Maybe something is shorted and drawing the current down to 12 Volts.

The Alternator is charging and maxing out the amps and that is causing the acid to boil; but is unable to go above 12 Volts because of the short circuit drawing it off. (In this by short circuit I am meaning something is on that should not be on.)

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84 300D, 82 Volvo 244Gl Diesel