1991 Honda Accord with flashing letter S on dashboard

if S button flashes and at the same time the speedometer is not functionning correctly, then the speedometer sensor is malfunction. The sensor is located at the transmission body above driveshaft.

please turn dashlight off

This is your transmission control unit telling you there is a problem. I have another post here (http://www.fixya.com/cars/t5554380-92_honda_accord_d4_light_flashing) with details on how to diagnose. This link says "flashing D4 light" but the issues it the same. 90-91 accords have flashing S lights for TCU errors, and 92-93 have flashing D4 lights.

usually the problem is the TCM or transmission control module. They are known to go bad. If transmission is working ok then this is most likely the problem.

you may need the electrics checked?or fuses an links

If the d4 light is flashing you have to check the transmission for diagnostic codes. once you have this we can go further!

The Flashing S indicates that there's a problem with your Transmission Control Module (TCM). Most likely the TCM has gone bad and needs to be replaced. You will need to get a new TCM from the dealer. Replacing the TCM should turn off the light

THE S LIGHT BLINKING ON THE DASH STANDS FOR SPORT NOT SERVICE IF YOU LOOK ON YOUR SHIFTER YOU HAVE A LITTLE BUTTON IF YOU PRESS THE BUTTON YOUR CAR WILL DRIVE ON SPORT IF YOU REASEASE THE THE BUTTON YOUR CAR SHOULD DRIVE REGULAR.
I hope this helps
Sam

the flashin s means in requires service asap....take to a repair shop as they will be only ones able to scan it for the code

A green flashing "S" is similar to a check engine light. It is telling you that something in or on your transmission is not functioning correctly, such as the speed sensor or shift control solenoids. If it starts flashing and the speedometer stops working or the transmission shifts weird, take it to a mechanic.