A 90 Air bag diagnostic monitor has single-digit error codes (not counting error code #10).
So, that means you have:
An error code of #3 - Air Bag Deployment Circuit - Low Battery Voltage or
Low Backup Power Supply Voltage.
It's likely that the very old
back up air bag battery has finally lost it's ability to store a charge. The newer systems use a capacitor or a "super-capacitor".
See:
Mustang 90-91 Air Bag Diagnostic Codes
Mustang 90-91 Air Bag Diagnostic Codes
http://www.veryuseful.com/mustang/tech/airBag/images/airBag89-91Code3.jpg
http://www.veryuseful.com/mustang/tech/airBag/images/airBagPartsLoc90.jpg
NOTE: BEFORE ANY AIR BAG SYSTEM REPAIR is attempted:
1) Disconnect the battery
2) Push on the brake pedal to help drain any stored charge
3) Wait 5 mins.
So, there are a few options.
1) Try to get another backup battery from an old Ford. If you can find one that still works, realize that it too will likely fail in a "few" years. But, you "might" get another 5 or 10 years from the battery.
NOTE the following options
ALL VIOLATE FEDERAL LAW AND IF YOU EVER SELL THE CAR OR ANOTHER DRIVER IS IN THE CAR AND GETS INJURED IN A CRASH, YOU COULD EASILY BE SCREWED FOR ALL YOUR WORTH BY EVEN THE DUMBEST LAWYER!
AGAIN: It VIOLATES FEDERAL, and many STATE, LAWS TO MODIFY, IN ANY WAY, AN SRS (e.g. air bag) system!!
2) Simplest: As long as you don't care about having an air bag system if all of the battery power is lost before a crash (IMHO, very very unlikely),
Then: Remove the bad air bag battery back battery (see diagram for location), and connect the two wires together.
3) Get a super-cap that is rated
for use in automotive systems, and replace the bad air bag battery backup with a super-cap (~$20). Splice in the wires as needed. This gives back the functionality of having an air bag in case the battery voltage is lost right before a crash. I guess if a transformer comes to life, grabs your car by the hood, takes the battery and alternator, and drops the car, then it would be good to have that functionality.
Last: Note I am NOT a lawyer, nor have any degrees or courses in law!
From what information was
easily found on the interweb, and after many forum "discussions" (LOL
), it seems that
Federal Law does not prohibit the
owner and sole driver of the car from making mods to an SRS system. Still, if someone stole the car and got injured, the owner would be sued and mega screwed.
From what I've
quickly seen, most
State Laws prohibit any mods regardless of who did them and the use of the car (such as a track only car).
Good Luck!
), bummer, I'm fixing this car to sell it. I better fix it right. looks like I got my work gut our for me. I'll check voltage at terminals 13 & 14 and go from there. 
