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Change gear ratio 3:73 to 3:55 in PCM

Wolverine

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Swapped my 2017 GTPP from the original 3.73 to a 3.55 (whole diff swap) last night. With help from @TigerFire13 we used Forscan to update the axle ratio code from 0175 to 0163. Additionally, I have been using shorter tires than OEM for several months now, and since we had Forscan connected we updated the tire height so my speedometer/odometer would be accurate. This meant changing the tire hex value to 07A1. Changing things in the BCM (Body Control Module) sets off DTC U0422 in the PCM (Powertrain Control Module).

Several test drives and Forscan tweaks later and I am still getting the same issue as OP. No power in 4th or 5th gear between about 2,500-4,000 rpm.
 
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2016s550gt

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Swapped my 2017 GTPP from the original 3.73 to a 3.55 (whole diff swap) last night. With help from @TigerFire13 we used Forscan to update the axle ratio code from 0175 to 0163. Additionally, I have been using shorter tires than OEM for several months now, and since we had Forscan connected we updated the tire height so my speedometer/odometer would be accurate. This meant changing the tire hex value to 07A1. Changing things in the BCM (Body Control Module) sets off a DTC in the PCM (Powertrain Control Module) which I think is U0422 or P160A.

Several test drives and Forscan tweaks later and I am still getting the same issue as OP. No power in 4th or 5th gear between about 2,500-4,000 rpm.
I switched from 373s to 331s. I am having the same issue. I changed it on forscan but didn’t fix it. I am wondering if I should get the Whipple flare tool but don’t want to spend the $300 and not being able to fix it
 

2016s550gt

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@2016s550gt Have you tried driving on your car's original OEM Forcan setup, with no gearing or tire changes in the programming? (the only change being the physical gears themselves). If so, did you still have the limp-mode issue?
Yes I have, that was the original problem. That’s why I tried forscan but same issue, might just order the Whipple flare tool and see how that goes or switch back to 373s
 

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Swapped my 2017 GTPP from the original 3.73 to a 3.55 (whole diff swap) last night. With help from @TigerFire13 we used Forscan to update the axle ratio code from 0175 to 0163. Additionally, I have been using shorter tires than OEM for several months now, and since we had Forscan connected we updated the tire height so my speedometer/odometer would be accurate. This meant changing the tire hex value to 07A1. Changing things in the BCM (Body Control Module) sets off a DTC in the PCM (Powertrain Control Module) which I think is U0422 or P160A.

Several test drives and Forscan tweaks later and I am still getting the same issue as OP. No power in 4th or 5th gear between about 2,500-4,000 rpm.
Have you accomplished the PCM re-learn procedure in FORScan?
 

Wolverine

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I have executed the PCM reset/relearn procedure a total of 3-4 times, using two separate methods as outlined on the Forscan forum.

Tonight I will be reloading the OEM settings back for both gear ratio and tire size changes, so this line will be as it was when stock. Will see if that has an affect on the limp-mode issue. If not, I need to figure out why the PCM relearn isn’t working properly.
 

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2016s550gt

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I have executed the PCM reset/relearn procedure a total of 3-4 times, using two separate methods as outlined on the Forscan forum.

Tonight I will be reloading the OEM settings back for both gear ratio and tire size changes, so this line will be as it was when stock. Will see if that has an affect on the limp-mode issue. If not, I need to figure out why the PCM relearn isn’t working properly.
Did the same thing but nothing. I will also probably try it again
 

Tommy V

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You need to.put the orginal data back in the pcm,when u do a gear swap,u.do not need to change anything in the pcm,mph is based off oss sensor in the trans.I have done muliply gear change swaps and have never had to change any parameters,verified my mph.to be exact with a gps also.The 11 to 14 got mph signal from abs so.changes had to be made when a gear swap,not the case with 15+.
 

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Last night I reverted back to stock Forscan settings in the BCM line 726-12-01. Stock gear ratio, and tire size, and check sum values. Tried another PCM reset and that did remove the U0422 code from the PCM, but the limp-mode issue is still present after driving it 2-3 miles. Maybe it needs more miles to self-learn?

Issue most prevalent in 4th gear (2,500-3,500 rpm) and 5th gear (1,750-3,000 rpm).
 

Tommy V

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Should not be going into limp mode just from a gear swap,did u do anything eles at the time.
 

Wolverine

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It’s all detailed in this thread, post #17 shows the conditions when this started.

I swapped pumpkins from 3.73 to 3.55 and updated Forscan line 726-12-01 from the stock gear ratio value (0175) to the new gear ratio value (0163), as well as stock tire size value (0858) to a new tire size value for my shorter tires (07A1). Forscan automatically calculates the new checksum value based on the first two number sets.

Any change to the BCM will trigger a DTC in the PCM because the PCM is now receiving information that is isn’t expecting to see from the BCM. It’s like throwing a red flag saying “Hey what’s the deal here?”. Now you should be able to reset/reflash the PCM somehow to force it to accept the new BCM values/edits as if they were the new stock values. Telling the PCM that my car came with the 3.55’s and short tires from the factory. From that point onward the PCM would expect (and see) the updated values I first put in and no DTC errors would arise again. That is my current understanding of how this is SUPPOSED to work. Unfortunately, that’s not how it’s actually working in my case.

When I put the BCM figures back to stock, the DTC I was getting from the PCM (U0422) goes away. Great! Except the limp-mode driveability issues remain. Not great.

My last remaining hope is that a Lund Ngauge will be able to cure the issue. I do not want to buy the Whipple Flare Tool for this silly of an issue.
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