08 GC Diesel Tire size change computer reset?

dsldave · 10-30-2009, 10:09 AM

I had a dealer install BF LT265/65R18 tires with 3.5in rock krawler lift kit on my 08 overland diesel and they had issues with check engine light some transmission code. They cleared it and said it had something to do with the computer re-learning the new tire size. I'm just not confident with dealers on technical issues because they rarely give me a straight answer.

Under moderate acceleration, the transmission takes a long time to shift even though you back off on the pedal it holds the gear for a couple of seconds. Does anyone know if re-chipping the engine [Easychip] would make it all work better and drivabilty would be more consistant. Tire size went from original 29.6"OD to 31.7" OD now

**LJS8510** · 10-30-2009, 01:28 PM

I have not had any problems
but
check this thread out

http://www.jeepforum.com/forum/f67/inmotion-tune-dyno-comparison-holy-844005/

INMotion Tuning® USA Homepage

suzieque · 10-30-2009, 02:43 PM

I believe, that the dealer can use a starscan to set the tire diameter based on some built in parameters. Anything outside those dealer accessible parameters requires custom chip programming. The difficultly is finding out what paramters are available.

dsldave · 10-30-2009, 03:14 PM

Thanks very much for the info LJS8510 and Suzieque. I can't believe the wealth of information and experience the people of this forum offer to help others out. Hopefully some day I can return the favour.

I am still under warrantee and am hesitant about inmotion because it appears they actually reprogram the ECU or whatever its called rather than a plug in module that can be removed prior to going in for service. It appears inmotion looks like the way to go and maybe I'll just forget about the warrantee because I can't stand the way the diesel/tranny shifts inconsistantly.

Again thanks so much, and to Suzieque, I hope you get all info from dash monitor working, I 've been following that thread as well. Yep, nothing like have this sofisticated engine and only a temp and fuel guage..

Robert J. yates · 10-30-2009, 03:24 PM

So my question is whether or not anyone else has had these issues when going larger on the tire size? I am getting ready to install a set of 265/60/18's which are 1" larger than stock and I have been planning on Starscan getting me close if not right on the mark.

justa4banger · 10-30-2009, 04:22 PM

INmotion tune won;t fix the shifting..... it will still be stupid sometimes... but with the added power you'll forget about the stupid shifting.....

I also believe the trans controller uses LOAD input from the engine to determine if the truck is working harder... if it meets this requirment it seems to hold the truck in a gear a bit longer....NOW this is MY THEORY............ but with my recent towing adventures i noticed the truck would really wind out before jumping to the next gear.... take the trailor off and ran it down the 1/4 mile and it worked awesome.... hook the trailor back up and i can see/feel the shifts HOLDING before upshifting...

MAybe the bigger tires add enough load to start this function in the TCM..... This is only a guess, and YES tires make a HUGE difference in performance and rotating mass......

justa4banger · 10-30-2009, 04:25 PM

Oh and no one should wait for an INMOTION TUNE.. if you do wait and then later get it...........you'll be kicking yourself for never getting the tune sooner... Oh and his "tune" is unreadable by Starscan. infact, unless they actually flash your ECM with an update the tune is invisbale and won;t bother a thing...

the "tune" is actually the original software, just rewritten the way it should have been done by MB/Chrysler

Yagro · 10-30-2009, 06:26 PM

Hey. I have a 2008 with the CRD Diesel engine and just recently put in a Superlift 4" lift kit and some BFG 285/75R17 tires and part of the kit was to flash the ECU with the new tire diameter. This fixed the speedometer error and shifts seem fine. I am wary installing a boost that just increases fuel pressurem but I was curious what kind of milage anyone with a similar setup is experiencing.

ctate · 10-30-2009, 07:06 PM

Quote:

Originally Posted by dsldave

I had a dealer install BF LT265/65R18 tires with 3.5in rock krawler lift kit on my 08 overland diesel and they had issues with check engine light some transmission code. They cleared it and said it had something to do with the computer re-learning the new tire size. I'm just not confident with dealers on technical issues because they rarely give me a straight answer.

Under moderate acceleration, the transmission takes a long time to shift even though you back off on the pedal it holds the gear for a couple of seconds. Does anyone know if re-chipping the engine [Easychip] would make it all work better and drivabilty would be more consistant. Tire size went from original 29.6"OD to 31.7" OD now

The dealer cant change the tire size to a tire that big. You will have to get a calibrator, like the procomp accu pro, to fix your shifting. The problem you are having with your transmission is the shift points. Once you calibrate your rig, then you wont have that problem anymore. I did not notice any power loss with mine when I lifted it and put bigger tires on. Your dealer has no idea what they are talking about with the computer relearning the tire size. They just cant change the computer to you size.

bytehoven · 10-30-2009, 07:29 PM

Quote:

Originally Posted by Robert J. yates

So my question is whether or not anyone else has had these issues when going larger on the tire size? I am getting ready to install a set of 265/60/18's which are 1" larger than stock and I have been planning on Starscan getting me close if not right on the mark.

Robert... I have run a +1-2" size tire on several Dodge and Jeep vehicles over the past 10 years, and never had any problems. I never had the dealer change the programming.

I also plan to run 265/60R19 on my 09 WK and I don't plan to have the computer updated unless the dealer agrees to make the change for free when I bring the Jeep in for the next oil change.

EDIT: Apparently the WK may be more sensitive to tire size changes, although I did not notice any problems while I was running a 30.5" set of LTX M/S2 tires for about a week.

ctate · 10-30-2009, 08:13 PM

On the WK, if you dont calibrate, you will get hard and uncomfortable shifting...like he is describing above.

bytehoven · 10-30-2009, 08:29 PM

Thanks...

I briefly had a set of 265/60R18 LTX M/S2 tires on my 09 WK and did not notice any odd shift behavior. Does the shift occur after a period of time or only with larger tire sizes?

Swayse · 10-31-2009, 08:08 AM

We have 265/70R17, 8% larger tire, our jeep shifts great. I guess we have one of the odd ones, I've always liked how the Jeep trans shifted. I'd love to have more power, but I'm scared it'll ruin the shifting by confusing the computer that controls shifting. But, going to a bigger tire size made the shifting better if nothing else.

**LJS8510** · 10-31-2009, 08:16 AM

Quote:

Originally Posted by Swayse

We have 265/70R17, 8% larger tire, our jeep shifts great. I guess we have one of the odd ones, I've always liked how the Jeep trans shifted. I'd love to have more power, but I'm scared it'll ruin the shifting by confusing the computer that controls shifting. But, going to a bigger tire size made the shifting better if nothing else.

Thats how I generally feel about mine, although I know my dealer mechanic did what he could when it came to tire size. My understanding is he used starscan to adjust for largest size tire it can, just not as large as we have.

I'm still wanting to check out Inmotion.
Just been to busy to do anything.

Swayse · 11-01-2009, 08:45 AM

Wonder if it would work to get the dealer to change the axle ratio that the computer sees with a starscan? If we went in with a starscan and changed the axle ratio from 3.73 to 3.54, it would get me really close to where it needs to be, I think. Could an independent shop have the tools to change the settings in the computer?