MR2 Overdrive Problem

[Nat Bozarth]

Just started driving a very well maintained 1987 MR2 that my sister bought new and always had maintained by a Toyota dealer. Just turned 100,000 miles. The car takes it self in and out of overdrive and although I've googled the problem and read about all I can find, I don't see anywhere a very concise explanation. Some comments about thermostats and operating temperature, others about overdrive solenoids and still others about exhaust temp sensors sending a signal to the overdrive solenoid. I gather it has something to due with operating temperature, but can anybody tell me where the sensor that relates to the overdrive actually is? Is it coolant oriented? Or exhaust oriented? Or is it more of an electrical function or malfunction??

thanks

Nat