transmission does not shift into overdrive on 1991 Dodge Ram Wagon B350

transmission does not shift into overdrive. the light in the switch is off, pushing the switch in does not change anything, is there a overdrive switch relay that could possibly be bad. If so where is it located?

2 answers
I have the same. Overdrive trouble could be solenoid left side trans above park/neutral swicth, Trany temp switch I have not located, coolant temp I have not checked and speed sensor that I changed. CC don't work
i just had a simuliar problem with my toyota highlander, I flushed the transmission fluid and change the filter if your car has one. THen I put a a lucas oil conditioner and Im good