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Old 04-17-2015, 11:19 PM
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I hate how my Jeep shifts

Does anyone know of anything you can do about the way the Jeep drives? I feel the shifting setup is obnoxious. I have an 08 Grand Cherokee Limited 5.7 hemi in it.

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  #2  
Old 04-18-2015, 09:53 AM
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Re: I hate how my Jeep shifts

You've sure got that right. The best way I've found to fix it, without tearing down the transmission, is to add the Sonnax Line Pressure Booster and purchase a Superchips Tuner with the Advanced Transmission Tuning Application. I've done both to mine and the crappy 545RFE is now about as good as it can get without tearing into it. Another option is to pick up a Transgo Shift Kit, but to intstall the complete kit will require some tear down. I think they also make valvebody kits that doesn't require major teardown. For now I am very happy with the way mine works.

The only thing I haven't been able to resolve is the WOT downshift (from about 60-65 mph) from 4th or 5th to 2nd. It has a terrible lag and seems to downshift only one gear at a time, and it's really slow in doing it. By contrast, my Hemi LX with the NAG1 transmission is instantaneous when doing the same WOT downshift.
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  #3  
Old 04-18-2015, 11:44 AM
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Re: I hate how my Jeep shifts

I have the Transgo shiftkit in my 5-45RFE (with 4.7L) and like the way it shifts now. This shiftkit also has a resistor (like the Sonnax, only it has to be soldered in), to increase the line pressure, which already gives crispier shifts and greater holding power for the clutches, but it also changes most of the springs in the accumulators with stroner ones, reducing the overlap time (the time that two clutches for diffrent gears are pressurized together), so reducing the clutch wear and lowering the transmission temperature.
To install the complete kit, the transmission must be lowered. There are some new springs for regulator valves in the pump housing and a stronger circlip for one of the clutch packs. But many just do the work on the valve body and leave the transmission in place.
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Old 04-19-2015, 10:33 AM
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Re: I hate how my Jeep shifts

Quote:
Originally Posted by jeepgcoman View Post
You've sure got that right. The best way I've found to fix it, without tearing down the transmission, is to add the Sonnax Line Pressure Booster and purchase a Superchips Tuner with the Advanced Transmission Tuning Application. I've done both to mine and the crappy 545RFE is now about as good as it can get without tearing into it. Another option is to pick up a Transgo Shift Kit, but to intstall the complete kit will require some tear down. I think they also make valvebody kits that doesn't require major teardown. For now I am very happy with the way mine works.

The only thing I haven't been able to resolve is the WOT downshift (from about 60-65 mph) from 4th or 5th to 2nd. It has a terrible lag and seems to downshift only one gear at a time, and it's really slow in doing it. By contrast, my Hemi LX with the NAG1 transmission is instantaneous when doing the same WOT downshift.
I have been thinking of getting the sonnax pressure booster as well. Can you install that without taking out the tune and trans app or should you uninstall the tune first?
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Old 04-19-2015, 11:40 AM
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Re: I hate how my Jeep shifts

It's been about 5 years since I installed mine, and I did not remove the tune. I do remember that it worked like crap for almost a week until the PCM/TCM figured out what was going on. Ever since then, it's worked fine and I've got over 40K on it since the install. I did not have the trans. app. installed on mine when I installed the Sonnax. I don't think the app. was even available when I did it. I don't think it would make any difference though. Maybe Frank can add his thoughts?

I see you have the SC trans. app. too. Do you have it set up the way you want it? What changes did you make to up/down shift schedules and torque management?
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Old 04-19-2015, 01:04 PM
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Re: I hate how my Jeep shifts

I just used superchips own version for the trans app for the up/down shift schedules. I was not comfortable enough to mess with it myself. My jeep has only 38K on it and I plan on keeping it awhile. Will the sonnax pressure booster help extend the life of the tranny? Is it something worthwhile to do?
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Old 04-19-2015, 02:25 PM
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Re: I hate how my Jeep shifts

Yes, it should help extend the life of the transmission, if you do regular services and address any irregularities as soon as you detect them. What the booster does is increase the line pressure by ~15-30 PSI, which increases the clamping pressure on the clutches. This helps keep slippage to a minimum.

