I have a 90' Honda Accord and have had transmission problems with my car for some time now. My "S" light had been saying on and I looked up solutions for it. I found that the TCU was the problem and needed a resistor and a capacitor changed. I changed them out and it worked for a little while, but occasionaly the "S" light would stay on and my car would be stuck in 3rd or 4th gear. One day my lights started to dim, my speedometer dropped out and my "S" light started blinking. My car started to lose power and I immediately knew it was my alternator was going out or was unplugging. So I checked it and the plug was falling out; that fixed the problem. But later my "S" light was blinking again, and it was staying on when driving. I went online to check solutions because I wanted to see if I could finally get this fixed. I found a post were a guy said his "S" light was on and his speedometer droped out. I put two and two together and went to check my alternator again and the plug was falling out again. I plugged it in and the "S" light turned off. To my amazment this was probably the problem all along.

So for you who have this "S" light problem, before going out and spending $400 on a rebuilt TCU or reparing it yourself. Check the alternator.