I was under the impression it's trans fluid temp. I noticed (since it's been in the 30s here) that my Kon's TC doesn't lockup for a while. Engine temps are at average (190-ish degF), and still no lockup after 10 minutes of driving up to 70mph. After about 15 - 20 minutes, it'll lock up. Pay attention to your rpms on cold days vs warm days. At 70mph, with 3.42s, lockup puts rpms at 1900. Just this week, I've noticed rpms at 2100-ish. After 20 minutes or so of driving, rpms drop to 1900 @ 70mph.

 

I have a fan from a Yamaha R1 on my tranny cooler, doesn't make a noticeable difference... disappointing.

 

TFT has to reach 80* and then the TC will lock up IIRC.