It might be the crank angle (photo optical thing) sensor.
I don't Know the test for this but getting the module for around $125 from CarQuest or other reputable dealer who carriers STANDARD BRAND electrical part,,,and they have to look it up in the big book ,,,not on the screen.

Very IMPORTANT and MOVE fast...There is a 90-91 federal emissions ( no egr) rebuilt ECU on EBAY ,,,,and the guy give really good diagnostic suggestions.
READ what he says before spending the big bucks. There is a rebuilder place in Houston you send yours with two day turnaround,,,NCM I think.
Dave , The Junk Yard Dog of Sacatomato

dave h.
'89 Raider V6 5spd;Aisin Hubs;; Gen2 LS: frt. brakes, rear coils;U.C.arms;R.trailing arms;idler arm; rear LSD axle w.disc brakes ;2 battery system for Dog's fan; relocated ECU; custom bumpers;J.Baker receiver;Conferr roofrack; t-bar crank.