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JLU RPM and 8 speed shifting issues?

Hard Rock Jeep

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Unfortunately no way to check fluid on the JL jeep.
Wow! Did not know that. I am a JK owner looking at the diesel. My first reaction to your comment was to see if today is April 1st. It seems hard to picture why that is a good idea on a Jeep.
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GrnSahara

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Ok, let's look at your facts. You have a new Sahara, you add a Mopar lift and 35" tires. You reprogram your computer and in an earlier post you say it ran good at first and then did not. Your engine in turning more RPM's than it theoretically should based on your differential ratio, your transmission gear ratio and the number of revolutions your tire has to make to achieve the speeds you are reporting. Let's assume your tach is correct (but we are still assuming something we don't know). Gears are gears and they are delivering a specific turning force to your tires. The next assumption is that you really do have a 3.45 gear ratio in your diff. Next we assume that your 35" tires are actually 109.9" in circumference yielding 576.52 tire rotations per mile. (Minor variations in tire size would not account for all of your reported problem.) If all of this is correct, you cannot be turning the RPM's you report at those speeds, UNLESS there is slippage (as noted above) in your transmission. Now, go check your transmission fluid. Is that OK? The only place you have "built in" slippage is in your torque converter. Low fluid could be preventing the transfer of power from your engine to your transmission at that point, necessitating over rev'ing your engine. If the fluid is correct, you need someone to open it up and see what is going on inside.
Thank you, I completely agree with you and have decided to let Jeep take care of this and hopefully send it to a transmission speciality shop for at least another opinion. I have contacted Jeep and they have 'escalated the case' but it likely won't lead anywhere unless a dealership wants to actually fix the problem.

I have taken it to two separate Jeep dealers, showing them the videos and complaining about the transmission. Neither of them would even consider taking it on a test drive with me, due to the fact that "the onboard computer is not throwing a code - therefore there is nothing wrong." Almost as if it is scripted, word for word from both 'master technicians'.

Their reasoning, without paraphrasing was "you put a lift on with 35s - so that basically voids the warranty and it's going to drive like that. The lift says it will accommodate 35's but really it's made for 33s - so the only thing to do is remove the lift and wheels and see how that goes for you"

The lift is MOPAR and the Wrangler has 800 miles on it. Hopefully @JeepCares continues to escalate this so I can actually drive it.
 

Hard Rock Jeep

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Thank you, I completely agree with you and have decided to let Jeep take care of this and hopefully send it to a transmission speciality shop for at least another opinion. I have contacted Jeep and they have 'escalated the case' but it likely won't lead anywhere unless a dealership wants to actually fix the problem.

I have taken it to two separate Jeep dealers, showing them the videos and complaining about the transmission. Neither of them would even consider taking it on a test drive with me, due to the fact that "the onboard computer is not throwing a code - therefore there is nothing wrong." Almost as if it is scripted, word for word from both 'master technicians'.

Their reasoning, without paraphrasing was "you put a lift on with 35s - so that basically voids the warranty and it's going to drive like that. The lift says it will accommodate 35's but really it's made for 33s - so the only thing to do is remove the lift and wheels and see how that goes for you"

The lift is MOPAR and the Wrangler has 800 miles on it. Hopefully @JeepCares continues to escalate this so I can actually drive it.
Your comment reminds me of a couple years ago at a Jeep dealer (but it could have been any auto dealer) where I commented that they needed to get a mechanic to look at something that was theirs that sounded funny. He said "we don't have mechanics anymore. We have technicians. They only know how to hook it up to the computer and read what the code says needs to be replaced. Nobody fixes anything anymore."

And I will add that you should not let them get away with saying a suspension lift (theirs or anyone else's) voids the warranty on non-suspension issues. Have a polite conversation with the people above the dealership and if they blow you off, get a lawyer. It is not a big deal. It is how our society resolves issues when two people cannot agree.
 

Stuckinthesand

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this is not true.
Technically you are correct. Just sucks to have to climb under it to do it. Should’ve kept the dip stick. I don’t mind it if the weather is nice but it’s freezing in NJ and I’m not climbing under there unless necessary.
 

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mwilk012

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Technically you are correct. Just sucks to have to climb under it to do it. Should’ve kept the dip stick. I don’t mind it if the weather is nice but it’s freezing in NJ and I’m not climbing under there unless necessary.
Too many idiots try adding incompatible fluids, unfortunately. If you’re having transmission problems, the dealer at a minimum needs to check the fluid level and condition.
 

jmccorm

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The transmission shifts off output shaft speed and has nothing to do with the ECU. More simply it's shifting due to RPM and your RPM will be correct regardless of what the speedometer says rightly or wrongly. Thinking the transmission shifts better after adjusting the speedometer is 100% placebo.
Please pardon the essay. I'll share with you my own story which started two or three months ago. Or you can jump forward to the end to read the conclusions.

