90 Lumina down shifting in cold weather

[wiley2000]

I've looked through the archives, but can't find any similar situation. Maybe it's because I'm not searching with the right terminology. I apologize if this is covered already.

My Lumina Euro has a specific problem during the colder months. (I live in the south, so cold isn't THAT cold.) After getting into overdrive, the car will periodically "downshift". I'll explain that in case I'm using the wrong terminology. The engine will all of a sudden run faster and the rpm's will increase by around 600. It will stay this way for anywhere from 30 seconds to 2 minutes, then the engine will go back to where it started. This happens only when the engine temp is below 150, and so never during warm weather.

There is no catching or lagging or slipping. The car runs fine the entire time this is taking place. My mechanic changed the transmission fluid and filter when I told him about it, but it hasn't changed anything.

Surely the temperature of the engine has something to do with this, right? Is there a sensor that needs to be changed?

Thanks for any insight!!

[dwalmop]

First thing's first. You're not feeling a "gear shift" what you're feeling is the torque converter lockout engaging and disengaging. A couple things caught my attention. It only does it when it's cooler out mainly. The torque converter will not lockout unless the engine is properly warmed up. You may simply need to replace your thermostat to fix the problem. With a 195 degree thermostat, your engine shouldn't be down to 150 unless you're just starting it. Since it's such a simple fix, I'd try that first. The same thing happened to me one December (I live in WI, where it does get 'cold') and I thought that there was a tranny issue as well, but all was ok afterward.

[jeffcoslacker]

dwal might be right, but 600 rpm's sounds like alot for a T/C lock/unlock. That sounds like a full blown gear change to me. If his suggestion doesn't help, there may (?) be a cable detent adjustment required to keep it from hunting in and out of overdrive.

[wiley2000]

Thanks a bunch for the suggestions - I knew I could get the information from someone who's been there, done that. I've got two things to bring to my mechanic and hopefully the cheaper one will work! I'll let you know which one does.

[dwalmop]

Yeah, 600 does seem like a lot, but my '90 does about that with tc lockout. My 96 doesn't, only about 300-400. Not to say I'm right though! Definately could be a gear change.