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Am an old timer have been around a few years. Driving my 81 around today and suddenly my radio shut off and antenna retracted. Then in a few seconds radio came back on and antenna extended. Kept doing this and I thought wtf. Started looking at my gauges and they would shut down when radio shut off and the voltmeter needle would go in the red at 18 volts. This all happened at a slow speed. Kicked it up to highway speed to get home and everything leveled out. When home I tightened battery side terminals of which I thought were tight enough. I got about a quarter turn more on them. I put a new alternator on the car a month ago. It was just like I unhooked the battery but the car ran fine. I have not taken it for a test drive yet. Just wondering if everyone thinks I am on the right track with the terminals or maybe a dead short somewhere of which I have no idea where to start.
That battery negative cable can be hiding the problem by being corroded internally under the insulation on either end or corroded at the frame ground....same with the positive cable (been there!)
New alternator maybe going bad?...over volt indication could be an intermittant/defective voltage regulator....(internal to the alt. on an '81)...Good Luck!