Not too long ago, I made a thread where I documented an issue with my JL's transmission. To help figure out what was going on, I coded a CAN bus recorder and (with the help of others) I started reverse engineering some of the traffic between modules.

The problem I was having with my vehicle seemed like intermittent hesitation. With a bit of trial and error, I finally found a way to reproduce the problem at will. The hesitation could be triggered by quickly flooring the accelerator.

The vehicle might be going 40mph in 8th gear, I'd floor it, and it'd take 1.6 seconds of coasting before it finished shifting gears and started to accelerate. The CAN bus logs (shared in that same thread) showed the transmission downshifting not once but twice, and it also showed the vehicle taking some extra time to fine-tune the rev-matching before finally shifting into gear.

As I reversed engineered more and more CAN bus messages, I found that the Wrangler maintains two major measurements of vehicle speed. The first is a GPS corrected speed, and that's what you see on your speedometer. The second was a calculated value based upon engine RPM, gear ratios, and tire size. It appeared to be used for mileage.

In my vehicle, the two measurements did not match. So I knew that my tire size was not set correctly. And right away, I confirmed at least one theoretical effect of this mismatch: my vehicle was adding up 20% more mileage than I was actually driving. (It effectively shaved an extra 1,000 miles from my warranty, but I wasn't going to try to push the issue.)

Now, I didn't know if fixing that issue would resolve the slow shifting, but I certainly wanted to correct it for it's own sake... if not to get my mileage right. So I slowly increased my tire size, bit by bit, until the vehicle's GPS assisted speed and it's calculated speed were the same. In the end, my size appeared to have been set about two inches too small. Today, I don't think I could have arrived at a more precise tire size setting any other way.

Now, the key question: What changes did I see?

Well, I saw three changes.

First, a reduction in time it took the transmission to shift gears under the same scenario. It went from a consistent 1.6 seconds to a consistent 1.0 seconds. My analysis of the CAN logs showed me that the double-downshifting was still happening, but the rev matching completed much more quickly. It seemed to know more precisely how many RPMs the engine needed to hit, and once it got there, it didn't need to spend any more time trying to fine-tune it.

Second, a completely unexpected finding, the automatic transmission would no longer automatically upshift into 8th gear at 40mph. It was an annoyance, but I thought that this was normal "eco behavior" for a Wrangler. Apparently not. With the tire size corrected and under similar circumstances, the Wrangler doesn't want to upshift to 8th gear unless the vehicle was traveling at least 50mph on level ground.

Third, the vehicle would now recorded it's mileage with a very high degree of accuracy. That was predicted and expected, so it was was good see the real-world confirmation.

So...

My own experimental data supports the claim that tire size and gear ratio settings do play at least two significant roles in the operation of the automatic transmission:

1. Rev matching​
2. Maximum gear to upshift into (increasing with vehicle speed)​

Additional effects are likely to present themselves when the mismatch is far greater, and again when there's a mismatch in the opposite direction (too big instead of too small).

The difference in when it upshifted was a bit of a surprise because I would have thought that it would have been based upon engine load and nothing more. Apparently not. But a mismatch having an impact upon rev matching? I can see how that happens.

CONCLUSION

It's a common fallacy on these forums that adjusting the speedometer will fix any transmission shifting issues.
On a literal level, as it turns out, I agree with you! There is no connection between speedometer and shifting issues. Transmission behavior is completely divorced from the GPS-corrected speed that you see on the speedometer. The number presented on the dash is for human benefit and nothing more.

However, a close cousin to that almost certainly is at play here. That's a computed vehicle speed (not normally shown to the driver) that is used by the transmission. Based upon real-world experimental data, at least to my own satisfaction, I believe I've demonstrated a strong connection between gear and tire size settings and some automatic transmission shifting issues.

For those with automatic transmission issues who are able to verify their vehicle's gear and tire size settings, it seems like a simple and worthwhile check to perform before moving on to larger issues.

Don't believe a word of it? My mother's from the "show-me" state, so I understand. Luckily, this is something that you can confirm or refute experimentally. Record your current settings and then start to drift away from those values. Go small at first, just to be safe. The degree to which you see an impact should be proportional to the degree of which you've moved away from the ideal values.
 

