Keeping Torque Converter Locked???

The converter slips when not locked and does less RPM than the engine. That's where heat is produced. So the heat is cooled by the fluid and the transmission cooler. The fluid must be clean to keep heat from being held in any particles present in fluid due to breakdown of components the transmission is made of and condensation caused by heating and cooling. Fully locked is no slipping and moving with the engine 1 to 1. Newer transmissions that are fully locked in all gears, use a wet system to switch gears by double clutching so fast that they switch to engine RPM without friction like torque converters produce by slipping in all gears except high gear. I've known transmission rebuilders to drill holes in the converter to allow more fluid through which fastens and gets closer to a lock than the slower fluid produced by factory design. So the heat registering is the engine working to hard, using more fuel to get the speed that a locked transmission would yield better MPG at cooler temps. Also transmissions will not go into high gear if the engine temperature is appearing to low through a bad temperature sensor signal from the engine block, causing heat buildup in transmission and engine and bad MPG.