Hey, I have a 82 Berlinetta with a 305. I recently got a new battery and all my problems started here. The battery I got worked for a few days then one day if i turned the key to crank all power would die, no lights, no door chime,and no noise of the engine trying to crank. Put the battery charger onto the start setting and she fired up. Idled for a few minutes, then tried to reverse out of my driveway and she died right away. Each time I started it, it would die under any load. Took it onto the highway and it died when i started to slow down, would not start back up and had to push it on the highway. The CEL came on half a second before it died. Under load it has a very low idle, around 350
The next day, it died again so I got a replacement battery which does the same thing. The second I turn the key the battery goes from 11.9v to 1.5v
Im thinking loose ground, what are you guys thinking?
Its rainy so I have a few days to think about where im going with this project.

 

I would check the ground and hot. Also could be the alternator not charging. Have it checked at your local autozone. Also might check the wire to the starter. A couple of days ago mine came loose there and would die on me. Hope this helps.

 

your alternator bushings could be bad

 

It sounds to me like you have a bad circuit...one that is shorting to ground and probobly a higher amp draw one. The CEL coming on could be the result of an engine diagnostic sensor that received low voltage triggering a light (and stored code). Start by placing a newly charged battery back in. If your battery was deeply discharged, you'll have to jump charge it so it can receive a full charge. This requires placing a good battery on the charger and using that battery to charge your deeply discharged one...as a deeply discharged battery will often not take a charge. At that point re-connect the battery, check volts with key off and then turn key on and re-check volts. If it drops, turn key off and start removing fuses from the block one by one until its isolated. i had a similar problem and it ended up being my hatch return relay, but it never dropped that much voltage...good luck!