UtahDirt

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Please pardon the essay. I'll share with you my own story which started two or three months ago. Or you can jump forward to the end to read the conclusions.

Not too long ago, I made a thread where I documented an issue with my JL's transmission. To help figure out what was going on, I coded a CAN bus recorder and (with the help of others) I started reverse engineering some of the traffic between modules.

The problem I was having with my vehicle seemed like intermittent hesitation. With a bit of trial and error, I finally found a way to reproduce the problem at will. The hesitation could be triggered by quickly flooring the accelerator.

The vehicle might be going 40mph in 8th gear, I'd floor it, and it'd take 1.6 seconds of coasting before it finished shifting gears and started to accelerate. The CAN bus logs (shared in that same thread) showed the transmission downshifting not once but twice, and it also showed the vehicle taking some extra time to fine-tune the rev-matching before finally shifting into gear.

As I reversed engineered more and more CAN bus messages, I found that the Wrangler maintains two major measurements of vehicle speed. The first is a GPS corrected speed, and that's what you see on your speedometer. The second was a calculated value based upon engine RPM, gear ratios, and tire size. It appeared to be used for mileage.

In my vehicle, the two measurements did not match. So I knew that my tire size was not set correctly. And right away, I confirmed at least one theoretical effect of this mismatch: my vehicle was adding up 20% more mileage than I was actually driving. (It effectively shaved an extra 1,000 miles from my warranty, but I wasn't going to try to push the issue.)

Now, I didn't know if fixing that issue would resolve the slow shifting, but I certainly wanted to correct it for it's own sake... if not to get my mileage right. So I slowly increased my tire size, bit by bit, until the vehicle's GPS assisted speed and it's calculated speed were the same. In the end, my size appeared to have been set about two inches too small. Today, I don't think I could have arrived at a more precise tire size setting any other way.

Now, the key question: What changes did I see?

Well, I saw three changes.

First, a reduction in time it took the transmission to shift gears under the same scenario. It went from a consistent 1.6 seconds to a consistent 1.0 seconds. My analysis of the CAN logs showed me that the double-downshifting was still happening, but the rev matching completed much more quickly. It seemed to know more precisely how many RPMs the engine needed to hit, and once it got there, it didn't need to spend any more time trying to fine-tune it.

Second, a completely unexpected finding, the automatic transmission would no longer automatically upshift into 8th gear at 40mph. It was an annoyance, but I thought that this was normal "eco behavior" for a Wrangler. Apparently not. With the tire size corrected and under similar circumstances, the Wrangler doesn't want to upshift to 8th gear unless the vehicle was traveling at least 50mph on level ground.

Third, the vehicle would now recorded it's mileage with a very high degree of accuracy. That was predicted and expected, so it was was good see the real-world confirmation.

So...

My own experimental data supports the claim that tire size and gear ratio settings do play at least two significant roles in the operation of the automatic transmission:

1. Rev matching​
2. Maximum gear to upshift into (increasing with vehicle speed)​

Additional effects are likely to present themselves when the mismatch is far greater, and again when there's a mismatch in the opposite direction (too big instead of too small).

The difference in when it upshifted was a bit of a surprise because I would have thought that it would have been based upon engine load and nothing more. Apparently not. But a mismatch having an impact upon rev matching? I can see how that happens.

CONCLUSION


On a literal level, as it turns out, I agree with you! There is no connection between speedometer and shifting issues. Transmission behavior is completely divorced from the GPS-corrected speed that you see on the speedometer. The number presented on the dash is for human benefit and nothing more.

However, a close cousin to that almost certainly is at play here. That's a computed vehicle speed (not normally shown to the driver) that is used by the transmission. Based upon real-world experimental data, at least to my own satisfaction, I believe I've demonstrated a strong connection between gear and tire size settings and some automatic transmission shifting issues.

For those with automatic transmission issues who are able to verify their vehicle's gear and tire size settings, it seems like a simple and worthwhile check to perform before moving on to larger issues.

Don't believe a word of it? My mother's from the "show-me" state, so I understand. Luckily, this is something that you can confirm or refute experimentally. Record your current settings and then start to drift away from those values. Go small at first, just to be safe. The degree to which you see an impact should be proportional to the degree of which you've moved away from the ideal values.
GPS assisted speedo Interesting, Where is that documented?
 

jmccorm

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...my vehicle was adding up 20% more mileage than I was actually driving. (It effectively shaved an extra 1,000 miles from my warranty, but I wasn't going to try to push the issue.)
Out of an abundance of caution, I realize that I should be more clear, just in case someone misinterprets this statement and it comes back to bite me in some unfair manner.

