Overdrive problem please help!!!

I have an 89 d50 sport cab with an auto tranny 2.6L with overdrive, overdrive does not work. When I first got the truck it would go in and out now and then now it doesnt go in at all. I changed the relay and it clicks like it should. When I bought the truck I was told it needed the transmission solenoid for the overdrive. So, I dropped the oil pan and the valve body and there is nothing there!! Where is this solenoid???? Any other suggestions???

There is a coolant temp switch on the intake manifold that controls the OD. Check on that.

does anyone have a pic or indication which sensor is the one for the O/D? I see 3 sensors attached to the thermostat housing but don't know which one I should test, and I'm afraid to ground anything, don't want to blow anything..