I wouldn't say I am the easiest on the transmission, plus mine has nearly 100 hp more than the stock Hemi, and mine still works great......knock on wood. I think, between the Sonnax and my reduction in torque management, the slippage is being kept to a minimum. I may consider doing the Transgo valve body upgrade that Frank was talking about (I know there are some weak points in it), but for now I am pretty happy with it.
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Old 04-19-2015, 03:13 PM
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Re: I hate how my Jeep shifts

Good to know thanks for the info.


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Old 04-19-2015, 04:48 PM
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Re: I hate how my Jeep shifts

I wonder if only increasing the line pressure will help extending the life of the transmission. I have no information about if clutches can/will slip once they are applied. What i do know however, is that the clutch overlap time is normally causing the biggest wear on the clutches. As mentioned before, the overlap time is the time when two clutches, with different speeds, are pressurized together. First the clutch which is going to apply will slip, somewhat later the clutch which is going to release starts slipping. The shorter that you can make the clutch overlap time, the less wear of the clutches and less heat is generated. Heat is the biggest factor which will reduce the live of the fluid, so the cooler it runs, the longer it will stay running.
Increasing the line pressure can reduce a bit the time it takes to complete a shift, but also the wear will be higher, because of the higher pressure on the slipping clutch. To reduce the time the shift will take, you actually have to change the springs in the accumulators with stronger ones.
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Old 04-19-2015, 06:17 PM
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Re: I hate how my Jeep shifts

Frank, do you think reducing the TM helps any? A hundred years ago when I worked on automatics, they were all totally hydraulically controlled with throttle pressure linkage instead of vacuum modulators and the myriad of sensors on today's automatics. Back then, to a point, more pressure was always good in reducing band and clutch slippage. I would think for sure, if you have more horsepower, increased pressure would help keep the clutches from slipping.

So, by what you're saying is that the valve body mods. would definitely help, maybe even more than the Sonnax and the reduction in TM?? If so, since I am pretty much done modding, maybe I'll give this a try. I see the valve body kit isn't very expensive either.
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Old 04-20-2015, 01:33 AM
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Re: I hate how my Jeep shifts

Eventhough previously the transmissions where mechanically controlled versus electronically nowadays, the internal principles didn´t change a lot over time. For one gear to work, two clutches needed to be pressurized at a time (one input clutch and one holding clutch). Making a shift means releasing one clutch and applying another one, but still keeping two clutches pressurized for each gear. Since you can not release one clutch first and apply the other one thereafter (or the engine rpm would rocket skyhigh), it means that there is a certain time that the two clutches from different gears (so different speeds) must be pressurized together, the overlap time.
I would not really expect reducing the TM to help a lot, since the problem is the two different clutches with each its own speed, from which one is being pressurized and the other being depressurized. The overlap time, the line pressure and the speed difference will determine mostly the heat generated (and the wear caused) during a shift. The speed difference between two clutches is a fixed value, determined by the gear ratios. Changing the line pressure will have influence, but difficult to tell how much. A too low line pressure will cause lower heat buildup during the shift, but will cause slipping clutches after the shift. A high line pressure will cause more heat buildup during the overlap time, but will create more holding power after the shift.
So best would be to reduce the overlap time. A side effect of reducing the overlap time is, that the shifts become more noticeable and firmer.
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Old 04-21-2015, 10:22 AM
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Re: I hate how my Jeep shifts

Thanks for your wisdom, Frank. I'll be picking up valve body shift kits for both my vehicles and install them at next fluid/filter change.....both will cost less than $150 US. I was unaware of the overlap. You're the man!!
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