Because my tire size was set incorrectly, my vehicle was registering 20% more mileage than it actually traveled. The impact is that my vehicle recorded ~1,000 more miles than was actually driven. As a result, with respect to warranty, is that I lose 1,000 miles of warranty coverage. Not the other way around.

That's yet another good reason to make sure that you have your tire size set right. Thinking of setting your tire or gear size wrong, but in the other direction? Don't. While I don't have proof, if they are so motivated, I can think of one or two ways Jeep might be able to figure that out.
 

jmccorm

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GPS assisted speedo Interesting, Where is that documented?
One of the many undocumented features built into your vehicle.

This might be limited to 8.4" uConnect radios. Some people say that the adjustment is instant, others say that self-adjustment takes a bit more time. You can check out this post back in July 2018 where yet another owner discovers this undocumented feature for themselves.

PS: In the end, they were wrong to believe that having your phone plugged into the vehicle was part of the correction process. It is not.
 

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jeepoch

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...
Don't believe a word of it? My mother's from the "show-me" state, so I understand. Luckily, this is something that you can confirm or refute experimentally. Record your current settings and then start to drift away from those values. Go small at first, just to be safe. The degree to which you see an impact should be proportional to the degree of which you've moved away from the ideal values.
Josh,

Kudos, very well thought out and written detail of your experience. I at least personally, really appreciate people who really dig down deeper than just veneer level in order to investigate root cause. Good job.

I have a 2019 Sport S 3.6L Auto but don't experience the two gear downshift out of 8th unless I really stomp on the accelerator. Of course it drops into 7th with almost any reason, (slope, load, or any throttle increase). Truthfully I'm no lead-foot and I'll have to experiment more to confirm your findings.

However, I have every reason to believe that I'll be able to reproduce your conclusions, with just one caveat; the speedometer reading determination.

My Sport has the lowest level UConnect 3 Radio of which I'm very confident has no GPS receiver. When I've lifted and added 35" inch rubber my tire size increased almost 3.5". From 31.5 to 34.78". My final result was based on both careful diameter and circumfrence measurements as well as some trial and error with my Tazer comparing it to my phone calculated GPS speed.

My speedo was consistently off before the final tire diameter adjustment. So I would state that your Jeep uses it's radio's GPS if it has one (via a UConnect 4). Or explicitly from the tire diameter calibration otherwise.

No criticism whenever with your observations. Just some clarity in why all the underlying confusion with the speedo.

Jay
 

jmccorm

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However, I have every reason to believe that I'll be able to reproduce your conclusions, with just one caveat; the speedometer reading determination. [...]My Sport has the lowest level UConnect 3 Radio of which I'm very confident has no GPS receiver.
Thank you for the kind response. And I agree with your correction! đź‘Ť

PS: The two-gear downshift (followed by yet another one or two gear downshift) may be a problem that's exclusive to 2.0T models without eTorque. Two weeks from now I'll be purchasing a throttle controller which is said to prevent this issue. I'll eagerly test that out too!
 

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On a literal level, as it turns out, I agree with you! There is no connection between speedometer and shifting issues. Transmission behavior is completely divorced from the GPS-corrected speed that you see on the speedometer. The number presented on the dash is for human benefit and nothing more.

However, a close cousin to that almost certainly is at play here. That's a computed vehicle speed (not normally shown to the driver) that is used by the transmission. Based upon real-world experimental data, at least to my own satisfaction, I believe I've demonstrated a strong connection between gear and tire size settings and some automatic transmission shifting issues.

For those with automatic transmission issues who are able to verify their vehicle's gear and tire size settings, it seems like a simple and worthwhile check to perform before moving on to larger issues.

Don't believe a word of it? My mother's from the "show-me" state, so I understand. Luckily, this is something that you can confirm or refute experimentally. Record your current settings and then start to drift away from those values. Go small at first, just to be safe. The degree to which you see an impact should be proportional to the degree of which you've moved away from the ideal values.
My own experiments contradict yours. However, I too have the 5' radio, so it's possible there's a difference because of configuration. I don't know if JLs have a GPS in them or not. I inherently don't believe they do. But I don't know for sure and have seen a variety of people on this forum/facebook say their speedometer needed no fixing after putting large tires on.

