Toyota Repair: Transmission, toyota sr5 pickup, temp gauge

Question
I have a 1988 Toyota SR5 Pickup. It has a 4 cyl 22Re EFI Engine. And an Automatic transmission with Overdrive.
The problem is the Transmission wil not go in to Overdrive.All other gears work fine. It would always take about 5 or 6 miles when cold before it would go in to overdrive. About 3 weeks ago when we had a cold spell in the weather it stopped working all together. Hope you can help.
Derek

Answer
Does the engine reach operating temperature to at least 1/2 on the temp gauge in a short time? The transmission computer has a temp sensor input that will not let the transmission go into overdrive until it sees a certain temperature,a couple of things that can cause this is a slow to open or stuck open thermostat or a bad ECU temp sensor on the engine.