When it runs too cold. Like in alaska and canada, when temps get to -40, the transmission will run too cold. The fluid may start to gel in some cases, and the lubrication properties drop severly when the fluid is subjected to temps that low.

 

Yep....At -60, if you dont have your radiator completely plugged up, you can gel your tranny fluid in your cooler. I have the big heavy duty cooler that goes in from of the radiator. If you wanna call BS, well come on up to Fairbanks, AK in about Jan or Feb next winter. It gets so cold, that you have to run 70w full synthetic in your differentials, b/c 90w freezes almost solid at -40. Its a whole different ball game up there. Oh yeah, there's a TSB that came out for these trucks that says below 0 you dont use overdrive.