Honda Repair: 91 Accord S light blinking, shift cable, speed sensor

Question
The "S" light started blinking on my 91 Accord LX 4 door sedan, automatic.  It still shifts and runs great, even in sport mode.  When I turn the key off it seems to reset and won't come back on for some time.  Is it just a sensor(ie. speed sensor)?  If so, will it do further damage without being repaired?  What is the approx. costs of one and how hard are they to replace?  Any other thoughts on a repair?Thanks for your help in advance!

Answer
Chuck:
I would take the car to a mechanic for a diagnosis, the only service I do on transmissions is changing the fluid. A speed sensor is about a hundred dollars and is located on the transmission housing by the shift cable and is not too difficult to replace. I would guess that it is not related to the speed sensor.
Sorry I can't be of more help.
Bill