Well you have the older OBD-1 system and doing various self-tests without use of a scan tool is fairly extensive and really requires you getting ahold of a workshop manual and doing the step by step procedures with the ignition key and a voltmeter. It's pages and pages of diagnostics and it's too much to post here, sorry.
I'd say in your case run it into a competent transmission shop that is all set up and pay them for the diagnostic time and then you'll at least know what's what. OBD-1 is a lot harder to use as a diagnostic tool than the OBD-II that came out a year later on your car.
Probably your problem is something related to this comment in the database:
"NOTE: When the overdrive indicator flashes on and off, a concern
has been detected in certain transmission related sensors or solenoids"