1996 Monte Carlo transmission problems

I have a 1996 Monte Carlo LS, 149K. The transmission sometimes will not up-shift from first when it is very cold outside. It can take from 5 to 20 minutes for this problem to correct itself and it does not always occur at all. It never happens in warm weather. There are no tranny codes.

I am due for a transmission fluid and filter change. Is there anything I can do to help this problem?

I have considered synthetic ATF. My concerns are there will still be some regular ATF in the torque converter and I may get leaks from the seals since I already have so many miles on the tranny.

Should be scanned for trans codes and commands to the shift solenoids. Check for good vacuum at the vacuum modulator, drivers side front lower part of trans.

Should be scanned for trans codes and commands to the shift solenoids. Check for good vacuum at the vacuum modulator, drivers side front lower part of trans.

There were no tranny codes. I turned off each solenoid one by one. They all seem to be working. Would the vacuum only be an issue in cold weather? I am not the primary user of the car, but I also noticed that the tranny can kick a little when abruptly engaged or when the car is being driven hard.

You used a trans capable bi directional scanner? Not many around at $3500. Are you sure it is first not in default?

You used a trans capable bi directional scanner? Not many around at $3500. Are you sure it is first not in default?

I am not positive that it is sticking in first. When the problem happens the engine revs to about 4000 rpm and the speed it limited to about 15-20mph so I am assuming that it is stuck in first. Otherwise, the speed would be higher.

I used a scanner that could communicate with the transmission, check for current codes, code history, and functionally test the 1->2 and 2->3 solenoids. I also put the car in drive, turned the solenoids off individually and observed the car shifting into 2nd, 3rd, and 4th as the solenoids were deactivated. I also did this with the TCC just to be sure.

Good, so you cannot command this upshift or it was working when you had the scanner on it, the problem wasn't there?

Good, so you cannot command this upshift or it was working when you had the scanner on it, the problem wasn't there?

It was working when I had the scanner on. The weather was mild outside and the tranny had already warmed driving to where the scanner is. I am not able to borrow the scanner and try scanning when the problem happens. Any other idea?

You can rule out the vacuum modulator, as this only controls fluid pressure for the transmission. Just guessing, but I'd say a fluid and filter change might help. I would try disconnecting the battery for a few minutes to reset the PCM, so it will re-learn it's shift points. If this doesn't help, it may be time to replace the PCM, as it controls shift points. Also, next time you have that scanner, look at the data display and check the throttle position sensor. To do this, pull up the sensor voltage display, and sloooowly depress the accelerator, with KOEO(Key on, engine off). Any dips in voltage while depressing the accelerator would indicate a bad TPS.