My son bought a 91 SR5 4x4 pickup with the V6 and auto tranny. 141k miles. The tranny will not shift into overdrive with cold starts even once the engine warms up. Tends to work better if you allow the engine temp to raise before driving. I've replaced the Thermostat with a new Toyota one, also replaced the ECT sensor as well as the tranny temp sensor. Serviced the tranny fluid and filter as well as the shift selenoids inside the pan. Also replaced the electric control box on the side of tranny for the gear selector. Sometimes it will shift into OD and while cruising down the interstate it will down shift and not go back to OD unless you pull over to a complete stop and take off again. This usually only works for a short time and must be repeated. Truck is to nice and runs to good to give up on it. Or maybe just my ego won't let me. Any help is greatly appreciated. Thanks farmalltim

 

There is a coolant temp sensor that keeps the trans from going into od until warm up that is separate from the dash gauge temp sensor check that.

 

I have replaced both the gauge sensor as well as the sensor that goes to the ECM. There is another sensor beside those two that is supposed to be for the fuel injection for starting. I have not replaced that one as I figured it had nothing to do with the tranny.
Thanks.