Put in a new battery in my 1980 911SC tonight - old one was shot cause I failed to unhook it while stored for winter. Anyway, now my alternator light is on.

BUT!
Only when the ignition is off - key in or out. As soon as I turn the key, it goes out and stays out.

What could be causing this?

 

Your alt. has shorted diodes in positive rectifier assembly. You may have installed
the battery backwards or jumped it backwards. The alt. has to be replaced.

 

how could it be short ? if you hook or jump, backward, I thought it's only toast the battery

Can you just replace the recifier diode in the alt ?

TC_SJ

 

It also "cooks" ALL electronic/electrical devices which are connected
directly to the battery. Yes, you can replace alt. parts, but it's not
easy to find them.