This question relates to my '91 with a 3.1 V6. My thermostat recently got stuck open (changing it tomorrow) and I noticed something interesting with my transmission and wanted to find out if it's related or not. So now that my thermostat is stuck open, the engine runs exceptionally cool. On a day like today, abut 40* out, the engine only runs about 125-130* going down the highway. So today I was out and noticed that the TC wasn't locking up. Overdrive was working fine but no TC. I get to where I'm going, take care of business, and then leave. At this point, while driving through slower traffic, the engine temp was up around 145-150* and the TC was locking up fine. Initially I thought the transmission was being weird and then I hit the highway again. I get on the highway and start cruising and noticed the TC was actually locking up like it should. Then as I'm cruising, the engine temp cools off to 125-130 again and I felt the TC come out of lockup. After this I started wondering if there was a relationship between the TC lockup and the engine temperature. Does the ECM control the TC lockup based on temperature as one of it's deciding factors on whether or not to lockup the TC? Or is my transmission possibly having other issues? Any thoughts on this would be greatly appreciated. TIA!

i do believe that the ECM wont lock the TCC until a certain temp has been reached...

Yep, The TCC will unlock, or not lock, when the coolant temp is below a certain threshold.

Interesting. That would explain my issue. Kinda sucks for gas mileage. Engine not at normal operating temp AND no TC lockup. Guess I need to get that thermostat changed.

In case there's anyone out there that hasn't found this out yet ... Murray thermostats SUCK!! I work for O'Reilly Auto Parts and that's all we sell. I've talked to several people that don't like Murray and I talked to a couple repair shops that quit using Murray and went with Gates or Delco because they got tired of replacing Murray thermostats. I went to a competitor and got a Delco today and tomorrow I hope my problem will be solved.

I agree with the others.
Also, there is a high temp switch on the trans valve body that will force the converter to lock if the fluid in the trans overheats. That helps protect the trans. Kind of like a "limp home mode" for the transmission. If you ever have the trans lock up, and stays locked up this could explain it.

Alot of T stats suck! I try to find a Robert Shaw, as I had good luck with them. They are getting harder to find now too. I understand they(Robert Shaw) manufacture stats for others.
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You can check out this article on the function of TCC, ECM, etc.
http://www.diy-efi.org/gmecm/papers/tcc.txt