Based on my experience, I would say that close to 40%, maybe more, of temperature related transmission issues are due to torque converter drain back, and loose valve body bolts (meaning not properly torqued to spec.). Torque converter drain back is commonly due to a poor trans pick-up filter. But also can be caused by loose valve body bolts (not just the main bolts, but the bolts that retain the two halves as well). Loose valve body bolts not only allow drain-back of trans fluid, but also allows torsional twist, binding and deformation of the valve body which can cause sticking or binding valves – especially when cold. However, after the vehicle is warmed up, the fluid fills up the torque converter and trans and operates normally.
Sometimes, depending on which bolts are loose and how the deformation of the valve body is, it can seal when cold, and leak, suck air and shift valves bind after warmed up Symptoms are typically harsh or delayed shift during cold operation. Quick verification of a drain-back issue is to check the trans fluid level “hot” the night before, then in the morning “before starting the vehicle,” check the fluid level again. If the fluid level is high, meaning well above the cold full mark (a little high is typical), then you have a drain-back issue with the trans. However, you can still have an improperly torqued valve body that may not exhibit drain-back and still allow binding shift valves.
Another area of cold shift issues, typically delayed shift issues, is the cooling system and related sensors. Most newer transmissions rely on a engine temperature sensor input to determine the proper shift points of the trans during cold operations. Commonly, shift points and are delayed as well as overdrive disabled until the trans reaches full operating temperature. This allows the engine to heat up quicker, transmission to circulate fluid and heat up quicker with minimal stress on the transmission.
The heaviest stress on an auto trans is when the trans is cold and it up-shifts quickly higher gears requiring heavy drive-train load. If you have a bad engine temp sensor, or a thermostat that is stuck open cold, it will provide a simulated cold operation value to the ECU and Trans telling it to continue to delay shift points.
There are still many other reasons for cold shift issues, but the above are common reasons for cold failures.