Dealer says tire size is creating computer fault? - JKowners.com : Jeep Wrangler JK Forum
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post #1 of 12 Old 03-08-2019, 06:40 AM Thread Starter
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Dealer says tire size is creating computer fault?

Hi all. I currently have my wife's 2012 JK in the shop at the local Jeep dealership for a recurring issue. She will be driving down the road normally, the engine will "surge" (her words), and the check engine light will come on. For the following couple of minutes she says there is no power, she will put the pedal to the floor and have no acceleration. After a couple of minutes the power comes back, but the check engine light comes on, and stays on for the next 50 or so miles.

The last couple of times it happened I couldn't get my hands on a code reader in time to check it before the light went off, but this time I did and it read 'P0730 Incorrect Gear Ratio'. Took it to the dealer since we have an extended warranty, and after a week of sitting on their asses they finally called me and the service writer is trying to tell me that I need to modify the computer to match the tires that we have on there. I'm running 315/70/17 Ko2's, so basically a 34.5" tire. He is trying to say this is throwing off the computer and they can't set the tire size that large in the computer, and I need to get an aftermarket programmer to set the proper tire size.

This sounds to me like they don't know what is going on and are just taking a shot in the dark.... anybody have any experience with this?
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post #2 of 12 Old 03-08-2019, 06:55 AM
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They are correct that the computer needs to be reprogrammed. I had a similar issue when I went to 275 tires.
Now as to if they are unable to program your Jeep, I don't know.
If you get a programmer, you have 2 options.
1. A simple programmer, which you can use your tire size, gear ratio, what happens when you hit lock on the fob, which lights come on for day time running lights. Costs around $180
2. A performance programmer, it does all of the above. Plus it comes several canned programs, one for performance, one for better MPG, etc. Some you can use to program shift points. These cost around $350
Another option is to find a custom Tuner which is a technician who will program your vehicle for how you drive, and any mods you've done. This starts at around $1000

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post #3 of 12 Old 03-08-2019, 07:11 AM Thread Starter
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I had a SCT programmer for my powerstroke years ago, and it did wonders. Aside from correcting the tire size I guess I didn't think there was that much performanc to gain from the JK motor. Any recommendations on a good programmer with custom tuning?
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post #4 of 12 Old 03-08-2019, 07:40 AM
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custom tuning for what?


get one of these or any of the others that reprogram for tire size changes. Just depends on how much you want to spend and what features you want. I got the diablo sport for my needs. Just don't update it.


https://www.extremeterrain.com/super...0716-3571.html
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post #5 of 12 Old 03-09-2019, 08:56 AM
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The dealership cannot reprogram the computer for oversized tires. The only way is from the factory or with an aftermarket device like a flash cal or procal. Also this will be paired to your jeep alone until you reset the jeep to stock and reset the module as well.
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post #6 of 12 Old 03-09-2019, 08:58 AM
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Oh I didn't know about those other methods posted above, to expensive for me anyway hahaha
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post #7 of 12 Old 03-10-2019, 11:50 PM
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Originally Posted by libordemise View Post
Oh I didn't know about those other methods posted above, to expensive for me anyway hahaha
If you can't afford a new ProCal or FlashCal, check Craigslist or wherever for a used one. But if you go used, do an in-person sale rather than shipped so you can plug it in and verify that it is not still VIN-locked to a previous vehicle.

Another option might be a smartphone app that I've seen talked about in a few posts. Something like $15 plus a cheap odb cable. No idea if it works, but might be worth looking into anyway.
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post #8 of 12 Old 03-11-2019, 02:28 PM
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Tire size shouldn't be causing that fault, they will make your speedo off but that should be it. With the improper gear fault, did you or someone else change gears? I know on 12s and newer you need a flashcal or other programmer to input gear change numbers when you change gears.
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post #9 of 12 Old 03-12-2019, 05:13 PM
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Quote:
Originally Posted by Chris Bohan View Post
Hi all. I currently have my wife's 2012 JK in the shop at the local Jeep dealership for a recurring issue. She will be driving down the road normally, the engine will "surge" (her words), and the check engine light will come on. For the following couple of minutes she says there is no power, she will put the pedal to the floor and have no acceleration. After a couple of minutes the power comes back, but the check engine light comes on, and stays on for the next 50 or so miles.

The last couple of times it happened I couldn't get my hands on a code reader in time to check it before the light went off, but this time I did and it read 'P0730 Incorrect Gear Ratio'. Took it to the dealer since we have an extended warranty, and after a week of sitting on their asses they finally called me and the service writer is trying to tell me that I need to modify the computer to match the tires that we have on there. I'm running 315/70/17 Ko2's, so basically a 34.5" tire. He is trying to say this is throwing off the computer and they can't set the tire size that large in the computer, and I need to get an aftermarket programmer to set the proper tire size.

This sounds to me like they don't know what is going on and are just taking a shot in the dark.... anybody have any experience with this?
Dealer is for the most part pointing you in the right direction. Get an AEV Procal or a tuner that has the tires size function and set your tires to the LOADED diameter at your street inflation pressure. Don't use the size marked on the tire, or in your case a guess at what actual size it is. The fault will clear itself.

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post #10 of 12 Old 03-13-2019, 09:34 AM
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There is an OBD JSCAN for ANDROID and IOS available for download for only $20; you will only need a compatible OBD reader (see YouTube and type OBD JScan). This programmer can do a lot of functions from changing your tire size, enabling camera function, enabling DRLs, etc. Check it out since either will save you a lot of $$$ from those programmer or going to the dealership. Let me know of it works out for you.
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post #11 of 12 Old 03-13-2019, 06:36 PM
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How long have you been running those tires?

I can't help but have a suspicion that there is something else going on besides just tire size. As mentioned, going up in tire size will make your speedometer read low and it could make your shift points a bit different. But you are only running 34"-ish tires. I can't imagine it's enough to put the thing into limp mode.

Tire size doesn't change the gear ratio that the transmission sees. The rotations in will still match the expected rotations out. From what I can tell, that fault has to do with internal transmission ratios, nothing to do with tire size. I think it more likely indicates an issue with shifting (like a failing or sticking valve), slipping, or a torque converter issue.

I could be wrong. Maybe there is something strange going on in the computer and all you need to do is program in the correct tire size. But I have a feeling there is another issue at play here.
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post #12 of 12 Old 03-14-2019, 05:38 AM
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Dealerships seem to follow the factory recommended flow charts to find a problem. It's sort of like "does power interrupt when door opens?" So they get you to put the doors on, "does power interrupt when hood is opened" then "does power interrupt because battery has been removed?" .
Instead of checking the fuse first.

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Running 37's as a daily driver is dumb
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