Charging issues

[MikeyLionHeart]

78 c20 I put a new alternator and battery in and on a cold start I get about 15 volts charging, but when I turn it off and start it again I'm down to 12. If I turn anything on electrical at this point it just drains the battery. Any ideas what's happening, she was fine one day then all this started.

 

[SirRobyn0]

It's a very simple charging circuit. You can look up a better diagram in the wiring diagram here >>> https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/ <<< I you want to know wire colors.

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Internally regulated alternator.
#1. is this the same thing it was doing with the old alternator
#2. Brand of alternator you purchased and was it new or reman'd. There are problems with cardone reman'd products and they reman a lot of alternators that end up being rebranded, for example NAPA reman alternators are cardones.

The 15 volt, then no voltage makes me think the alternator you purchased likely has a defective regulator in it, but that's just kind of my gut feeling.

Here is some troubleshooting info.

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