You don't need a scan tool to get codes from and early 90s GM. All you need is a paper clip. Short the A and B terminal together of the scan tool connector and then count the flashes. It will flash 1 - 2 three times and then go into your codes. After it is done flashing each code three times it will flash 1 - 2 an additional three times before starting over. Get the codes from a Chilton's guide or go to the parts store. They will usually tell you what they mean.
