Hi. I have a 91 accord wagon AT and the S light will just randomly start blinking as im driving down the road... Also, the speedometer will drop to "0" while I am driving down the road (if this has anything to do with it).

There is no CEL on mind you, just the S light. It blinks and I don't know why.

 

S-light flashing means there is a transmission code stored. Most likely the vehicle speed sensor is intermittent, which disables the speedometer and causes the trans code to set and S-light to flash.

Go to DIY for instructions on obtaining the Trans code.

good luck