Thread: New transmission not working correct - possible PCM issue?

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?

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.

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.

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

Originally Posted by Trannyman95

Unless your drive and driven sprockets were replace with the correct ones it will throw an incorrect gear ratio code as I mentioned above. Did you check the "Current gear ratio" in 3rd gear yet? Without knowing the data here there is no reason to look into anything else yet.

Trannyman... the transmission builder tells me "the gears have 33 tooth drive and 37 tooth driven" for a ratio of 1.1212. The HD differential has a 3.29 ratio. This leads me to believe that the transmssion has a final drive ratio of 2.93 (3.29/1.1212=2.93). Is that correct? If it is correct... then I will have to send back to the transmission builder because I asked him to build me a HD transmission with the GT gearing so that I would not have to change my PCM. If the transmission had the GT gearing then I would assume the drive and driven gears would have the same tooth count - right... say at 35 (half way in between the 33 and 37)?

Question... how do I determine the differential ratio? Do I take the tooth count from the ring and divid by the tooth count of the sun gear?

NOTE. I have not changed the PCM from the stock GT.

Originally Posted by Trannyman95

If you truely have a 33 Drive and 37 Driven then your builder went in the wrong direction!!! This will give you a 3.69 overall ratio. The HD transmissions have a 37 Drive and 33 Driven, giving you a 2.93 overall ratio. The GT / NA 3800 cars had a 35 Drive and 35 Driven sprocket arrangement. Again, PLEASE check the current gear ratio on a scanner in 3rd gear, there is nothing more to talk about until you check that number unless your builder admits to putting in the wrong gears, which will be proven with a scanner.

Trannyman95. You were correct, my math was wrong. They did put in a 33 Drive and 37 Driver for a 3.69 overall ratio. The builder gave me the option of changing out the gears or putting in a new PCM from zzperformance. I went with the new PCM, because I kinda like having the stronger acceleration with the new gears. Everything is working perfectly now.

Originally Posted by Trannyman95

Glad to hear you got it sorted out and everything is working right now

Trannyman95... there is one odd behaviour that I am assuming has to do with changing the stock torque converter with a 2800 stall torque converter. When cruising on the highway at 60 miles an hour... at the slightest change in grade (going up hill and I do mean slightest) it seems the transmission slips because the revs will go up about 500 rpm for about a second and then back again, and I can feel the car get weighted back. It will continue to do this untill the grade levels off again. Is this PCM again or something else?