98 GP GT 3800 Auto 123k coupe - TRANNY?

I was driving my gp home from work last night just out the blue it did a funny downshift thing when I got off the hwy. I had to stop then when I took off it shifted out of 1st and felt like it was slipping/shuddering then seemed to go again but was shifting weird. I babied it a bit to get it home. I live on a bit of a hill and it was shuddeirng or slipping or whatever trying to get up the little hill so I pulled it down in 1st maually and it went right up into the garage. I checked the fluid before shutting it off and it was full and looks fine. I didn't have time to look at it last night or this morning. I rode with my wife to work but will try to look more tonight. What can I do or should I try? I was going to run by and get some sea foam transmission stuff and put in it and try to drive it some to see what it does but I have a feeling it is toast. I have not done a tranny fluid and filter change on it yet.
I had a funny shifting issue with my Montana and found all of the info on the sea foam then pan drop and filter with shift kit from ZZp. It worked great so far on my Montana but this seems different for sure. I think is slipping. I will know more tonight but would you all suggest trying the sea foam or not even bother?
Rob

In getting ready for the worse....what years of trannys will fit in this thing. I know someone parting out a 97 and a 99. I have been reading other posts on here and it looks like the 97 have a different tranny. All are GT's that I'm talking about.
Thanks much for any advice on this...I'm really bummed out about it. I love my car.
Rob

97's have 4t60e's, I think 98's are the split year, and 99+ have 4t65e's.

Better find out what one yours is.

i will tell you from experience that putting a used tranny in a GP is nothing but a nightmare. reason being is the PCM controls vacuum and alot of other things in the 4T65E. if you put a used tranny in the car you must get the PCM flashed so that it will learn the new codes for the tranny. even after this is done there is no guarantee that the tranny will work the same.

i just rebuilt the tranny in my 98 and has been nothing but a nightmare. put a used one in it to begin with...that tranny lasted about 2 months, so i decided to rebuild it. i now have problems with the TCC not unlocking. wish i would have kept the original tranny :angryfire

my suggestion to you...get the original one rebuilt.

Jerad, have you checked all the inputs the the PCM for the TCC?

If I were you, I'd ohm sweep the TPS, check for flat spots or drop outs, and also step on the brakes to make sure your brake lights come on. If they dont, your brake switch could be bad causing no signal to be sent to the PCM to command out of TCC lockup.

The biggest mistake people do is not check the gear ratio when putting in an used transmssion,it has to be the right gear ratio or it will not shift right.The book says they will not fit on the 4t65Es,WRONG! The final drive has to be changed and sometimes the electrical.Some have the netrual safety switch inside and some of them are outside.About 1999,GM did switch the line fittings to jiffy tites which I used to hate them and like them now since I have the special release tool now.They take quicker to remove by rotating the release tool 60 degrees after it seated in than a flare nut fitting and do not leak

Would the correct gear ratio be able to be programmed into the PCM with a tech 2?

And I think it was the 04+ GP's that have the internal neutral safety switches. Which was probably about the dumbest thing GM could ever do.

We should probably get back on topic. Does anyone have any suggestions for fritcr?

Jerad, have you checked all the inputs the the PCM for the TCC?

If I were you, I'd ohm sweep the TPS, check for flat spots or drop outs, and also step on the brakes to make sure your brake lights come on. If they dont, your brake switch could be bad causing no signal to be sent to the PCM to command out of TCC lockup.

checked the TPS already...tested out just fine. did a light check my last oil change, and all the lights worked fine. the only time i have problems is when its in OD. if i just lightly get on the gas i get the "jigging" effect. another problem i have noticed is that if i get on the gas real hard...the car will rev for for like a second or so then finally downshift...kind of like a delay. i told my dad i wanna put a shift kit in and see if it helps my issues, but he said i really shouldnt because he is determined that is what caused the last tranny to fail.

The final drive has to be changed on the 4t65E's including the 4t60s and the transmission will work if the gear ratio is right.Another thing yo check on them is the inside color of the big outside plug,wrong color,will not work

WOW this all sounds like a big mess!. I did check out the car a bit more last night when I got home. I tried to post from home last night we have a dish for our internet and it was storming here really bad. In short the car drove and shifted fine. I put a can of Sea Foam Trans Tune in it and drove it around. I'm going do a pan drop and filter change and hope for the best. I'm sure it will do it again but I guess I have to try.
Rob

FritCR, check your PM's

For the record, you cannot swap a 4t60e into a car that was originally a 4t65E or vice versa without a lot of cobbling. The 4t60e still has a vacuum modulator that controls part throttle shifts, where the 4t65e is fully electronic with no vacuum modulator. The transmission wiring harness connector is wired differently also, along with other differences that will make that swap attempt more trouble than it's worth IMO.

FYI for everyone, the OP's car has the 4t65e transaxle.

Just to let everyone know. I ran Seafoam Trans tune in it for about 100 miles then did a pan drop, filter fluid and a shift kit from ZZpro and it shifts great now. No problems at all. Of course since I have now posted this it probalby will not make it home after work. LOL
Rob