Why would the Voltage light come on? '93 Toyota Corolla?

There are 269,000 miles on this car. I have done all regular maint. It was a lien sale no frills vehicle before I got it. I just got a new battery as it needed one anyway and had the alternator checked. It registered no problems. When this light comes on my head lights with brighten. I have no owners manual etc. Also now it seems to not be shifting properly when going 65. It feels like it isn't going into overdrive (or whatever it is called). I do not want to ruin the car, but they want so much to just diagnose problems that I wouldn't be able to repair the vehicle spending what little I have to figure out the problem! Please advise if possible! Thank you :)