I have my transmission preheater bypassed since I'm usually offroading in the summer. But recently I finished my gauge cluster and now I can see my transmission temps (sensor is in the trans cooler line before the cooler). I see temps from 80-140, 140 is only when really getting after it or stop and go in the city. But for long cruises at 60+mph I regularly see the fluid temp drop to below 100. Per the FSM the normal operating temps are 112-176 F. I'm thinking that perhaps it would be beneficial to install a thermostatically controlled mixing valve. that would send fluid through the radiator preheater to maintain a set temp when its cold out then bypass it when the transmission doesn't need it. Does anyone know of a 12 volt programmable mixing valve? I believe I saw that Mr. T had added a mixing valve to the 200 series just for this purpose but can't remember the part number.