A friend of mine has a '90 honda accord and there is a green "S" light in the lower right dash. What does this light mean?

 

The "S" light is the service light, similar to the Check Engine Lighton newermodels.

 

The S light is just like the check engine light, but it is for problems with the transmission. If the S light is flashing, then the trans control unit has detected a problem. Just like the check engine light flashes when the engine control unit detects a problem.

 

the s light means Sport, it's sport mode for the tranny. there is a switch on teh gear shifter to turn it on and off. it trims the gear shift times in sport mode.