1990 Ford T-bird not starting, battery seems ok

[allstarr]

I have a 1990 Ford T-Bird. My car has been having starting issues for a while. I'm not sure if the seat belt light being on whenever I have the car on is part of the issue, or not. Note that it's on, even though I have it buckled.

When using Cen-tech digital multimeter, it reads 12.5 - 12.7.

Earlier today, I hooked up a Everstart 750a jump starter. Once red and black clamps were applied, and I turned the Everstart on, the engine icon keeps flashing. Doesn't become solid. It reads 12.7 - 12.9 v. All 4 battery bars are full.

Overhead light comes on when reattach battery, and re-enter the car. However, the check engine light comes on for a second, and other lights, then all goes out. OR it doesn't come on. Makes a clicking noise.

From this, it indicates that the battery seems to be ok. Alternator? Starter?

[aleekat]

So you try to start and nothing happens? Or you try to start and it just turns over? If so, check cranking voltage. You can't really test the alternator without the car running. Unless you pull it and take it to get tested. What model, engine?