My OD off light began flashing today in my 95 Explorer. The only code it spat out was 639, which is a fault in the transmission speed sensor circuit. I did drive the truck on a muddy wet trail earlier...is it possible that messed up the sensor? Is it okay to drive the truck? What/where is this sensor and what kind of money damage am I looking at? The truck shifts fine and the fluid is good.

 

I don't have any info on OBD-I Explorers, so I can't help with any specifics.

If it were mine, I would get under there and open all of the transmission electrical connectors, clean/dry them out and apply dielectric grease to the pins before reassembly. Also do the same at the transfer case VSS connector. It might fix the problem and, even if it doesn't, it is not a wasted effort. You need to waterproof all connectors under the truck for use in wet/muddy conditions

 

Thanks for the advice. After digging around online, I have narrowed the problem down to the input speed sensor, since the speedometer is still working. I'm going to do some cleaning/greasing tomorrow.

 

I think the sensor just got some moisture in its connection yesterday. Apparently it dried out overnight because I could not get the light to come on for anything. I will still clean and grease the connectors under the truck though.