1992 ford ranger wont shift into overdrive

The problem is most likely not in the transmission. Since it is doing the same thing with both of them. I would start with checking speed sensor circuit. The speed sensor send signal to the processor and the processor in turn controls shift point etc. You need a really good scanner and monitor PIDS. Watch what the speed sensor is reading and look at fuel trim to make sure the vehicle is seeing what the vehicle is really doing. Your OBD is very primitive compared to todays OBDII, which holds and keeps information. Therefore you have to monitor while driving and situation happens. If you can, try a KNOWN GOOD PCM first. However, don't go out and buy a new one without verifying that it is the PCM

might be the linkage not letting it go all the way down. like a bind up

could have faulty governor.throttle cable need adjustment.the 83 - 90 models you need to adjust front and rear bands in the transmission.

Blinking O/D light is a warning to the driver that they should take it to a qualified repair shop. Also for the technician that DTC'S - diagnostic trouble codes are stored that will point a qualified tech to a problem for testing , electrical circuit's ,componets etc.... You have no idea of the complexity of these systems . Your best bet ,take it to a qualified repair shop !
Transmission Control Indicator Lamp (TCIL)
The transmission control indicator lamp (TCIL) is located in the instrument panel and is labeled O/D OFF. It is illuminated in conjunction with the TC switch.
The TCIL will flash if the EPC solenoid circuit is shorted or a fault has been detected in a monitored sensor used for transmission operation.

Sounds like TPS (Throttle Positioning Switch) They develop "hot spots" or wear spots usually at throttle positions that are commonly used alot IE: cruising speed. When they get hot spots they fluctuate looking for solid connection. Replace TPS.

This problem could be one of several it is hard to pin point the car needs to be hoisted up and checked out further for torque converter which is between the motor and transmission or it could be a internal transmission valve this is why it needs to be hoisted up preferably at a transmission specialist.

The D inside of an O is for overdrive. It is an automatic overdrive transmission. There isn't an overdrive button on it. If you want to disable the overdrive you have to shift down a gear.

these cars are a 4 speed auto
it you here the car change 4 times then it is going in to od
best way to try this is by following

1. go to an open rood where you can do 100kph (60mhp)
2. have your overdrive turned off (there should be a light on the dash board that comes on)
3. then turn over drive on and see if the RPM drops

if it dose not work let me know and i will try my best to help you
ricko

Sounds like the shift kit is warn out in the transmission. The overdrive line may not be able to provide enough hydrolic pressure to engage the gears to move the truck. When you switch to regualr drive, that line is able to build the hydrolic pressure to engage the gears. At a certian speed (for the transmission, certian hydrolic pressure) there is enough pressure to engage the gears in overdrive, but go below that, and you have to start in drive again. If this is the case, there is no easy fix for that and I recomend taking it to a transmission shop to have your shift kit examined. Opening up an automatic transmission requires specalized tools, clean shop enviroment, and knowlage more then the average back yard machanic has.

The blinking overdrive button is telling u that a serious fault has occurred in the electronic controls or a mechanical fault such as excess slippage is occurring.