Hi All

This is my first post... I have owned my Grand Prix GT "40th Anniversary" since new... I have loved every moment.

Currently I am working towards modifying the car to be a better performing daily driver. To start my modifications I have replaced the stock 4T65E transmission with a performance 4T65E-HD transmission in preparation for a turbo install some time next year. Anyways the new transmission was installed by a respected transmission builder... but there are issues with hard shifting or not shifting correctly under WOT. Below is a list of issues I have encountered...

1. Shifts hard through all gear shifts.
2. Under WOT, shifting from 1-2... car hits 6000rpms and then jumps a couple of times before shifting. Seems like a rev limiter engages before the actual shift occurs.
3. Under WOT, shifting from 2-3... car will not shift unless I pull up on the accelerater. At 6000rpms it seems like a rev limiter engage again (car jumps) but it will not shift until I pull up on the accelerater.

My transmission installer has checked the transmission over and according to him everything is in good working order with regards to the mechanicals of the transmission. He believes the PCM is the problem. He tells me that the maintained transmission pressure is high at park, "current out of the PCM is sitting at 0.12 to the EPC solenoid, which means high pressure" He says that as the car is put into drive the current jumps up to .88, but then returns to 0.12 and maintains the high pressure.

The question is... is it likely that I have a PCM problem or is it likely it is something else?

What year car did the donor trans come from?

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

Your problem is that the HD transmission has a different overall gear ratio of 2.93:1, your GT trans has 3.29:1 gearing. You need custom pcm tuning to correct your problem. If you or your transmission guy has a scanner you will find that by looking at the "Current Gear Ratio" info it will read .88 in 3rd gear. It MUST read 1.00 give or take .01 to work properly or it WILL throw an incorrect gear ratio code. When this happens you will have very harsh shifts as the pressure is maxed under most conditions and lockup comes on very hard all the time which is hard on the trans. The gearing changes is made by changing the drive and driven chain sprockets inside the transmission and requires taking the transmission half way apart to do this so its a pretty involving task. Having the pcm reprogrammed will take care of this. There are other ways to get around this but reprogramming the pcm is cheaper and easier to do.

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www.TripleEdgePerformance.com
4T65E Transmission Info
Voted GPF.net Community Hero

You'll need to change "Gear ratio" and "Pulses per revolution" in your bin file. I had similar problesm last year i went from 2.93's to 3.29's. The tranny didn't know where the torque converter was and it (tc) wouldn't lockup. The tranny constantly overheated because the torque converter wouldn't lockup. Also the shifts were really hard, now matter what i changed to soften them up. Once i put in the correct values all the symptoms went away.

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L26,Xp, zzp trans,3.29's,2700 conv, slp headers/catback, diy ported heads/LIM, 3 angle valve job,.052 MLS hg, outlet ported Gen V, 3.6, N*, LQ4, 4" FWI, agx's, ssc's, f-bodys, gmpp sway's +stb's

Thank you for the comments. I think I need to clarify a few things... The transmission was built specifically for the car and has the stock GT 3.29 gears installed. Since the car was a GT stock, I would think that PCM reprogramming would not be necessary, unless having the HD differential also changes overall drive ratio?

Another note... 245mm torque converter was also installed. Stall set at 2800.

Trannyman95... from other posts I have noticed that you have extensive knowledge of transmissions... I am hoping you might have more to say after I clarified that my new transmission has stock GT 3.29 gears and a 245mm torque converter set at 2800 stall.

To become 3.29, the HD tranny would need to have it's chain and sprocket set replaced with some from a non-HD. The HD would then be 3.29 rather than 2.93.

Still knowing what year would be good.

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

The transmission came from a 2002 year GP. It does have the 3.29 gears instead of the 2.93 gears. I had it built that way, specifically to not have to reprogram the PCU until I do a turbo next year.

Okay, my lack of GP knowledge here... what year is your car... a "40th" version? Put simply, trannies from up to 02 aren't compatible with 03+ PCMs and vice versa. That's why I'm checking.

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2004 Impala LS - got some mods
NAIOA Moderator

Making you think twice before picking on Impalas

2002 Grand Prix GT "40th Anniversary"... so should be no problem with matching this transmission with this car - right?