Torque converter won't lock up when it's cold

[thezentree]

My 97 Yukon's TC won't lock up in overdrive when it's cold out anymore. It takes like 40 miles of highway driving before it'll finally lock up. What controls TC lockup? My transmission guy reprogrammed the computer so it locks up all at once, not gradually like stock, but that was probably 150,000 miles ago. Is there some kind of sensor or vacuum switch that tells the computer when to lock up, or is there a solenoid that's gone bad?

Disclaimer: my terminology is probably incorrect, due to my belief that no mortal being understands automatic transmissions and that they're purely magical. Feel free to point out and correct me where I'm wrong.

 

[Cornfield creations]

Our local dealer uses Lube Guard (red bottle) added to the fluid to help with that. It would be a cheap thing to try before digging in deeper

 

[michael0584]

Hows the engine temp? It won't lockup till the engine is up to normal operating temps. I think around 180. If your thermostat is sticking open it will keep it from locking up.

 

[6.2Blazer]

It's pretty common on computer controlled trucks that the torque converter will not lock up until everything gets up to a certain temperature. However 40 miles when it's only 40 degrees outside seems rather excessive. As mentioned above, what is the engine temp like?

 

[thezentree]

The engine temp stays pretty cold, so I'm fairly certain the thermostat is sticking open. Guess I'll give that a try. Thanks guys.

 

[noJeepshere]

They're set to 160 degrees engine temp. If your engine is running cold for so long something is wrong. Even on the coldest nights in Rexburg, Id, coming home from work, say -15f, it wouldn't be more than 2 miles before the motor warmed up and the torque converter locked up.

How many miles does that thing have now?

 

[sweetk30]

ya gm used engine temp for convortor lockup temp range.

bet the t-stat is getting weak and not letting the vehicle warm up as fast. and now winter is setting in you will see the longer warm up time on a whiped t-stat.