about six months ago the speed sensor started acting up and not letting the truck shift into overdrive, so i pulled out the sensor and cleaned it out real good and it worked fine for 6 months, bout a week ago it started acting up again and cleaning it didnt work this time, so i was satisfied with the six months i got out of it so i got another one, but it still didn't work, i drove for about three days without the speed sensor so im guessing thats why i got a code 15, the odometer is working, its just not shifting into overdrive... any suggestions on what to look for next?

 

Did you reset the PCM after the new sensor was installed?
If your positive the sensor is your problem then look into the wiring.
Since your truck is a 91 the speedo is cable driven not electronic working off of the speed sensor.

 

How do you reset the pcm

 

Unplug the battery, negative side will do for about 1 min.