Yeah, something is wrong with the computer. The 5v reference voltage is screwed up. You should be seeing 5v on either of the circuits you tested. Is the check engine light real dim looking?
The best place to check the 5 volt reference is at the MAP/Baro Sensor. It is on the same wire as the engine mounted sensor like the TPS and EGR.
Look for an orange/wire wire on the MAP/Baro sensor which is mounted on the firewall near the upper intake manifold.
Diagrams courtesy of Tmoss & Stang&2birds
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info Everyone should bookmark this site.
Man you guys really know your stuff. Thanks to all. After the last few days I feel I know this gt pretty good. My last one was a primo 89 gt vert with 62000 original. Never had a hiccup.
This is a triple black 91 vert wit 82000 on it. Way rougher though.
I'll take some pics and post it up now that we have been cleaning, polishing and repairing. It's gonna be our summertime hot rod.
Gonna do 3:73 gears, aluminum heads, new intake, cam, ect. 5 lug 4 wheel disc conversion and some suspension.
UPDATE
Got a 87 maf a9l maf computer used for a great price. Plugged it in and vroooom! Thanks to all. Appreciate all the help. Hope I can be as helpful to folks that come looking for answers.