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Old 05-10-2014, 02:37 PM
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Transmission won't upshift at higher RPM

The transmission in my 2001 Grand Cherokee Ltd. 4.7l was rebuilt 20,000 miles ago and has probably had this problem since then, but I hardly ever use full throttle and so I didn't notice the problem for quite a while. The transmission operates and shifts perfectly in all situations, with the exception that at full throttle/high RPM's, it wont upshift. To get it to upshift, I have to back off the throttle and let the RPM's fall. The guy who rebuilt the transmission is no longer around, but before he left he said it was probably the throttle position sensor. I replaced the TPS, but nothing changed. I have always made sure the transmission fluid is completely full, so that's not the problem either. I found an old post about the same issue. There, it was suggested that the transmission control module needed to be re-flashed. I messaged the OP to see if that worked, but it's been over a week and I haven't heard from that person. So, I'm throwing it out here for all to see in hopes that one of you knows whats going on. I'll appreciate your help.
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Old 05-10-2014, 03:27 PM
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Re: Transmission won't upshift at higher RPM

This one is new to me. Sofar i couldn't find a lot of information about the kickdown function. The TCM at least uses information of vehicle speed and TPS position to calculate the moment for he downshift. It will go into second prime gear during kickdown. I presume that it will use the vehicle speed information and/or engine rpm (could eventually be the tranny input speed sensor) to decide to upshift again (or the TPS when you would release the accelerator a bit) . The TPS would also have been my first thought, but thinking about it more would not make that so obvious anymore. This because if you keep your foot on the floor, then the TPS % will not change, but the transmission should upshift at a certain speed/rpm.
You could try and swap the tranny input speed sensor with the output speed sensor. Both are mounted on the left side of the tranny housing. If that doesn't help, try to reflash the TCM.
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Old 05-11-2014, 03:37 PM
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Re: Transmission won't upshift at higher RPM

Trans replacement time seems like
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Old 05-11-2014, 06:13 PM
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Re: Transmission won't upshift at higher RPM

Quote:
Originally Posted by hoffmanestates View Post
Trans replacement time seems like
Is that the only advice you can give?
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Old 05-11-2014, 06:14 PM
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Re: Transmission won't upshift at higher RPM

Well more then likely thats whats wrong
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Old 05-11-2014, 06:19 PM
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Re: Transmission won't upshift at higher RPM

Quote:
Originally Posted by hoffmanestates View Post
Well more then likely thats whats wrong
You base that on what? Transmission is shifting perfectly, only doesn't upshift after kickdown. This doesn't look as a transmission fault.
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Old 05-12-2014, 10:45 AM
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Re: Transmission won't upshift at higher RPM

Quote:
Originally Posted by Frango100 View Post
This one is new to me. Sofar i couldn't find a lot of information about the kickdown function. The TCM at least uses information of vehicle speed and TPS position to calculate the moment for he downshift. It will go into second prime gear during kickdown. I presume that it will use the vehicle speed information and/or engine rpm (could eventually be the tranny input speed sensor) to decide to upshift again (or the TPS when you would release the accelerator a bit) . The TPS would also have been my first thought, but thinking about it more would not make that so obvious anymore. This because if you keep your foot on the floor, then the TPS % will not change, but the transmission should upshift at a certain speed/rpm.
You could try and swap the tranny input speed sensor with the output speed sensor. Both are mounted on the left side of the tranny housing. If that doesn't help, try to reflash the TCM.
Thank you, Frank. I appreciate your response. Swapping the input and output sensors seems worth a try. I won't be able to try it right away, but when I can, I'll let you, and everyone here, know what happens. I'm assuming that after swapping the sensors, if the transmission upshifts correctly at full throttle/high RPM, then installing a new sensor should would be in order. If nothing changes, then I should try the re-flash of the Transmission Control Module at a dealership.
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Old 05-12-2014, 10:56 AM
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Re: Transmission won't upshift at higher RPM

Quote:
Originally Posted by hoffmanestates View Post
Well more then likely thats whats wrong
Thank you, hoffmanestates, for your response. As the transmission has already been rebuilt fairly recently, and because it functions flawlessly in every way except on those rare occasions when I use full throttle, I don't think another rebuild is the answer in this case. Stay tuned, and we'll all find out if replacing a sensor, or re-flashing the TCM solves the problem.
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