So my gauge is reading 12v which would mean it's just running off the battery. The thing is that it's been running like this for the past several days since I bought it so the alternator has to be working at least enough to keep the battery charged. I took it to Autozone to have the battery and alternator tested. The first time they said the battery was too low to test the alternator. I charged the battery and brought it back. This time they said the alternator was bad. Took out the alternator and had it tested out of the car and it passes. In the car it fails but outside it passes.

 

Is the two wire connector plugged into the back of the alternator? What is the condition of the wiring?

 

The little wires are plugged in and they seem to be ok. All the wiring I see is in good condition. I started the car and unhooked the battery and it stays running. The gauge still says it's only putting out 12v. While the battery was disconnected I turned on the headlights which shut all other electronic things in the car off for a split second.

 

When tested with a multimeter, what voltage is shown for alternator output? I assume you're not only going by the in-car gauge.

 

NEVER, EVER DISCONNECT THE BATTERY IN A COMPUTER CONTROLLED VEHICLE WHILE IT IS RUNNING! Be glad you have not fried anything in the car...

If you are not testing with an actual meter, then you may not even have a problem. The voltmeter in the car is only a general idea of what is happening.