[RIP]

The fact your light is on indicates the PCM has stored a fault code for you. Not sure about a 1990 but you may be able to download fault codes using a DRBII scanner. Take it to Autozone or the like and they'll do it free of charge and tell you what the codes mean. The scanner will reset the light. Or...for less difinitive codes try this: cycle your ignition switch from off to on (not start) three times leaving it on. Watch your "maintenance light" and count the series of flashes. A code 23 would look like flash, flash, pause, flash, flash, flash. If there's more than one code there will be a longer pause between codes. I think Allpar.com describes the codes. Keep in touch.