1980 Vette not charging battery
 

New 120 amp. 3-wire alt., new Optima red top, replaced "main" ground wire, old one corroded to pieces. On start the volt meter only goes to 13 V. and drops as load is added. Checked alt. with digital volt meter and the alt. reads 12.2 V. Any ideas?

Take the new alternator to a Battery Specialist dealer, or the like, and have it tested or put a known good alternator on your can and see if the problem is still there.

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See the attached photo - check for 12v using a test light or meter on terminal "2" as in the diagram. With the key switched to "on", there should be 12v to energize the field and allow the alternator to charge. With the key "off", there should not be voltage here (or it will drain your battery). Hope this helps...
JD