no overdrive

I have an '88' pickup with an efi 22r a/t. after warm up the o/d would kick in. Now o/d doesn't kick in. I changed fluid, replaced thermostat and still no o/d. The on/off indicator light works but no o/d. I also checked all fuses and all good. I've heard a number of possible causes. Anybody have any info on most common and likely cause, it would be greatly appreciated.

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Check the coolant temperature sender for the ECU.

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The ECU delays shifts and prevents shifting into overdrive when the engine is cold to make the engine run at higher RPM. Running the engine at higher RPM warms the engine up more quickly so the catalytic converter and O2 sensors reach operating temperature (and go into closed loop mode ) more quickly, lowering emissions more quickly.
The ECU uses voltage from the coolant temperature sensor to determine whether the engine is cold or hot. If the coolant temp sensor is bad or disconnected, no voltage reaches the ECU. The first generation of OBD did not have a way to monitor the function of the coolant temp sensor so it wouldn't know if the sensor was working or not.
It is possible that the OP's transmission has internal problems, but the question was what the most common cause was, which is a problem with the engine coolant temp sensor and related circuit.
I probably should have added the second most likely cause, which is a bad shift solenoid.