I have a 1991 Explorer with the 4.0L engine, automatic with 4wd. My problem with it is that all of a sudden, it doesn't seem to want to shift out of low gear until it warms up a bit. If I put it is Low2, it will shift to it but not into drive. From L2, If I shift into Drive, it will shift back to Low1. If I go fast enough, it will finally shift into drive and then overdrive. When it does shift, shifting is positive with no slippage. After it warms up, everythin works fine. I have heard conflicting stories as to what can be the problem. One is the speed sensor needs to be replaced. Two, the modulator is going bad. Personally, I am leaning to the Speed Sensor as speedometer readings are reading high by about 10 percent as well. Passing gear works fine. My next question is, how difficult is it to change the speed sensor for an old backyard mechanic? Where is it located? How about the modulator. In some of my reading, one of these items has some kind of pin you don't want to lose. Any help would be appreciated. Thank You

Variable Speed Sensors (VSS) are easy to replace. They are usually held on by one bolt and are often located on the transmission, close to the firewall. I would try replacing the VSS first. Go to AutoZone.com to see what it looks like.

OK! I finally got the speed sensor replaced. Like you said, an easy job. However, it didn't cure the problem. Could it be the modulator, even though it kicks down into passing gear etc. I would wonder about a vacuum leak to the modulator but engine runs fine. I still have to gun the engine and get up to speed before it will shift out of low when it is cold. Once it shifts, it is fine. Where do the wires go from the speed sensor?

Personally, I would drop and clean the pan, change the filter, and refill with Mobil 1 Synthetic Multi-Vehicle ATF, before trying anything else. This link should help:

https://www.askmehelpdesk.com/cars-t...ml#post1873132