This is the index page for code removal. Electronic Transmissions is at the bottomleft column, page 33-22.This is page 33-22 for clearing codes on all Toyota Electronic Transmissions up to 2006 Another page described how to check codes for Electronic Transmission but it only stated 1987-1995 two years short of your model without a scan tool.
This page states remove the Stop fuse (brake light) or fuse marked EFI in the engine compartment fuse box, for ten seconds. Have key in off position.Maybe this will help.
Stay away from expensive shops and drop in to mycarspace or google rearwheeldrive 1990 Q-45