiRtyD]

'22 Rubicon Diesel. I'll start with saying I recently went with 35s and size was set through Tazer.

I've been noticing if the truck is in 7th when I set the cruise, it stays in 7th and won't shift to 8th, but if I disengage cruise it will immediately shift to 8th, and then I can engage cruise again and it will stay in 8th. It has been cold out, so at first I was assuming it was not shifting until the trans warmed up, but even after reaching temp (monitored with ScanGauge) it won't shift to 8th on it's own while in cruise.

I understand a gasser would maybe want to hold 7th with 35s, but a diesel shouldn't care. Thought I'd ask on here if there is a common issue with this, maybe a TSB update that I need, etc. Forum search didn't come up with anything, Google search came up with the same from a couple other forums, one was a Cherokee and one was a Durango, but no causes or solutions and the discussions died out.

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[arosen1997]

Do you by chance have the Shifter pushed over to the manual control? If you do it will hold that gear with cruise on as best it can.

 

[JTdiRtyD]

Do you by chance have the Shifter pushed over to the manual control? If you do it will hold that gear with cruise on as best it can.

Nope. Always in D

 

[arosen1997]

Nope. Always in D

Huh weird. Instead of taking it I and out of cruise you could also move the Shifter to M and put it in 8th yourself.

 

[JTdiRtyD]

Huh weird. Instead of taking it I and out of cruise you could also move the Shifter to M and put it in 8th yourself.

I could, but if there IS something going on that doesn't resolve it. It otherwise performs just fine, so I think it's just the cold or maybe needs a relearn. Unsure if these have a relearn process, but I figured I'd disconnect batteries overnight this weekend and see what happens. But thought I'd see what others have to say.

 

[Minty JL]

all comes down to the 1s and 0s programmed in the computer

 

[kevman65]

Aren't 7th and 8th like a blonde P hair different?

 

[Maximus Gladius]

I am going to suggest a “quick learn” as my Mucar CDE900 pro reader, full diagnostic FCA SGW unlock license tool says it.

I performed a quick learn (relearn) on my 8 speed auto yesterday because I had recently changed my tire size to 315/79/17 and had the odometer calibration done at the dealership. I asked if the calibration changes the shift points in the transmission and they didn’t know. He did say it corrects the odometer. So, since getting this new pro reader and purchasing the FCA SGW license, I started researching what others were saying about calibrations, why do them and when to do them and doing a relearn on the transmission is something that was said a lot when changing tire size. ….so I did last night and I found going to work today and back home (about 60k) the transmission found 8th gear a lot more than it did before. Ran a bit cooler too. I’m happy.

 

[Maximus Gladius]

One more point I’d like to make about the MUCAR CDE900 Pro reader, when I was flipping through the “special functions” concerning the transmission, It shows our transmissions learning process as “long learn” and I’m thinking way back to when my first transmission was having problems and I took it in to a transmission specialist, he told me the transmission usually takes 10k kms to “learn”. So if you have that or more, and then change tire size, the transmission is “well learned” and conditioned to operate as it always did, which makes sense that your transmission wants to stay in 7th with the new, larger sized tires.

Of course there may also be the need to regear (which also requires the need to recalibrate) but I do think and got to see if you only did a quick learn on the tranny, it made a difference for me.

Others can chime in if they know calibrating the new tire size with JSCAN or taser not only fixes the odometer but also does the relearn for the transmission. I recall the dealership calibrating my tire size used the AlfaOBD2 reader for that, but as I said, he didn’t know if that changed the shift points but after doing my own quick learn, the Alfa system didn’t do it.

 

[ncl]

On my gasser I went from stock 245/75/17 to the standard rubi size 285/70/17. I had to fiddle with the tire size calibration a little for it shift the way I felt was correct. 1st setting it shifted to 8th too soon. 2nd time it wouldn't hit 8th much like you. Mind you truck came set to 31.9 from factory. I think I settled on 32.5 for tire size? Can't remember. But maybe fiddle with the size and go to 34.5 ot 34.75 and see if that helps. My speedo was always pretty accurate during the different settings as well.

 

[Sandman 4x4]

'22 Rubicon Diesel. I'll start with saying I recently went with 35s and size was set through Tazer.

I've been noticing if the truck is in 7th when I set the cruise, it stays in 7th and won't shift to 8th, but if I disengage cruise it will immediately shift to 8th, and then I can engage cruise again and it will stay in 8th. It has been cold out, so at first I was assuming it was not shifting until the trans warmed up, but even after reaching temp (monitored with ScanGauge) it won't shift to 8th on it's own while in cruise.

I understand a gasser would maybe want to hold 7th with 35s, but a diesel shouldn't care. Thought I'd ask on here if there is a common issue with this, maybe a TSB update that I need, etc. Forum search didn't come up with anything, Google search came up with the same from a couple other forums, one was a Cherokee and one was a Durango, but no causes or solutions and the discussions died out.

In most of the later models I’ve owned, cruise control is more of a convenience tool than mileage gaining on highway. I have always averaged more mpg by just barely maintaining the speed by feathering the gas pedal, than using cruise control. I find CC downshifting too soon and holding a lower gear, while the average mpg drops. But using the adaptive cruise control in bumper to bumper traffic works amazing at both convenience and mileage.

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