What I can say for certain, is that transmissions manually know what speed a vehicle is going due to sensors, gear ratio, etc. It's often said on here that the ECU needs to know your speed for the transmission to properly shift. This is true, but the input is coming from the transmission itself, not the tire calibration in the ECU. Another poster in this thread spoke how gearing needs to be accurate for transmission shifts, which is true. In the context of his post though, he was suggesting this means tires need to be calibrated too. Little does he know he proved his ignorance on the subject. Gearing needing to be calibrated accurately is exactly why tires don't need to be calibrated- gearing is part of the mathematical equation for the transmission to know speed, so it's vital to be correct. In contrast, tire calibration simply doesn't. Tire calibration is just there for the speedometer, the speedometer is there for us to read but it's not there to help the transmission shift.

So yes, it's 100% a mechanical fact that transmissions manually know vehicle speed. It's why a bad VSS, or Governor, or false gearing in the ECU will cause horrible shift logic.
 

jmccorm

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My own experiments contradict yours. However, I too have the 5' radio, so it's possible there's a difference because of configuration. I don't know if JLs have a GPS in them or not.
Just a couple of messages earlier, @jeepoch also corrected my explanation of an auto-adjusting speedometer not being available on the 5" radio, and I agree with you and he both on that point.

What I can say for certain, is that transmissions manually know what speed a vehicle is going due to sensors, gear ratio, etc. It's often said on here that the ECU needs to know your speed for the transmission to properly shift. This is true, but the input is coming from the transmission itself, not the tire calibration in the ECU.
FWIW, there might be a few ways this could go south, but I think there's a decent way of testing this that might support one point of view over the other. (Concerning the influence of tire size settings on transmission behavior.)

The simplest way would be for a person to significantly misconfigure their tire size and then to check for any differences in behavior.

As a datapoint, here are two observations made when the tire size was set two inches smaller than they actually were:

1. Upon sudden full throttle, the downshift took 0.6 second longer when the tire size was set 2 inches smaller than it's actual value.​
2. On level ground and steady throttle input, the transmission would upshift to 8th gear at 40mph (instead of requiring 50mph to hit 8th gear) when the tire size was set 2 inches smaller than it's actual value.​

If someone were to intentionally misconfigure their tire size by an even greater amount, that might provide significant evidence one way or another.

If I understand your position correctly, and I *do* want to give it a fair representation, such a change should have no impact on transmission behavior. Conversely, my position is that such a setting would see changes similar to (or greater than) those provided in my observations above.

I think it would be ideal if the test was run apples-to-apples with it being on a 2.0T non-eTorque engine, but ultimately, I think these behaviors should manifest across all engine types.

I think there's some small risk for an incomplete experiment to be performed (for example, a module reset might be required for a change to fully take effect), but again, I'm willing to see how things turn out without too many strings attached to it.
 

Hard Rock Jeep

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My own experiments contradict yours. However, I too have the 5' radio, so it's possible there's a difference because of configuration. I don't know if JLs have a GPS in them or not. I inherently don't believe they do. But I don't know for sure and have seen a variety of people on this forum/facebook say their speedometer needed no fixing after putting large tires on.

What I can say for certain, is that transmissions manually know what speed a vehicle is going due to sensors, gear ratio, etc. It's often said on here that the ECU needs to know your speed for the transmission to properly shift. This is true, but the input is coming from the transmission itself, not the tire calibration in the ECU. Another poster in this thread spoke how gearing needs to be accurate for transmission shifts, which is true. In the context of his post though, he was suggesting this means tires need to be calibrated too. Little does he know he proved his ignorance on the subject. Gearing needing to be calibrated accurately is exactly why tires don't need to be calibrated- gearing is part of the mathematical equation for the transmission to know speed, so it's vital to be correct. In contrast, tire calibration simply doesn't. Tire calibration is just there for the speedometer, the speedometer is there for us to read but it's not there to help the transmission shift.

So yes, it's 100% a mechanical fact that transmissions manually know vehicle speed. It's why a bad VSS, or Governor, or false gearing in the ECU will cause horrible shift logic.
I don't understand "transmissions manually know what speed a vehicle is going due to sensors, gear ratio, etc.." and you say it has nothing to do with tire size. If the premise is that a transmission needs to know what speed it is going, how does a transmission know what speed it is going without knowing how many revolutions per minute the tire is making? Load changes due to both conditions and speed. Tire RPM is determined by axle RPM's. Speed is determined by tire RPM's and circumference. At the same number of axle RPM's a bigger tire is going to go "faster" in MPH. I suppose I understand that in a modern computerized transmission, it can determine load and can of course know RPM input from the torque converter, and therefore can figure out "when to shift" without actually knowing vehicle speed, but I don't see how it can measure speed without knowing tire size or having a GPS.
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