'88 Transmission Problems?
 

My '88 Camry has some issues when it comes to a cold transmission.
The other day, when I was leaving for work, the auto. tranny shifted fine going into 2nd and 3rd gears. It took 6 miles before it upshifted into 4th. The whole time the engine was racing.

This happened two other times each taking 6-8 miles before upshifting into 4th.

Today, I ended up driving it the whole way to work (35 miles) in 3rd gear. It never upshifted into 4th. I talked to one of the other mechanics at work about it and he suggested a tranny flush. So, a tranny flush and $115 later, after work I drove it out to where my dad works and it shifted fine. After I left my dad's work, I drove it the 15 mi. back to town. It did it again! The whole way into town in 3rd gear.

Is there a clog in the system that only happens when the fluid is cold, or could there be a sensor that is going bad?
Any Ideas would be appreciated.
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This trans has a coolant temp sensor that disables the O/D feature below coolant temps of 109F. The sensor activates an electronic solenoid on the trans.

The water temp sensor appears located on the coolant manifold the upper radiator hose attaches to. It appears to be the outboard sensor nearest the drivers side of the car. Wire color may be Yellow/Black.

Check the resistance of the sensor. If the coolant temp is below 109F it should be 0 ohms. If 131F and above infinite resistance.

Or could be the solenoid on the trans. This item should be removable without taking out the trans. Wire colors may be Red/Blue and Pink/Blue. The solenoid should have a resistance around 13 ohm.

Thank you. I was wondering about the solenoid too.